Embryology. Germ cells.
Fertilization. Cleavage.
1. Philogenesis and
ontogenesis conception. Human embryogenesis periods.
2. Progenesis: oogenesis and
spermatogenesis. Meioses.
3. Spermatozoon
microscopic and submicroscopic characteristics. Biologic significance in
fertilization.
4. Oocytes types.
Ultrastructure and functions of a human oocyte.
5. Fertilization, stages
and biological significance.
6. Early stages of human
development: syncarion, zygote, conceptus.
7. Cleavage. Different kinds of cleavage. Blastulas types.
8. Human zygote cleavage, its characteristic features.
9. Blastocyst morphofunctional characteristics.
10. Tubal period of embryonic life of a human blastocyst.
11. Implantation site, significance and stages.
12. Abnormal implantation sites of blastocyst.
13. Histiotrophic and haematotrophic embryogenesis periods.
14. Extracorporal fertilization, medical and social significance.
Decrease of
population in highly developed countries on the one hand and its noncontroled
increase in the countries of the “third world” on the other hand is one of the
most important problem of the contemporary medicine. That's why for future
doctors it is necessary to know the hum-an embryogenesis. It begins with
progenesis which results in gametogenesis. Different influences can destroy
these processes and result in infertility or development of congenital
malformation. At the same time there may be some reasons which prevent
fertilization. Deep knowledge of all these processes will help doctors to make
a correct diagnosis and prescribe an adequate treatment to the patients with
sterility and to prevent nondesired pregnancy.
Embryology is the science about the embryo origin and
development (from Greek “embryon”–embryo and “logos”–study or science). Modern
biologic sciences recognize general and special embryology. The first one
studies the most common features of embryo origin and development. Special
histology – science about the individual development of particular group of
animals,classes and so on. Teratology studies abnormalities of embryo
development.
Main tasks of embryology learning are the next:
1.
To understand the development of different tissues and organs from
single cell (zygote).
2.
To make rational explanation of adult structures location and relationships.
3.
Embryology explains development of extraembryonic organs (fetal
membranes and others), which connect embryo (fetus) and mother and whose normal
functions are absolutely necessary for normal embryogenesis.
4.
The knowledge of normal development allowes to understand the mechanisms
and results of congenital abnormalities, which is the basic point of
their prevention.
5.
Embryology is general science, which interconnects anatomy and
histology, physiology and pathology, surgery and physical diagnosis, pedyatrics
and obstetrics.
In the course of human life there are 3 principal periods:
progenesis, prenatal and postnatal periods. Progenesis includes development of
germ cells. Prenatal period is subdevided into embryonic and fetal periods.
Progenesis or gametogenesis
is a process of a germ cells production (ovogenesis for oocytes and
spermatogenesis for spermatozoa). These processes take place in male
and female sex glands (ovaries and testes). Spermatogenesis begins in puberty
(13-16 years) and lasts till the old age. Oogenesis begins before the
birth then it is stopped and is renewing in puberty, and continuous to
menopause.
MEIOSIS
Meiosis is the cell division that takes place in
the germ
cells
to
generate male and female gametes, sperm and egg cells, respectively.
Meiosis requires two cell divisions, meiosis I and meiosis II, to
reduce the number of chromosomes to the haploid number of 23
(Fig. 1.3). As in mitosis, male and female germ cells (spermatocytes and primary
oocytes)
at the beginning of meiosis I replicate their DNA so that each of
the 46 chromosomes is duplicated into sister chromatids. In contrast to mitosis,
however, homologous chromosomes then align themselves in pairs, a
process called synapsis. The pairing is exact and point for point except for the
XY combination. Homologous pairs then separate into two daughter cells.
Shortly thereafter meiosis II separates sister chromatids. Each gamete then contains
23 chromosomes.
Crossovers, critical events in
meiosis I, are the interchange of chromatid segments between paired homologous
chromosomes. Segments of chromatids break and are exchanged as homologous
chromosomes separate. As separation occurs, points
of interchange are temporarily united and form an X-like structure, a chiasma.
The approximately 30 to 40 crossovers (one or two per
chromosome) with each meiotic I division are most frequent between genes that are
far apart on a chromosome. As a result of meiotic divisions, (a)
genetic variability is enhanced through crossover, which redistributes
genetic material, and through random distribution of homologous chromosomes
to the daughter cells; and (b) each germ cell contains a haploid number
of chromosomes, so that at fertilization the diploid number of 46 is restored. Polar Bodies Also during meiosis one
primary oocyte gives rise to four daughter cells, each with 22 plus 1 X
chromosomes. However, only one of these develops into a mature gamete, the
oocyte; the other three, the polar bodies, receive little cytoplasm and
degenerate during subsequent development. Similarly, one primary spermatocyte
gives rise to four daughter cells, two with 22 plus 1x.
Maturation of Sperm begins at Puberty. Spermatogenesis, which
begins at puberty, includes all of the events by which spermatogonia are
transformed into spermatozoa. At birth, germ cells in the male can be recognized in
the sex cords of the testis as large, pale cells surrounded by supporting cells (Fig.
1.21A). Supporting cells, which are derived from the surface
epithelium of the gland in the same manner as follicular cells, become sustentacular
cells, or Sertoli cells. Shortly before puberty,
the sex cords acquire a lumen and become the seminiferous tubules. At about the same time,
primordial germ cells give rise to spermatogonial stem cells. At regular
intervals, cells emerge from this stem cell population to
form type A spermatogonia, and their production marks the initiation of
spermatogenesis. Type A cells undergo a limited number of mitotic divisions to
form a clone of cells. The last cell division produces type B spermatogonia, which then divide to form
primary spermatocytes. Primary spermatocytes then enter a prolonged prophase (22 days)
followed by rapid completion of meiosis I and formation of secondary
spermatocytes. During the second meiotic division, these cells immediately begin to form
haploid spermatids. Throughout this series of events,
from the time type A cells leave the stem cell population to formation of
spermatids, cytokinesis is incomplete, so that successive cell generations are
joined by cytoplasmic bridges. Thus, the progeny of a single type A spermatogonium
form a clone of germ cells that maintain contact throughout
differentiation. Furthermore, spermatogonia and spermatids remain embedded in deep
recesses of Sertoli cells throughout their development. In
this manner, Sertoli cells support and protect the germ cells, participate
in their nutrition, and assist in the release of mature spermatozoa.
Spermatogenesis is
regulated by luteinizing hormone (LH) production by the pituitary. LH binds
to receptors on Leydig cells and stimulates testosterone production, which in turn
binds to Sertoli cells to promote spermatogenesis.
Follicle stimulating
hormone (FSH) is also essential because its binding to Sertoli cells stimulates
testicular fluid production and synthesis of intracellular androgen receptor
proteins.
Spermiogenesis The series of changes
resulting in the transformation of spermatids into spermatozoa is spermiogenesis. These
changes include (a) formation of the acrosome, which covers half of the
nuclear surface and contains enzymes to assist in penetration of the egg and its
surrounding layers during fertilization condensation of the nucleus; formation
of neck, middle piece, and tail; and (d) shedding
of most of the cytoplasm. In humans, the time required for a spermatogonium to
develop into a mature spermatozoon is approximately 64 days.
When fully formed, spermatozoa enter the
lumen of seminiferous tubules. From there, they are pushed toward the epididymis
by contractile elements in the wall of the seminiferous tubules. Although
initially only slightly motile, spermatozoa obtain full motility in the
epididymis.
In preparation for possible fertilization, both male
and female germ cells undergo a number of changes involving the chromosomes as
well as the cytoplasm. The purpose of these changes in meiosis is twofold:
1. Reduction of chromosomes number to
half that in the normal somatic cells, i.e., from 46 to 23. This is
accomplished by two specialized divisions, known as meiotic or maturation
divisions. The reduction in the number of chromosomes is necessary, since
otherwise fusion of a male and female germ cell would result in an individual
with twice number of chromosomes of parent cells.
2. To alter the shape of germ cells in
preparation for fertilization. The male germ cell, initially large and round,
loses practically all of its cytoplasm and develops a head, neck and tail. The
female germ cell, on the contrary, gradually becomes larger as the result of an
increase in the amount of cytoplasm. At maturity the oocyte has a diameter of
about 120 mm.
CLINICAL CORRELATES
Chromosomal
abnormalities, which may be numerical or structural, are important causes of birth
defects and spontaneous abortions. It is estimated that 50% of conceptions end in spontaneous abortion and
that 50% of these
bortuses
have major chromosomal abnormalities. Thus approximately 25% of conceptuses have a
major chromosomal defect. The most common chromosomal abnormalities in
abortuses are 45,X (Turner syndrome), triploidy, and trisomy 16.
Chromosomal abnormalities account for 7% of major birth defects, and gene
mutations account for an additional 8%.
Numerical Abnormalities
The normal human somatic
cell contains 46 chromosomes; the normal gamete contains 23. Normal
somatic cells are diploid, or 2n; normal gametes are haploid, or n.
Euploid refers to any exact multiple of n, e.g., diploid or triploid. Aneuploid refers
to any chromosome number that is not euploid; it is usually applied when an
extra chromosome is present (trisomy) or when one is missing (monosomy).
Abnormalities in chromosome number may originate during meiotic or mitotic
divisions. In meiosis, two members of a pair of homologous chromosomes
normally separate during the first meiotic division so that each daughter
cell receives one member of each pair.
Sometimes, however, separation does not occur (nondisjunction),
and both members of a pair move into one cell (Fig. 1.5, B and C
). As a result of nondisjunction of the chromosomes, one cell
receives 24 chromosomes, and the other receives 22 instead of the normal
23. When, at fertilization, a gamete having 23 chromosomes fuses with a
gamete having 24 or 22 chromosomes, the result is an individual with
either 47 chromosomes (trisomy) or 45 chromosomes (monosomy).
Nondisjunction, which occurs during either the first or the second meiotic
division of the germ cells, may involve the autosomes or sex chromosomes. In
women, the incidence of chromosomal abnormalities, including
nondisjunction, increases with age, especially at 35 years
and older.
Occasionally
nondisjunction occurs during mitosis (mitotic nondisjunction) in an embryonic cell
during the earliest cell divisions. Such conditions produce mosaicism, with
some cells having an abnormal chromosome number and others being normal.
Affected individuals may exhibit few or many of the characteristics of
a particular syndrome, depending on the number of cells involved and their
distribution.
Sometimes chromosomes
break, and pieces of one chromosome attach to another. Such translocations
may be balanced, in which case breakage and reunion occur between two
chromosomes but no critical genetic material is lost and individuals are
normal; or they may be unbalanced, in which case part of one chromosome is
lost and an altered phenotype is produced. For example, unbalanced
translocations between the long arms of chromosomes 14 and 21 during meiosis
I or II produce gametes with an extra copy of chromosome21, one of the causes
of Down syndrom.
Translocations. Loss of the short arms is
not clinically significant, and these individuals are
clinically normal, although they are at risk for
producing offspring with unbalanced
translocations. B. Karyotype of translocation of chromosome
21 onto 14, resulting in
Down syndrome. Karyotype of trisomy 21 (arrow), Down
syndrome. are particularly common between chromosomes 13, 14, 15, 21,
and 22 because they cluster during meiosis.
Down
syndrome is usually caused by an extra copy of chromosome 21 (trisomy). Features
of children with Down syndrome include growth retardation; varying
degrees of mental retardation; craniofacial abnormalities, including upward slanting
eyes, epicanthal folds (extra skin folds at the medial corners of the eyes),
flat facies, and small ears; cardiac defects; and hypotonia. These individuals
also have relatively high incidences of leukemia, infections, thyroid
dysfunction, and premature aging. Furthermore, nearly all develop signs of Alzheimer’s
disease after age 35.
In 95% of cases, the syndrome is caused by
trisomy 21 resulting frommeiotic nondisjunction, and in 75% of these
instances, nondisjunction occurs during oocyte formation. The incidence of Down
syndrome is approximately 1 in 2000 conceptuses for women under age 25.
This risk increases with maternal age to 1 in 300 at age 35 and 1 in 100 at
age 40. In approximately 4% of cases of Down syndrome, there is an
unbalanced translocation between chromosome 21 and chromosome 13, 14, or
15 .
The final 1% are caused by mosaicism resulting from mitotic nondisjunction. These
individuals have some cells with a normal chromosome number and some that are
aneuploid. They may exhibit few or many of the characteristics of
Down syndrome.
TRISOMY 18. Patients
with trisomy 18 show the following features: mental retardation,
congenital heart defects, low-set ears, and flexion of fingers and hands.
In addition,
patients frequently showmicrognathia, renal anomalies, syndactyly, and malformations of the
skeletal system. The incidence of this condition is approximately 1 in 5000
newborns. Eighty-five percent are lost between 10 weeks of gestation and
term, whereas those born alive usually die by age 2 months.
TRISOMY 13. The
main abnormalities of trisomy 13 are mental retardation,
holoprosencephaly, congenital heart defects, deafness, cleft lip and
palate, and eye defects, such as microphthalmia, anophthalmia, and
coloboma. The incidence of this abnormality is approximately 1 in 20,000 live births, and over 90%
of the infants die in the first month after birth.
KLINEFELTER SYNDROME. The
clinical features of Klinefelter syndrome, found only in males and
usually detected at puberty, are sterility, testicular atrophy,
hyalinization of the seminiferous tubules, and usually
gynecomastia. The cells have 47 chromosomes with a sex chromosomal
complement of the XXY type, and a sex chromatin body (Barr body: formed by condensation of
an inactivated sex chromosome; a Barr body is also present in normal females) is
found in 80% of cases. The incidence is approximately 1 in 500 males. Nondisjunction
of the
XX homologues is the most common causative event. Occasionally, patient with Klinefelter syndrome
have 48 chromosomes: 44 autosomes and four sex chromosomes
(XXXY). Although mental retardation is not generally
TURNER SYNDROME. Turner syndrome, with a 45,X karyotype, is the only monosomy
compatible with life. Even then, 98% of all fetuses with the syndrome
are spontaneously aborted. The few that survive are unmistakably
female in appearance (Fig. 1.12) and are characterized by the absence
of ovaries (gonadal dysgenesis) and short stature. Other
common associated abnormalities are webbed neck, lymphedema of the
extremities, skeletal deformities, and a broad chest with widely spaced
nipples. Approximately 55% of affectedwomen are monosomic for the X and chromatin
body negative because of nondisjunction. In 80% of these women,
nondisjunction in the male gamete is the cause. In the remainder of women,
structural abnormalities of the X chromosome or mitotic nondisjunction
resulting in mosaicism are the cause.
TRIPLE X SYNDROME. Patients with triple X
syndrome are infantite, with scanty menses and some degree of mental
retardation. They have two sex chromatin bodies in their cells.
Structural Abnormalities
Structural chromosome abnormalities, which involve one or more
chromosomes, usually result from chromosome breakage. Breaks are caused by environmental factors,
such as viruses, radiation, and drugs. The result of breakage depends on what
happens to the broken pieces. In some cases, the broken piece of a
chromosome is lost, and the infant with partial deletion of a chromosome is abnormal.
A well-known syndrome, caused by partial deletion of the short arm of
chromosome 5, is the cri-du-chat syndrome. Such children have a catlike
cry, microcephaly, mental retardation, and congenital heart disease. Many other
relatively rare syndromes are known to result from a partial chromosome
loss. Microdeletions, spanning only a few contiguous genes, may
result in microdeletion syndrome or contiguous gene syndrome. Sites
where these deletions occur, called contiguous gene complexes, can
be identified by high-resolution chromosome banding. An example of a
microdeletion.
Inheriting the deletion on the maternal
chromosome results in Angelman syndrome, and the children are mentally
retarded, cannot speak, exhibit poor motor development, and are prone to
unprovoked and prolonged periods of laughter. If the defect is inherited on the
paternal chromosome, Prader-Willi syndrome is produced; affected
individuals are characterized by hypotonia, obesity, mental
retardation, hypogonadism, and cryptorchidism.
Characteristics
that are differentially expressed depending upon whether the genetic material is
inherited from the mother or the father are examples of genomic imprinting. Other contiguous gene
syndromes may be inherited from either parent, including Miller-Dieker
syndrome (lissencephaly, developmental delay, seizures, and
cardiac and facial abnormalities resulting froma deletion at 17p13) and
most cases of velocardiofacial (Shprintzen) syndrome (palatal defects,
conotruncal heart defects, speech delay, learning disorders, and schizophrenia-like
disorder resulting from a deletion in 22q11). Fragile sites are regions of
chromosomes that demonstrate a propensity to separate or break
under certain cell manipulations. For example, fragile sites can be revealed by
culturing lymphocytes in folate-deficient medium. Although numerous fragile
sites have been defined and consist of CGG repeats, only the site on the long
arm of the X chromosome (Xq27) has been correlated with an
altered phenotype and is called the fragile X syndrome. Fragile X syndrome is
characterized by mental retardation, large ears, prominent jaw, and pale blue
irides. Males are affected more often than females (1/1000 versus 1/2000),
which may account for the preponderance of males
among the mentally retarded. Fragile X syndrome
is second only to Down syndrome as a cause of mental retardation because
of chromosomal abnormalities.
Gene Mutations
Many congenital formations
in humans are inherited, and some show a clear mendelian pattern of
inheritance. Many birth defects are directly attributable to a change in the
structure or function of a single gene, hence the name single gene mutation. This type of defect is
estimated to account for approximately 8% of all human
malformations.
With the exception of the
X and Y chromosomes in the male, genes exist as pairs, or alleles, so
that there are two doses for each genetic determinant, one from the mother and
one from the father. If a mutant gene produces an abnormality in a single
dose, despite the presence of a normal allele, it is a dominant mutation. If both alleles must be
abnormal (double dose) or if the mutation is X-linked in
the male, it is a recessive mutation. Gradations in the effects of mutant genes
may be a result of modifying factors. The application of
molecular biological techniques has increased our knowledge of genes
responsible for normal development. In turn, genetic analysis of human
syndromes has shown that mutations in many of these same genes are
responsible for some congenital abnormalities and childhood diseases. Thus, the link
between key genes in development and their role in clinical syndromes is
becoming clearer.
In addition to causing
congenital malformations, mutations can result in inborn errors of
metabolism. These diseases, among which phenylketonuria, homocystinuria, and
galactosemia are the best known, are frequently accompanied by or cause various
degrees of mental retardation.
Diagnostic Techniques for
Identifying Genetic Abnormalities
Cytogenetic analysis is used to assess
chromosome number and integrity. The technique requires
dividing cells, which usually means establishing cell cultures that are
arrested in metaphase by chemical treatment. Chromosomes are stained with Giemsa stain to reveal light and dark
banding patterns (G-bands) unique for each chromosome. Each band
represents 5 to 10×106 base pairs of DNA, whichmay include a
fewto several hundred genes. Recently, high resolution metaphase banding techniques have been developed that demonstrate greater
numbers of bands representing even smaller pieces of DNA, thereby
facilitating diagnosis of small deletions.
New molecular techniques, such as fluorescence in situ
hybridization (FISH), use specific DNA probes to identify
ploidy for a few selected chromosomes. Fluorescent probes are
hybridized to chromosomes or genetic loci using cells on a
slide, and the results are visualized with a fluorescence microscope. Spectral
karyotype analysis is a technique in which every chromosome is
hybridized to a unique fluorescent probe of a different color. Results are then analyzed by a computer.
The proper embryogenesis means the development of an embryo.
More commonly it occupies all the period of pregnancy. But to be exact, this
period may be subdivided into such stages: the early period of conceptus
(first 7 days), the proper embryonic period (2th -8th weeks),
and fetal period (9th week - to the birth).
Postembryonic
period includes neonatal period, childhood, pubertation, youth, adult, an old
age, which are studied by different sciences, for example: neonatology,
pediatrics, gerontology and so on.
The early period of conceptus and embryonic period and progenesis are
the subjects of a proper embryologic investigation.
All the individual development of organism from the very beginning
(origin) and to the death means ontogenesis in
opposite to phylogenesis – historical
development of some species.
The interconnection between these two terms was described at the end of
XIX c. in the biogenetic law of Gekkel-Muller:
ontogeny is the short repeating of the phylogeny.
Methods
of research
Modern embryology uses different methods of research.
The most important of them are the next:
1. Watching and
descriptions.
2. Comparing.
3. Evolutionary
method.
4. Experimental
method.
The simplest
is the method of observation and description of embryo in certain animals or
humans. On this basis, there descriptive embryology.
Much
later method of comparison was began to use, which made
comparative embryology . With this method K.M.Ber (1828)
discovered the law of embryos similarity.
The
gradual accumulation of detailed and accurate information about the structural
changes that occur to the embryo during its development prepared the basisi for
evolutionary embryology, the founders of which were O.Kovalevskyy and
I.Mechnykov. The development of evolutionary embryology led to the discovery of
biogenetic law by E.Hekkel and F.Muller.
Experimental
embryology emerged due to the development and improvement of the experimental
technique. The founders of this method is V.Ru , H.Drish and H.Shpeman.
Experimental embryology is trying to identify control and regulatory mechanisms
of development.
Brilliant
success of molecular biology have given rise to the development of a special
section , which was named biochemical embryology.
Teratology
- a section of embryology, which studies malformations.
There are two contrasting theories of embryogenesis in modern embryology,
which describe the human development– preformation and epigenesis.
The basic point of preformation theory is the next: from the very
beginning of the embryogenesis the whole body of the human being is presented
in embryo and after fertilization his little organs can enlarge. There are two
trends in this theory. Spermatics support Hippocrat, who suppose the
spermatozoon been principle material of the future human body and oocyte only
gives nutrition to this. Their opponents – ovists – consider the oocyte
contains the main structures of human being and the biologic meaning of
spermatozoa is to activate them in definite way. This theory was restored and
especially widespread in XVII-XVIII c.c.
The XVIII-XIX c.c. was the period of an active “struggle” between the
different points of scientific wiev on the human embryogenesis. In 1775 Lazano
Spallanzani showed that both the ovum and sperm are necessary for new
individual development. At the middle of XIX c. molecular biology had begun to
develop. The structure and functions of DNA were described and in 1759 Caspar
Wolff had grounded the theory of postformation or epigenesis, which
principal ideas were firstly created by Aristotel many centuries ago. The main
points of this theory are:
1. Development
of new organism begins after the merging of oocyte and spermatozoon.
2. Characteristic
set (collection0 of chromosomes is renewed.
3. Genetic
information begins its realization.
Heinrich Christian Pander later had described three germ layers of
embryo (blastoderm), from which different structures appear.
Active development of medicine and embryologic investigations of the
past hundred years have demonstrated an epigenetic nature of embryo
development. The fertilized egg, possessing a simple form and exhibiting an
apparently undifferentiated structure, undergoes a series of developmental
changes, which result in the spatial differentiation of the mature organism
with its specialized types of cells, tissues and organs. But this
differentiation is directly connected with genes, which are present in the
chromosomes of zygote nucleus. So, special features of human body in future
depend on the heredity information of oocyte and spermatozoon, which
participate in fertilization.
The basic processes of embryogenesis
From the fertilized egg (zygote) multicellular organism is developing.
In the zygote genes are repressed (inactive) and are in the form of
heterochromatin . Heterochromatin - a repressed genetic material
(DNA chromosomal protein bound histones). Start of development is characterized
by expression ( activation ) of certain groups of genes that dismissal of DNA from
histones associated with it. Euchromatin - is expressed (functioning) genes. At
first are derepressed genes that cause an increase in cell number (proliferation) and regulate general cell metabolism. Specifical
genes for tissues are activated in the stage of gastrulation. In the process of
organo- -and histogenesis the other genes are included that regulate a specific
function of differentiated cells.
Zygote has the possibility to form a whole body. Some cells which appear
as a result of first few divisions after fertilization retain this ability. These
cells are called totypotential. With the growth
of the cells they gradually lose their ability to form all types of cells that
are present in the adult body. When cells are developing they loose some
possibilities. This reduction of choises is called determination. Determination
- a process in which a group of cells selects only one of many possible ways of
development. Gene expression lies in
base of determination as a result of this cell (or group of cells) becames a
specialized cell type.
Differentiation is the outward
manifestation of determination which is accompanied by a gradual restriction of
potencies in cell . Differentiation is the
morphological and functional expression of the part of the genom, which
remained in the possession of a cell or group of cells. As a result of
differentiation, some cells acquire new properties (specialized) , but also
lose some of its previous capacity. For example, functional differentiation is seen
as developing the ability to contractions in myocytes or conductivity in
neurocytes .
From the morphological point of view the final
differentiation resulting in the formation of numerous specialized cells and
structures. Thus, the difference between the cells that have the same set of
genes is determined by differentiated repression and expression of genes. Different cell types
express different genes.
Due to the fact that protein synthesis is dictated by the expression of
genes , the genes are divided into two categories. Genes of the first - operate
in all body cells , determining their basic metabolism and insensitive to the
direct effects that cause differentiation. Genes of second group are controlling
the synthesis of proteins that define the difference between cells during
development ( their specialization ) and responding to factors that induce
differentiation (inductors).
The question is, what is the reason of expression and repression of different
genes? In cleavage of zygote daughter
cells receive identical sets of chromosomes (46 for humans ), but not always
cytoplasm is divided evenly between daughter cells either in quantity or in
quality. It is known that the unequal distribution of different parts of the
fertilized egg cytoplasm between daughter cells leads to differentiation of the
latter into cells of various types. Thus, the cytoplasm activizes mechanism
that makes the impact on the nucleus for inclusion (expression) of some and
exclusion (repression) of other genes.
An essential feature of differentiation is that the differentiation of
cells usually occurs after their proliferation. Cells that are dividing quickly
are nondifferentiated (eg mesenchymal cells). In contrast, highly
differentiated cells usually lose their ability to proliferate (eg neurocytes ,
cardiomyocytes, red blood cells ).
Differentiation is irreversible and unidirectional - from less
differentiated to more differentiated structure.
Morphogenesis - the embodiment
of the spatial plan of the embryo. Morphogenesis is the implementation of
various processes: growth, induction of directed cell migration , cell-cell
interactions, cell death.
Growth - increase
in mass and linear dimensions by increase of cells amount, morphofunctional units of organs, organs and
organ systems. The increase in mass without cell division observed in
hypertrophy in normal (hypertrophy of the myometrium during pregnancy) and
pathological conditions. The body produced numerous humoral factors (growth
factors) that stimulate the growth and proliferation of various cell types.
Specific cell types that emerged as a result of the determination, are forming the tissue. The process in which individual
tissues during differentiation acquire their characteristic appearance called histogenesis. Organs originate of the various tissue
cells. Induction is an important process of
organogenesis. Induction means the influence of one embryo tissue (inductor) to
another target tissue, resulting in the formation of qualitatively new
structure. Examples of primary embryonic induction is the formation of the
nervous system of the dorsal ectoderm under the influence of the inductor - chordomezoderm.
Hans Shpemana for the discovery of organizer and the phenomenon of primary embriological
induction in 1935 won the Nobel Prize. All subsequent induction processes in
the embryo are called secondary induction.
Repeated movement or migration of
individual or groups of cells from one part to another areb typical for the
development of embryo. Often embryonic cells appear and multiply in one place,
and differentiate and make their function in another (eg gonoblasts are formed
in the yolk sac and migrate to the testes and ovaries, neural crest cells
migrate and differentiate into melanocytes of epidermis, adrenal chromaffin
cells, neurocytes of autonomic and spinal ganglia and so on).
The concept of positional information explains
the nature of cellular interactions in morphogenesis. The overall plan
structure of the body is determined very early in the embryonic development.
Later, during the whole period of the formation of a body or body parts,
morphogenesis specified by positional information signals. Due to this concept,
the cell knows its place in the body and differentiates in accordance with this
provision. Cell receives positional information from other cells. Moreover, the
cell reaches the final differentiation in the case of receiving of series of consecutive signals of positional
information. The field within which positional information signals are well working,
is called morphogenetic field. During the next
several divisions, cells of morphogenetic fields keep the memory of their
initial appointment.
The death of cells - a necessary component of embryonic development.
Natural (geneticically programmed) cell death during the early stages of
ontogenesis is realized by apoptosis. For example , from 25% to 75 % of the
total population of neuroblasts are dying at different stages of nerve system
development
Germ
cells
There are some principle differences between germ cells and somatic
ones. First of all, germ cells contain 23 chromosomes, they have possibility to
assimilation and without fertilization they die in a short period of time.
Somatic cells have 46 chromosomes, they can not merge but they are still alive
without fertilization.
Principle differences between germ cells and somatic ones are shown
below
|
Differences |
Germ
cells |
Somatic
cells |
|
Chromosomal
set |
23
(haploid) |
23x2
(diploid) |
|
Possibility
to assimilation |
Have |
Have
no |
|
Without
fertilization |
Die |
Live |
Prenatal period
Prenatal (antenatal period) begins from male and female germ cells
meeting, which result in zygote formation. This period includes next processes:
1. Fertilization
–fusion of germ cells.
2. Cleavage
– series of rapid zygote devisions with short interphase, which result
in formation of multicellular organism – blastula.
3. Gastrulation –
process of germ layers (fetal membranes) formation (ectoderm, endoderm and
mesoderm).
4. Neurulation –
formation of axial organs: notochord, neural tube and premordial gut.
5. Histogenesis –
period of tissues orogin and formation.
6. Organogenesis
means the appearance and and development of recognizable organs.
|
Stages and processes of embryogenesis |
||
|
Process |
Result |
Place
and tyme |
|
Fertilization |
Zygote
(unicellular organism) |
Fallopian
tube ampula |
|
Cleavage |
Blastula
(multicellular organism) |
Fallopian
tube 2nd -4th days and uterine cavity 5th-7th
days |
|
Gastrulation |
Gastrula
(bilayered organism) |
7-14th
days, endometrium |
|
Neurulation |
Neurula
(threelayered organism with nerve tube) |
14th
-17th days … |
|
Histogenesis |
Tissues |
|
|
Organogenesis |
Organs |
|
|
Systemogenesis |
Systems
of organs |
|
postnatal period
Postnatal period occurs after the birth and
includes infancy, childhood, puberty, adolescence, adulthood.
Infancy includes the
first year after the birth. Newborn is infant of 1 month or less.
Childhood is period
from 13 month untill puberty. During this period primary and secondary teeth
are appeared.
Puberty is period from
12 to 15 years, during which secondary sex characteristics are developing.
Puberty end in female with the first menstrual cycle and in male with first
pollution.
Adolescence
is the period from about 11 to 19 years of age, during which rapid physical and
sexual maturation occurs. There are active ossification (bones formation),
growth of the body and organs.
Adulthood is the
period of full growth and maturity (18-21 years).
Spermatozoa
Spermatozoa – the male germ
cells were exactly described by Levenguk and his pupil Gamm in 1677. They are
60-70 mkm long and have a very specific shape: each spermatozoon has a head,
neck, body and tail.
>
The head of spermatozoon is of oval flattened shape. It contains
elongated nucleus with condensing chromatin. A small amount of cytoplasm in the
head makes a special shape with perforatorium- sharp top of the spermatozoon
head. It is the most suitable shape for passage forward due to the moving of
flagella. There are 23 chromosomes inside – 22 autosomes and the last one – sex
chromosome (x or y). Accordingly, there are two types
of spermatozoa: androspermia and gynecospermia. Nuclear envelope has no
nuclear pores.
There is a special structure – acrosome –
at the top of spermatozoon head. This acrosomic cap, or galea capitis is a
specifically changed Golgi apparatus, which contains enzymes hyaluronidaze,
trypsin-like, glucosidases and other. Bilaminar acrosomal cap covers the
anterior two-thirds of the nucleus. Acrosomal enzymes facilitate spermatozoon penetration
of the corona radiata and zona pelucida in fertilization.
The neck is narrow: it contains a funnel-shaped basal body and a
spherical centriole. The basal body is also called the connecting piece because
it helps to establish an intimate union between the head and remainder of the
spematozoon. Axial fiber (axonema) is the principal component of the body and
tail. It is composed of 9 pairs of peripheral microtubules and one
central pair. The axial filament begins just behind the centriole. It passes
through the middle piece and most of the tail. The middle piece is long
cylinder (1x7 ìm) with a lot of mitochondria, which
are helically wrapped around the axonemal complex for energy supply of
spermatozoon.
Sperm is a male seminal
fluid (semen) – complex mixture produced by testis, the seminal vesicles and
prostate gland. 3-5 ml of semen contains 200- 300 million
spermatozoa (20-200 mln/ml). 75 % of them are alive, 50 % actively
moving, 60-70 % have normal structure. Secretions derived from the male
accessory glands. There are some growing, maturing cells like spermatocytes and
spermatids and some epitheliocytes and leucocytes in the sperm. The fluid of
the sperm is presented with water and some organic and mineral components. The
chemical reaction of a sperm is alkaline, just in such medium spermatozoa are
most active.
Biologic significance of
spermatozoon in the fertilization.
1. Active
passage toward to the ovum.
2. It gives 23 “paternal”
chromosomes.
3. The sex of future baby
depends on spermatozoon type.
4.Gives the centriole.
5. Introduces special cleavage
signal protein.
6. Gives mitochondrial DNA.
7. Promotes oocyte meiosis.
OOCYTE
Oocyte (ovum) – female
germ cell was described by Karl fon Ber in 1827. It is the largest cell
(130-150 mkm in diameter) in human body which can not move.
Oocytes are developing in ovary in special vesicles – follicles, which
undergo prominent changes due to maturation of ovum. Mature follicle
(Graaphian) wall has special follicular (granulose) cells and antrum is filled
with estrogens. Mature oocyte (II) is settled over cumulus oophorus and is
covered with a few tunics. From outside they are: corona radiata (tunica
granulosa), zona pellucida and oolemma. The last tunica oolemma is the
proper cytolemma. Zona pellucida is a special chemical membrane, which contains
much glycoproteins and glycosoaminglicans. The outer layer consits of numerous
small follicular cells, which give their processes to the ovum through the
previous tunic. (
Ovulation is the process by which an oocyte is released from the ovary.
Beginning from puberty, usually one follicle matures each month and ovulation
occurs. After ovulation, oogenesis is completed outside the ovary, in the
uterine tube. In the case of fertilization the secondary oocyte completes
second meio.
Nucleus of human ovum lies eccentrically and contains 23-d chromosomes.
There are all the organells of a general meaning in the ooplasm: mitochondria,
endoplasmic reticulum, few lysosomes. But cell center is absent, so oocyte
needs this organell for successful maturation (in fertilization spermatozoon
will bring it). Smooth endoplasmic reticulum and Golgi apparatus is
especially well developed and produce yolk inclusions. These inclusions are
arranged in small granules, which are dispersed in cytoplasm.
In the peripheral layer of cytoplasm just under the plasmalemma numerous
cortical granules may be seen. They take an active part in the process of
fertilization.
Oocytes of different species may be classified
due to the amount and disposition of yolk inclusions.
1. Alecytal
ones have no inclusions (insects).
2. Oligolecytal
cells contain moderate yolk inclusions. In primary isolecytal oocytes nuclei
lie in the center of cells, in secondary isolecytal (female) nuclei are
disposed eccentrically.
3. Fish and birds have polylecytal oocytes with great volume of yolk.
According to location of this inclusions there are centrolecital and
telolecital oocytes.
Human oocyte is
oligolecital secondary isolecital.
Biologic significance of
oocyte in the fertilization process.
1.
They give half of a necessary amount of chromosomes.
2.
Supply the nutrition of embryo.
3. Protection of embryo at the early stages of
development.
Oocyte
surface wiev in scanning microscope.
FERTIIZATION
Fertilization
is the first stage of embryogenesis, the process by which the male and female
gametes fuse and result in zygote appearance.
The
biological significance of fertilization are the next:
1. The restorations of the full chromosomal set.
2. Determination of the embryo sex.
3. Initiation of a cleavage.
In female this process takes
place in the ampullar portion of the Fallopian (uterine) tube. Some specific
processes occur in the course of fertilization, which includes distant and contact stages.
Distant stage
of fertilization means special interaction of ovum (in uterine tube) and
spermatozoa, which enter female vagina. Contact
stage begins after their meeting in uterine tube.
Only 1% of sperm deposited in the vagina
enter the cervix, where they may survive for many hours. Movement of sperm
from the cervix to the oviduct is accomplished primarily by their own
propulsion, although they may be assisted by movements of fluids
created by uterine cilia. The trip from cervix to oviduct requires a
minimum of 2 to 7 hours, and after reaching the isthmus, sperm become less motile
and cease their migration. At ovulation, sperm again become motile,
perhaps because of hemoattractants produced by cumulus cells surrounding the
egg, and swim to the ampulla where fertilization usually occurs.
Spermatozoa are not able to fertilize the oocyte immediately upon arrival in the
female genital tract but must undergo capacitation and the acrosome reaction to
acquire this capability. Capacitation is a period of conditioning in the female
reproductive tract that in the human lasts approximately 7 hours.
Much of this conditioning, which occurs in the uterine tube, entails
epithelial interactions between the sperm and mucosal surface
of the tube. During this time a glycoprotein coat and seminal plasma
proteins are removed from the plasma membrane that overlies the acrosomal
region of the spermatozoa. Only capacitated sperm can pass through the corona
cells and undergo the acrosome reaction.
So, distant
stage of fertilization includes next processes: capacitation and taxis of
spermatozoa. This stage begins with special activation of spermatozoa and
continues with their active movement forward the ovum). Spermatozoa in vagina
are not active. In the female genital way they have to be activated.
Scheme of female reproductive system.
Capacitation
means physiological changes or
special activation of spermatozoa, which occurs with them in female sex way. It
is the process by which enzymatic secretions of the uterus and uterine tube of
the female genital tract strips glycoproteins from sperm cell membrane.
Cholesterol of sperm plasma membrane is removed during capacitation, which
results in the increased fluidity of the membrane that is required for the
fusion of acrosomal membrane with the sperm cell membrane. Only after such
changes they begin to move toward. Capacitation lasts about 7 hours.
Taxis – active passage of spermatozoa. Chemotaxis (chemotropism)
– movement toward to the oocyte, which produces special chemical substances –
gamones. Reotaxis – is the sperm ability for advance against the
current of fluid secreting by the epithelium, which lines the cervix, uterus
and uterine tube (against the mucus flow in the uterine tube). Stigmotaxis – against the peristaltic contractions
of Fallopian tube.
Distant interaction of germ cells is regulated by prostaglandins,
fertilizing proteins (androgamones, hynogamones), progesterone and pH.
Seminal fluid of semen contains prostaglandins, which have
pharmacodynamic action on the smooth muscle of the uterus and uterine tube.
Spermatozoa
in uterine tube.
Prostaglandins stimulate
uterine contractions, rhyth mic contractions of uterine tube muscles and smooth
muscles at all. Thanks to these peristaltic contractions the ovum is
transported along the Fallopian tube to the uterine space.
Oocyte and spermatozoa are moving from the opposite sites of female
genital tract. Spermatozoa are much rapid, however, to be counted for by their
intrinsic motility.
Prostaglandins assist in spermatozoa movement through the uterus and
tubes to the site of fertilization in ampullar portion of the Fallopian
(uterine) tube.
Fertilizing proteins promote
chemotaxis. The chemical substances, which are produced by the ovum and sperms,
are known as gamones.
The oocyte secretes 2 types of hynogamones: the first one stimulates spermatozoa
in opposite to second one, which agglutinates them.
Androgamones
are produced by spermatozoa. Androgamones of firs type blocks the movement of
male germ cells and androgamone II lyses the oocyte membrane.
Progesterone is secreted
by the corpus luteum of the ovary after ovulation and stimulates the secretion
of nutrient-rich fluid by mucosal epithelium of female reproductive tract
(glandular epithelium of uterine tube and cervix, and uterine glands - crypts).
PH of surrounding environment has
grait influence on the process of fertiluization. The matter is, the average
speed of spermatozoa in female body is at about 2-3 mm per minute but it varies
very much with different pH.
Sperms move slowly in the acidic environment of the vagina, but move
rapidly in the alkaline medium of the uterus.
It was found that few motile spermatozoa may appear in the ampulla of
the uterine tube 5 minutes after their deposition near the external uterine os,
however, it took up to 45 minutes to other spermatozoa to complete the journey.
Only about 200-500 sperms usually reach the site of fertilization. Most of them
degenerate and are resorbed by the female genital tract.
Contact stage of fertilization includes: acrosomal reaction, denudation, penetration, cortical
reaction and zonal reaction.
Stage 1 – denudation, stage2 – penetration,
stage3 – zonal reaction and monospermy.
When spermatozoa get in contact with oocyte (contact
stage of fertilization), they are binding
to it.
The last ones have two regions:
1) the sperm receptors that recognise intergral
proteins of their plasmalemma;
2) the other region of ZP-3 molecule binds to receptor proteins located
in the head of the sperm, triggering the acrosomal reaction.
Acrosome membrane fuses with corona radiata of oocyte and acrosomal
enzymes are released from acrosome. The effusion of enzymes results in
digestion of intercellular junctions. 200 – 500 spermatozoa reach the site of
fertilization and introduce in the tunica granulosa. First of all denudation – dispersion of the corona radiata
cells – occurs. During 12 hours due to the contraction of spermatozoa tails the
oocyte rolls to the uterus with the speed 4 times in a minute and loss
follicular cells.
Then penetration of zona pellucida
begins, most important enzymes are hyaluronidase and trypsin-like protease,
acrosin, which lyses zona pellucida, permitting the flagella movement of the
sperm to propel the sperm toward the oocyte. Penetration is accomplished by
limited proteolysis of the zona pellucida in front of the advancing sperm.
Additional sperms may be found attached to zona pellucida. Several sperms may
penetrate zona pellucida, but only one sperm completes the fertilization
process (monospermy).
At last, the oolemma (the proper cell membrane of oocyte) is solved and
spermatozoon enters ooplasm. The sperm binds to receptors of oocyte plasma
membrane. Cell membranes of gametes fuse and break down at the area of fusion.
The sperm nucleus with 23 chromosomes, neck with centriol and mitochondria of
middle piece enter cytoplasm of oocyte. The sperm’s plasma membrane remains
behind.
Scheme of fertilization
1 – oocyte cytoplsm, 2 – oocyte nucleus, 3 – zona pellucida, 4 – corona radiata cells, 5 – head of spermatozoon, 6 – neck of spermatozoon, 7 – tail of spermatozoon, 8 – oolemma.
9 – tunic of fertilization. 10 – female pronucleus, 11 – male pronucleus , 12 – mitotic spindle, 13 – fusion of pronuclei.
The fusion of germ cells is responsible for
several postfusion reactions: fast block of
polyspermy, cortical reaction and zona reaction.
The fast block of polyspermy includes changes of membrane resting
potential of oolemma that prevent contact between oocyte and another sperms. It
consists of large and long-lasting (few minutes) depolarization of the oolemma.
Cortical reaction is
slow component, after oolemma polarity changes Ca++is released from
ooplasmic stores and promotes effusion of the oocyte cortical granules content
outside and its reaction with remnant of zona pellucida and oolemma. As a
result of this cell size decreases thus forming perivitelline
space. Enzymes within the cortical granules (proteases) act to
hydrolyze ZP-3 molecules in zona pellucida, the sperm receptors, thus
preventing additional sperms from reaching the oocyte. These enzymes degrade
the glycoprotein oocyte receptors for sperm binding and make it impermeable to
other sperms (monospermy).The content of cortical granules, which are released
into perivitelline space, also causes
changes if the plasma membrane.
These enzymes form the perivitelline barrier by cross-linking proteins
on the surface of zona pellucida. This event promotes final and permanent block
to polyspermy. So, tunica of fertilization
appears. There are such mechanisms of cortical reaction:
1. Modification of cell membrane potential (minus
to plus).
2. Releasing of the Ca++ from the depot to
hyaloplasm.
3. Cortical granules exocytosis.
4. Storage of the water between the ovolemma and
zona pellucida in perivitellin space.
5. Hardening of zona pellucida with formation of
fertilization tunica.
Sperm nucleus enters the secondary oocyte and the ovum completes its
second meiotic division. This result in two haploid cells formation: the ovum
and polar body. Polar body with 23 s-chromosomes is extruded into the
perivitelline space between the oolemma and tunica of fertilization.
Chromosomes, which remain in ovum reconstitute into the female pronucleus.
Within the cytoplasm of the oocyte, the nucleus of the sperm enlarges to
form the male pronucleus, it turns
around; at that time 2 nuclei may be seen in the cell: so called male and
female pronuclei. Since that time till the moment of their fusion such unicellular organism is named “synkarion”. Two
pronuclei soon meet approximately the center of the ovum. They fuse, forming a
zygote with the diploid chromosomes number. After conjugation mitosis begins.
Zygote is genetically unique because it contains new combination of chromosomes
that is different from that in the cells of parents.
Chromosomal sex of embryo and child is determined at the time of
fertilization. It is clear that one chromosome of each of the 23 pairs is
derived from the mother and the other from the father. Fertilization by the
X-(gynecospermium) or Y-bearing ( androspermium) sperms will result in origin
of female embryo in the first case and male in the second one.
CLEAVAGE
Cleavage (fissio) is the
next process after fertilization – special mitotic divisions (short G1 phase)
without growth of daughter cells. It results in rapid increase in the
number of cells with diminution in their size. These embryonic cells are called
blastomeres. Cleavage directly depend on the amount and distribytion of yolk
inclusions in the oocyte (type of ovum) and varies according to whether the
eggs are alecital, oligolecital and polilecital.
Process of cleavage is characterized by the next features: all zygote is
dividing or part of it, size of appearing cells-blastomeres and time of their
appearance.
Cleavage may be full (complete or holoblastic) and partial
(meroblastic).
Complete cleavage means full division of zygote into blastomeres. Such
cleavage is typical for oligolecital primary isolecital cells, which have
moderate amount of yolk inclusions dispersed in cytoplasm. Partial cleavage
includes division of animal pole of blastula, vegetative pole of blastula with
nutrients is not dividing. It is typical for birds polilecital and telolecital
eggs – a lot of yolk inclusions are condensed together, blastula is known as
diskoblastula.
Complete cleavage may result in origin of similar or different
blastomeres – so called equal and unequal cleavage. The last one takes place in
amphibian blastula. Some time cells are similar but not absolutely the
same–subequal cleavage (female ovum).
In the case of synchronic cleavage all the blastomeres are dividing at
the same time and in the case of asynchronic one – at a different time.
Different blasules
a) celoblastulà, b) amphiblastula, c) discoblastula.
d) periblastula, e) blastocyst.
Human embryo cleavage
Cleavage of human zygote begins 30 hours after fertilization in uterine
tube (tubal period) and lasts during next 6 days (till 7th). Since
5-6 day embryo enters the uterine space (uterine period). As a result
multicellular organism is developing. His name is “blastula” which consists of
blastomers. During all this period of time blastula has a constant size and is
surrounded by tunica of fertilization.
Cleavage process directly depends on the type of oocyte, his yolk
inclusions volume and disposition). Human oocyte is olygolecital II isolecital
that is why human zygote has full (complete)
subequal asynchronic cleavage. It means all the zygote is dividing
into blastomers (fully) and all the blastomers, which appear during the
cleavage, are almost equal in size. The differences between blastomers may be
seen just after the first division: one of the blastomers is a little bit
smaller and lighter than the other one. The blastomeres are oval and lie
parallel to each other. The nucleus of each cell becomes invisible about 1st
hour before the next division.
The division of two-cell stage is dichotomus, but the blastomeres do not
divide synchronously. At this stage, as at subsequent stages, the larger light
cell divides first. The larger cell divides 40 h after fertilization and
three-cell stage is formed. So, different blastomers are dividing at different
time (asynchronically), their amount changes in such way: 2, 3, 5 …
After the nine-cell stage the blastomeres change their shape and tightly
align themselves against each other to form a compact ball of cells. This
phenomenon is called compaction. It is mediated by cell surface adhesion
glycoproteins (ovomorulin). Compaction permits greater cell-to-cell
interaction. When there are 12 to 32
blastomeres, the developing conceptus is called morula.
The spherical morula exists about 3 days after fertilization been
rolling in the uterine tube.
Scheme
of cleavage
About 72 hours after fertilization human morula contains 12 blastomeres.
The 12-cell stage consists of: 11 small light peripheral cells, which surround
a larger (dark) centrally placed one. Smaller cells are more numerous and divide
more rapidly. They form outer cell mass. Larger cells divide slower and they
few in number. Internal cells of the morula are called inner cell mass.
The morula is surrounded by tunica of
fertilization, which is of great importance in keeping the blastomeres in a
restrained and compacted cluster. Morula remains enclosed by
abovementioned tunica through which it is nourished by diffusion of oxygen and
low molecular metabolites from uterine tube secretions (histiotrophic
nutrition).
During 2-3-4th days blastula
consists of blastomers (till 32), which are tightly connected one to each other
and it looks like a raspberry. It is morula.
It rolls along the uterine tube to uterine cavity and reach it at the 5th
day. At that time blastula consists of larger dark blastomers (embryoblast), which are surrounded with
smaller slightly flattened and light ones – trophoblast
and contains the space inside (blastocel).
The blastocyst cavity (blastocel) appears between the trophoblast and
the inner cell mass and separates them except of one side where they remain in
contact. Such type of blastula is called “blastocyst”, it appears as a result
of full subequal asynchronic cleavage. The wall of blastocyst (trophoblast) is known as
blastoderm. It has abembryonic pole (over embryoblast) and opposite vegetative pole.
The next 2-3 days is the stage of free blastocyst because it has no tight contact with
maternal organism. During this time it roles over the surface of endometrium
untill abembryonic pole will attach to mucosa between ducts (necks) of
uterine glands, which are actively producing mucous now being regulating by
progesterone, which is produced by corpus luteum of ovary.
Human
blastocyst has wall – trophoblast, cavity – blastocoel and embryoblast.
Human cleavage normally occurs as the zygote passes along the uterine
tube toward the uterus. Blastomeres become smaller with each cleavage division
until they rich nuclear-cytoplasmic ratio typical for somatic cells. In the
blastocyst stage zona pellucida becomes thinner and disappears. The
trophoblastic cells, which have the capacity to invide the mucosa, to come into
direct contact with the endometrium.
IMPLANTATION
Implantation.
By the end of first week of development, the human zygote has passed through
the morula and blastocyst stages and has begun its implantation into the
uterine mucosa. Implantation is a process of
blastocyst introducing into the endometrium, usually it occurs along the
posterior or anterior wall of the uterine body.
The wall of the uterus consists of three
layers: (a) endometrium or mucosa lining the inside wall; (b)
myometrium, a thick layer of smooth muscle; and (c) perimetrium,
the peritoneal covering lining the outside wall. From puberty (11–13 years)
until menopause (45–50 years), the endometrium undergoes
changes in a cycle of approximately 28 days under hormonal control by the
ovary. During this menstrual cycle, the uterine endometrium passes through three
stages, the follicular or proliferative phase, the secretory or progestational
phase, and the menstrual phase. The proliferative phase begins at
the end of the menstrual phase, is under the influence of
estrogen, and parallels growth of the ovarian follicles. The secretory phase begins
approximately 2 to 3 days after ovulation in response to progesterone produced by
the corpus luteum. If fertilization does not occur, shedding of the
endometrium (compact and spongy layers) marks the beginning of the menstrual phase.
If fertilization does occur, the endometrium assists in implantation and
contributes to formation of the placenta.
At the time of implantation, the mucosa of
the uterus is in the secretory phase, during which time uterine glands and
arteries become coiled and the tissue becomes succulent. As a result,
three distinct layers can be recognized in the endometrium: a superficial compact
layer, an intermediate spongy layer, and a thin basal layer.
Normally, the human blastocyst implants in the endometrium along the
anterior or posterior wall of the body of the uterus, where it becomes
embedded between the openings of the glands.
If the
oocyte is not fertilized, venules and sinusoidal spaces gradually become packed with blood cells,
and an extensive diapedesis of blood into the tissue is seen. When the menstrual
phase begins, blood escapes from superficial arteries, and small pieces of
stroma and glands break away. During the following 3 or 4 days,
the compact and spongy layers are expelled from the uterus, and the basal
layer is the only part of the endometrium that is retained. This layer, which
is supplied by its own arteries, the basal arteries, functions as the
regenerative layer in the rebuilding of glands and arteries in the proliferative
phase.
There are two phases of implantation: adhesion
and invasion. Till that moment
tunica of fertilization dissappears and at first blastocyst attaches to the
uterine mucosa with embryoblast pole. The trophoblastic cells begin to
penetrate between the epithelial cells of endometrium. The penetration
and erosion of the mucosa cells results from proteolytic enzymes produced by
trophoblast.
Hence, by the end of the first week of development, the human zygote has
passed through the morula and blastocyst stages and has begun its implantation
in the uterine mucosa.
Implantation, or nidation, involves
penetration through the uterine epithelium, with little signs of necrosis of
the connective tissue stroma and blood vessels of the endometrium.
This type of human embryo implantation is called interstitial implantation, in which the blasocyst
comes to lie entirely within the endometrium.
Process of implantation continues (lasts) at about 40 hours. Till the
end of second week the embryo is completely embedded in the endometrial stroma
and covered with epithelium.
Embryo
is embedded unto utrine mucosa.
The adhesive mechanisms are those which attach the blastocyst to
endometrium. Uterine mucosa contains numerous simple nonbranched uterine glands
(crypts), usually embryo attaches between their openings by the embryonic pole,
containing embryoblast. At this period blastocyst actively absorbs fluid from
uterine secretion and expands to 0,25 mm in diameter. Contacting with
endometrium trophoblast rapidly proliferates and begins to invade inside. Its
outer layer syncytiotrophoblast is multinucleated cytoplasmic mass with
irregular villi over the surface. It contains a lot of lysosomes, whose
lytic activity erodes the maternal tissues. Inner layer of cytotrophoblast
consists of mitotically active mononucleated ovoid cells.
The fingerlike processes of syncytiotrophoblast extend through the
endometrial epithelium and invade the connective tissue and blood vessels. The
blastocyst sinks into the endometrial stroma through the implantation window.
Blastocyst penetration causes firstly defect of endometrium, thus forming
spaces around villi (lacunae) and secondly the rupture of the endometrial blood
vessels, with overflow of blood into these lacunae. Later they become filled by
a mixture of maternal blood and secretions from eroded uterine glands. At this
stage of embryogenesis the earlier histiotrophic
nutrition takes place. Embryo is nourished by diffusion.
Endometrial surface over implanting embryo is gradually closed by
fibrin and by prolipheration of adjacent epithelium.
In 12-day embryo adjacent lacunae fuse to form network and maternal
blood flows into lacunae oxygen and nutrients become available to the embryo.
So hematotrophic nutrition of embryo
begins.
Stromal cells of endometrium are transformed into decidual cells that
accumulate glycogen and lipids.
Process of embryo implantation is under endocrine control: progesterone
of ovarial corpus luteus and estrogens. The syncytiotrophoblast produces
chorionic gonadotrophin, which enters the maternal blood and maintains the
hormonal activity of corpus luteum. By the way, determination of this hormone
in urine allows identifying pregnancy at the early stages.
It has to be noticed that successful implantation
occurs “fifty-fifty”.
Abnormal implantation sites (ectopic
pregnancy)
Occasionally implantation in the uterus itself may lead to serious
complications. This is particularly so when the blastocyst implants close to
the internal os. At later stages of development the placenta overbridges the os
(placenta previa), and causes severe bleeding in the second half of pregnancy
and during delivery. Not infrequently implantation sites are found outside the
uterus, resulting in extra-uterine or
ectopic pregnancy. This may occur at any place in the abdominal cavity, ovary
or Fallopian tube. Most frequently in the abdominal cavity the blastocyst
attaches itself to the peritoneal lining of the recto-uterine cavity (
Sometimes the blastocyst develops in the ovary proper, causing a primary
ovarian pregnancy. More commonly an
ectopic pregnancy is lodged in the Fallopian tube (tubal
pregnancy). In the latter case, the tube ruptures at about the
second month of pregnancy, resulting in severe internal hemorrhaging by the
mother.
So, there are two types of ectopic pregnancy
intra-uterine and extra-uterine, the last one includes tubal, ovarial and
peritoneal ones.
Ectopic pregnancy
usually leads to death of embryo and severe bleeding by the mother during the
second month of pregnancy.
HISTIOTROPHIC AND HEMATOTROPHIC
PERIODS OF EMBRYOGENESIS
In different stages of embryogenesis embryo has different types of
nutririon. During the first 30 hours after fertilization it posseses
nutrients from the yolk inclusions of the oocyte. Without fertilization oocyte
may be alive three days thanks to the yolk. It is so called “vitelotrophic nutrition” (vitelum – from
At the begining of the 5th week trophoblast enzymes solve the
wall of uterine mucosa vessels and new type of nutrition begins. It is “hematotrophic nutrition”, because the embryo
receives all the necessary products from the maternal blood.
specific processes of embryogenesis
Ooplasmatic segregation –
redistribution of the oocyte cytoplasm components which result in formation of presumptive zones (portion of an oocyte from which
some exact structure may develop).
Proliferation is a process
of cells amount increase by means of their devision.
During the embryogenesis the cells may influence one to each other and
produce some differences between themselves. Thus the induction results in differentiation.
At the same time being developing in some way the embryonic cells have lost their
possibility to be changed in different way. This process is named a comition. Gens expression and repression are the
basic points of these processes.
EXTRACORPORAL
FERTILIZATION
During last period of time in medical practice of many countries of the
world so-called extracorporal (artifitial) fertilization is used very
often in the case of male or female infertility.
It was in 1976 in
There are some advances of such extracorporal fertilization:
1. It is possible to choice the future
sex of baby.
2. The sperm may be reached with
spermatozoa, abnormal onces may be picked out.
3. Abnormal sites of implantation are
almost impossible in such case (f.e., tubal pregnancy).
Real-life
situations to be solved
1.
A lot of spermatozoa with spherical head were revealed in
histological research of spermatozoa smear. What may it cause?
2.
After spermatozoon head introduction
into oocyte content of cortical granules of oocyte is released outside. What is
the name of this process?
3.
At distant stage of fertilization
spermatozoa move against fluid flow toward oocyte. What is the name of this
addressed movement?
4.
At some stage of embryogenesis human
embryo looks like vesicle with space. Its wall consists of light small
blastomeres and larger dark ones at one pole. What is the name of embryo at
this stage?
5.
Cleavage of zygote result in blastula
formation. Which type of blastula is typical for human embryogenesis?
6.
Due to expression of cell genome
components embryonic cells obtain characteristic morphological, biochemical and
fuctional peculiarities. Name this process.
7.
Histological specimens are stained
with hematoxylin and eosin. They have different oocytes. Which one is human oocyte?
8.
In
endoscopy of uterine space embryo was revealed. It was not attached to
endometrium. What is this stage of embryogenesis?
9.
In
experiment in vivo it is fixed a moment in which spermatozoa surround oocyte
and corona radiate disappears. What is the name of this process?
10.
In first crucial period of
embryogenesis (5th -6th day) tunic of fertilization
suddenly disappear. Embryo at this time was placed in uterine tube. What may it
result in?
11.
In histological research of spermatozoa smear dense structures
are revealed at the top of heads. What are these structures?
12.
In histological specimen it is seen
oocyte at moment of fertilization by spermatozoon. What is main result of this
process?
13.
In histological specimen it is seen
oocyte at moment of fertilization by spermatozoon. What is usual place of
fertilization?
14.
Inner sex organs
of woman were excised in
surgical operation. Embryo which consists of two cells was found in
histological research of these organs. What is usual localization of such
embryo in normal condition?
15.
Inner sex organs of
woman were excised in
surgical operation. Embryo which consists of 16 tightly packed blastomeres was
found in histological research of these organs. What is the name of this
embryo?
16.
Inner
sex organs of woman were excised in surgical operation. Embryo which consists of
16 tightly packed blastomeres was found in histological research of these
organs. It is cowered with special tunica. What is the name of this tunica?
17.
Multiple penetrations of spermatozoa
into one oocyte are impossible. Why? What is it due to?
18.
Process of embryo implantation into
uterine mucosa consists of two stages. What is morphological evidence of first
stage?
19.
Process of spermatozoon head
penetration into oocyte is seen in histologic specimen. What is the name of
this process?
20.
When spermatozoa are placed in vagina
after ovulation they begin to move faster. What is the name of this process
(special activation of spermatozoa)?
21.
A lot of spermatozoa with spherical
head were revealed in histological research of spermatozoa smear. What may it
cause?
22.
After spermatozoon head introduction
into oocyte content of cortical granules of oocyte is released outside. What is
the name of this process?
23.
At distant stage of fertilization
spermatozoa move against fluid flow toward oocyte. What is the name of this
addressed movement?
24.
At some stage of embryogenesis human
embryo looks like vesicle with space. Its wall consists of light small
blastomeres and larger dark ones at one pole. What is the name of embryo at
this stage?
25.
Cleavage of zygote result in blastula
formation. Which type of blastula is typical for human embryogenesis?
26.
Due to expression of cell genome
components embryonic cells obtain characteristic morphological, biochemical and
fuctional peculiarities. Name this process.
27.
Histological specimens are stained
with hematoxylin and eosin. They have different oocytes. Which one is human
oocyte?
28.
In
endoscopy of uterine space embryo was revealed. It was not attached to
endometrium. What is this stage of embryogenesis?
29.
In
experiment in vivo it is fixed a moment in which spermatozoa surround oocyte
and corona radiate disappears. What is the name of this process?
30.
In first crucial period of
embryogenesis (5th -6th day) tunic of fertilization
suddenly disappear. Embryo at this time was placed in uterine tube. What may it
result in?
31.
In histological research of spermatozoa smear dense structures
are revealed at the top of heads. What are these structures?
32.
In histological specimen it is seen
oocyte at moment of fertilization by spermatozoon. What is main result of this
process?
33.
In histological specimen it is seen
oocyte at moment of fertilization by spermatozoon. What is usual place of
fertilization?
34.
Inner sex organs
of woman were excised in
surgical operation. Embryo which consists of two cells was found in
histological research of these organs. What is usual localization of such
embryo in normal condition?
35.
Inner sex organs of
woman were excised in
surgical operation. Embryo which consists of 16 tightly packed blastomeres was
found in histological research of these organs. What is the name of this
embryo?
36.
Inner
sex organs of woman were excised in surgical operation. Embryo which consists of
16 tightly packed blastomeres was found in histological research of these
organs. It is cowered with special tunica. What is the name of this tunica?
37.
Multiple penetrations of spermatozoa
into one oocyte are impossible. Why? What is it due to?
38.
Process of embryo implantation into
uterine mucosa consists of two stages. What is morphological evidence of first
stage?
39.
Process of spermatozoon head
penetration into oocyte is seen in histologic specimen. What is the name of
this process?
40.
When spermatozoa are placed in vagina
after ovulation they begin to move faster. What is the name of this process
(special activation of spermatozoa)?
41.
A lot of spermatozoa with spherical
head were revealed in histological research of spermatozoa smear. What may it
cause?
42.
After spermatozoon head introduction
into oocyte content of cortical granules of oocyte is released outside. What is
the name of this process?
43.
At distant stage of fertilization
spermatozoa move against fluid flow toward oocyte. What is the name of this
addressed movement?
44.
At some stage of embryogenesis human embryo
looks like vesicle with space. Its wall consists of light small blastomeres and
larger dark ones at one pole. What is the name of embryo at this stage?
45.
Cleavage of zygote result in blastula
formation. Which type of blastula is typical for human embryogenesis?
46.
Due to expression of cell genome
components embryonic cells obtain characteristic morphological, biochemical and
fuctional peculiarities. Name this process.
47.
Histological specimens are stained
with hematoxylin and eosin. They have different oocytes. Which one is human
oocyte?
48.
In
endoscopy of uterine space embryo was revealed. It was not attached to
endometrium. What is this stage of embryogenesis?
49.
In
experiment in vivo it is fixed a moment in which spermatozoa surround oocyte
and corona radiate disappears. What is the name of this process?
50.
In first crucial period of
embryogenesis (5th -6th day) tunic of fertilization
suddenly disappear. Embryo at this time was placed in uterine tube. What may it
result in?
51.
In histological research of spermatozoa smear dense structures
are revealed at the top of heads. What are these structures?
52.
In histological specimen it is seen
oocyte at moment of fertilization by spermatozoon. What is main result of this
process?
53.
In histological specimen it is seen
oocyte at moment of fertilization by spermatozoon. What is usual place of
fertilization?
54.
Inner sex organs
of woman were excised in
surgical operation. Embryo which consists of two cells was found in
histological research of these organs. What is usual localization of such
embryo in normal condition?
55.
Inner sex organs of
woman were excised in
surgical operation. Embryo which consists of 16 tightly packed blastomeres was
found in histological research of these organs. What is the name of this
embryo?
56.
Inner
sex organs of woman were excised in surgical operation. Embryo which consists of
16 tightly packed blastomeres was found in histological research of these
organs. It is cowered with special tunica. What is the name of this tunica?
57.
Multiple penetrations of spermatozoa
into one oocyte are impossible. Why? What is it due to?
58.
Process of embryo implantation into
uterine mucosa consists of two stages. What is morphological evidence of first
stage?
59.
Process of spermatozoon head
penetration into oocyte is seen in histologic specimen. What is the name of
this process?
60. When spermatozoa are placed in vagina after ovulation they
begin to move faster. What is the name of this process (special activation of
spermatozoa)?
61. There is a cross section of
spermatozoon in anelectron micrograph.. Mitochondrial sheath wrapped around the
axial filaments is seen. What part of spermatozoon is sected ?
62. In histologic specimen of a human
oocyte there is a small amount of yolk inclusions, settled uniformly in
cytoplasm. What type of oocyte is this?
63. Human oocyte was fertilized with
spermatozoon with Y-chromosome. What sex will baby have?
64. Human spermatozoa are moving
toward to oocyte and tunics of both cell join into contact. What process takes
place?
65. After spermatozoon penetration
into human oocyte other ones can't do that. What kind of fertilization is this?
What structure prevents penetration of numerous spermatozoa into oocyte?
66. At the 6-7-th day of
embryogenesis em brio implantation had occurred in the uterine tube. What will
happen in such case of abnormal implantation site?
67. Cavity appears in human embryo
and blastomers differentiation begins. What structures will appear as a result
of this differentiation?
Summary
With each ovarian cycle, a number of
primary follicles begin to grow, but usually only one reaches full maturity, and only
one oocyte is discharged at ovulation. At ovulation, the oocyte is
in metaphase of the second meiotic division and is surrounded by the
zona pellucida and some granulosa cells. Sweeping action of
tubal fimbriae carries the oocyte into the uterine tube.
Before spermatozoa can fertilize the oocyte, they must undergo (a)
capacitation, during which time a glycoprotein coat and seminal
plasma proteins are removed from the spermatozoon head, and (b)
the acrosome reaction, during which acrosin and
trypsin-like substances are released to penetrate the zona pellucida. During
fertilization, the spermatozoon must penetrate the corona radiata, the zona pellucida, and
the oocyte cell membrane. As soon as the spermatocyte has entered the
oocyte, the oocyte finishes its second meiotic division and forms
the female pronucleus; the zona pellucida becomes
impenetrable to other spermatozoa; and the head of the sperm separates
from the tail, swells, and forms the male pronucleus. After both
pronuclei have replicated their DNA, paternal and maternal chromosomes
intermingle, split longitudinally, and go through a mitotic division, giving
rise to the two-cell stage. The results of fertilization are (a) restoration
of the diploid number of chromosomes, (b) determination of chromosomal sex, and (c) initiation
of cleavage. Cleavage is a series of mitotic
divisions that results in an increase in cells, blastomeres, which become smaller with
each division. After three divisions, blastomeres undergo compaction
to become a tightly grouped ball of cells with inner and outer layers.
Compacted blastomeres divide to form a 16-cell morula. As the morula enters the
uterus on the third or fourth day after fertilization, a cavity begins to appear,
and the blastocyst forms. The inner cell mass, which is formed at the time of
compaction and will develop into the embryo proper, is at one pole of the
blastocyst. The outer cell mass, which surrounds the inner.
The uterus at the time of implantation is
in the secretory phase, and the blastocyst implants in the endometrium along the
anterior or posterior wall. If fertilization does not occur, then the menstrual
phase begins and the spongy and compact endometrial layers are shed. The
basal layer remains to regenerate the other layers during
the next cycle.
Student’s Practical Activities
Task
No 1. Students must know and illustrate such a histologic
specimens.
Specimen 1. Human
spermatozoa (smear).
Stained with nigrosin and
eosin.
Find spermatozoa
at a low magnification. They have fibers-like structures and lie irregularly.
Then watch male germ cells at high magnification. Pay a special attention on
flagellate form of these cells. Their heads are oval shaped and small, tails
are much longer.
Illustrate and indicate: 1. Head. 2. Nucleus. 3. Acrosome. 4. Neck. 5. Tail.
Specimen 1a. Spermatozoa
of guinea pig (smear).
Stained with nigrosin.
Find spermatozoa
at a low magnification. They have fibers-like structures and lie irregularly. Then
watch male germ cells at high magnification.
Their heads are oval shaped and small, tails are much longer. Pay a
special attention: these cells may have two tails.
Illustrate and indicate: 1. Head. 2. Nucleus. 3. Acrosome. 4. Neck. 5. Tail.
Specimen 2. Oocyte of Graafian follicle. (Cat ovarium).
Stained with hematoxylin
and eosin.
Ovarian surface is
covered with flat epithelium, which rests on tunica albuginea (dense connective
tissue). Ovary of cat usually has a lot of follicles at the same time. At low
magnification find mature follicle in the cortex of ovary. It is large and has
space inside. Two well prominet layers are seen in its wall: the inner
one consists of numerous small follicular cells and the outer one – theca
externa has two sublayers – innermost with blood vessels and the outer –
without, consists mainly of fibrous connective tissue. Find some follicle with
ovum, which rests on the cumulus oophorus (is connected with wall of follicle).
Ovary of cat (low
magnification).
Oocyte in follicle (high
magnification)
At high
magnification explore the structure of oocyte and surrounding tunics. Pay
attention - nucleus is situated eccentrically. There are some yolk inclusions
in cytoplasm. The female sex cell is surrounded with transparent tunic of a
rose colour, which refract the light very much - zona pellucida. Follicular
cells (small cells with violet nuclei) and their processes form the radiant
crown (corona radiata) of oocyte.
Illustrate and indicate: 1. Oocyte. 2. Zona pellucida. 3. Crown radiant.
4. Follicular epithelial cells.
Specimen 3. Fertilized oocyte of
ascaridae .
Stained with Iron
hematoxylin.
In cross section
of horse acaridae uterus at a low magnification numerous oocytes are seen, most
of them are fertilized. At high magnification you can see one or two residual
cells in perivitelline space (the space, which originate due to cortical
reaction after spermatozoon penetration in oocyte). This space lie between
tunic of fertilization and proper oolemma. Find the cell with two nuclei. One
of them is larger - female pronucleus and the smaller one - male pronucleus.
So, synkarion is
unicellular organism, surrounded by tunic of fertilization and containing two
pronuclei with haplod chromosomal each one.
Polar body is
ovalshaped basophilic structure, which lies in the perivitelline space between
oolemma and tunic of fertilization.
Illustrate and indicate: 1. Synkaryon. 2. Male pronucleus. 3. Female pronucleus.
3. Polar
body.
Task
No 2. Students must know and
illustrate such a scheme.
Scheme 1. Human blastocyst.
Blastocyst consists of two different kinds of
blastomers: dark large (embryoblast) and small light, which surround the
embryoblast and is called trophoblast. There is a cavity with fluid in central
part of blastula - blastocel.
Illustrate and indicate: 1. Embryoblast.
2. Trophoblast. 3. Blastocel.
Scheme 2. Implantation.
At the first stage of implantation
(adhesion) blastocyst attaches to the surface of endometrium, mainly between
the uterine glands. The second phase (invasion) of blastocyst into endometrium
continues under the influence of trophoblast ferments, which are destroying
mucosa and blastocyst falls into pit and is covering with connective tissue and
then epithelium.
Illustrate and indicate: 1. Embryoblast.
2. Trophoblast. 3. Trophoblast villi. 4. Uterine epithelium. 5. Uterine
connective tissue. 6. Uterine gland.
Task
No 3. Students should be able to indicate elements in the
electron micrographs:
1. Flagella (tail of the
spermatozoon).
2. System of
microtubules doublets.
References:
1. Medical Embryology.
Langman J. Baltimore. Wilkins Co. 1969. P. 3-34.
2. L. Carlos
Junqueira, Jose Carneiro, Robert O. Kelley. Basic Histology. - 7th ed.
Appleton and Lange.
3. Inderbir Singh. Textbook of Human Histology
with colour atlas. – 4th ed. Jaypee Brothers Medical Publishers (P)
LTD, 2002.
4. Victor P. Eroschenko. Atlas of Histology with
functional correlations. - 9th ed. Lippincott Williams and Wilkins,
2000.
5. Wheater P.R., Burkitt H.G., Daniels V.G.
Functional Histology: a text and colour atlas. - 2nd ed. Longman
Group UK Limited, 1987.
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http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
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