GENERAL AND COMPARATIVE EMBRYOLOGY
1. Embryology (the term, varieties)
2. Periods of ontogenesis
3. Germ cells
4. Stages and processes of embryogenesis
5. Extraembryonic organs (fetal membranes…)
6. Types of embryo nutrition
7. Critical (crucial) periods
8. Extracorporal fertilization
Embryology is the science about the embryo origin and development. Contemporary 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.
In the course of human life there are 3 principal periods: progenesis, embryonic and postembryonic period.
Progenesis or gametogenesis is a process of a germ cell production (ovogenesis for oocytes and spermatogenesis for spermatozoa). These processes take place in male and female sex glands. 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 is twofold:
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 embryology research.
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: ontogenesis is the short repeating of the phylogenesis.
There are some principle differences between germ cells and somatic ones. First of all, germ cells contain 23 chromosomes, they and without fertilization they die in a short period of time. Somatic cells have 46 chromosomes, they caot merge but they are still alive without fertilization.
Main differences between germ cells and somatic ones are the next: germ cells karyotype is haploid, somatic cells- diploid, germ cells have possibility to assimilation producing thus new cell (zygote) and without assimilation they are dying, somatic cells survive without merging.
Spermatozoa – the male sex 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. A small amount of cytoplasm in the head makes a special shape with perforatorium- shurp 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: androspermium and gynecospermium. There is a special structure – acrosome – at the top of spermatozoon head. It is a specifically changed Golgi apparatus, which contains enzymes hyaluronidaze and trypsin. Proximal centriol lies in the neck of spermatozoon. Axial fibre is the principal component of the body and tail. In the first ones there are many mitochondria which supply energy to him.
Biologic sugnificance of spermatozoa in the fertilization.
1. Active passage foward to the ovum.
2. They give 23 “father” chromosomes.
3. The sex of future baby depends on spermatozoon type.
4. Give the centriol.
5. Introduce special cleavage signal protein.
6. Mitochondrial DNA.
7. Influence on the oocyte mejosis.
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 caot move.
It is covered with a few tunices. From outside they are: corona radiata (tunica granulosa), zona pellucida and ovolemma. The last one is the proper cytolemma. Zona pellucida is a special chemical membrane which contains much glycoproteins and glycosoaminglicans. The outer layer consits of numerous follicular cells which give their processes to the ovum through the previous tunic. Their functions are protection and nutrition of the oocyte and regulation of its maturation.
Nucleus of the ovum lies excentrically and contains 23 chromosomes. There are all the organells of a general meaning in the ovoplasm. Apparatus Goldgy is especially welldeveloped and produces yolk inclusions.
In the peripheral layer of cytoplasm just under the plasmolemma numerous cortical granules may be seen. They take an active part in the process of fertilization.
Classification of oocytes due to the amount and location 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.
3. Fish and birds have polylecytal oocytes with great volume of yolk.
Biologic sugnificance 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.
In the course of embryogenesis there may be recognised such stages and periods of it: fertilization, cleavage, gastrulation, neurulation, histogenesis, organogenesis, systemogenesis.
Fertilization is the first stage of embryogenesis, the process by which the male and female gametes fuse and result in zygote appearence.
The biological sugnificance of fertilization are the next:
1. The restoration of the full chromosome complect.
2. Determination of the emryo sex.
3. Initiation of a cleavage.
In female this process takes place in the ampullar portion of the Fallopian (uterine) tube. There are two stages of fertilization: distant and contact ones.
Capacitation means special activation of spermatozoa which occurs with them in female sex way. Only after such changes they begin to move foward.
Taxis – active passage of spermatozoa. Chemotaxis (chemotropism) – toward to the oocyte, which produces special chemical substance – gamones. Rheotaxis – against the fluid flow in the uterine tube. Stigmotaxis – against the peristaltic contractions of Fallopian tube.
When spermatozoa get in contact with oocyte (contact stage of fertilization), acrosomal reaction begin and acrosomal enzymes exfuse and begin to solve the oocyte tunices. First of all denudation – dispersion of the corona radiata cells – occurs. 200 – 500 spermatozoa reach the site of fertilization and introduce in the tunica granulosa. During 12 hours due to the contraction of spermatosoa tails the oocyte rolles to the uterus with the speed 4 times in a minute and loss follicular cells. Then penetration of zona pellucida begins, and at last, the oolemma (the proper cell membrane of oocyte) is solved. As a result of cortical reaction, which includes passage of the oocyte cortical granules content outside and its reaction with remnant of zona pellucida, tunica of fertilization appeares. 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.
After the introducing of 1 spermatozoon (monospermy) into the oocyte the ovum complite its mejosis. Polar body with 23 s-chromosomes is exfused into the perivitellin space between the ovolemma and tunica of fertilisation.
Father nucleus turnes around, at that time 2 nuclei may be seen in the cell: so called father and mother pronuclei. At the moment of their fusion such unicellular organism is named “synkarion”.
Cleavage (fissio) is the next process after fertilization – special mitotic division without growth of doughter cells. It begins 30 hours later fertilization and lasts during next 6 days (till 7th). 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 with 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 subequal asynchronic cleavage. It means all the zygot 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. Different blastomers are dividing at different time (asynchronically), their amount changes in such way: 2, 3, 5 …
During 2-3-4th days blastula consists of blastomers (till 16) which are tightly connected one to each other and it looks like a raspberry. It is morula. It rolles 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 light ones – trophoblast and contains the space inside (blastocel). Such type of blastula is called “blastocyst”, it appears as a result of full subequal asynchronic cleavage.
The next 2-3 days is the stage of free blastocyst because it has no tight contact with mother organism.
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. There are two phases of implantation: adhesion and invasion.
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.
Process of implantation continues (lasts) at about 40 hours. Till the end of second week the embryo is complitely embedded in the endometrial stroma and covered with epithelium.
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. 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. Ectopic pregnancy usually leads to death of embryo and severe bleeding by the mother during the second month of pregnancy. Most frequently in the abdominal cavity the blastocyst attaches itself to the peritoneal lining of the recto-uterine cavity (Douglas pouch). This also may take place in the peritoneal covering of the intestinal tract or to the omentum.
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.
In different stages of embryogenesis embryo has different types of nutririon. During the first 30 hours after fertilization it posseses nutritive substances from the yolk inclusions of the oocyte. It is so called “vitelotrophic nutrition” (vitelum – from greece “yolk”). After that “histiotrophic nutrition” begins thus during the first month of pregnasncy embryo is feeded with secretion of surrounding tissues. At that period of embryogenesis hormone progesteron is necessary very much because of it influence on this secretory process.
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 mother blood.
Gastrulation is a process of bilaminar and trilaminar germ disc formation, it is the next stage of embryogenesis after the cleavage. It occurs in the embryo in the course of implantation and lasts from 7th till 17th days. There are two stages of implantation: early (7-14 days) and later (14-17 days). As a result of first ones two germ layers appear – ectoderm and entoderm). The third layer – mesoderm is developed during the later gastrulation.
The type of gastrulation directly depends on the type of oocyte and cleavage. That’s why there are such 4 types of early gastrulation: invagination,epyboli, migration and delamination.
1. Invagination may be observed as a result of full equal synchronic cleavage of zygote which is developed from olygolecital I isolecital oocyte. Because of active cells prolipheration the vegetative pole of blastula “moves” to the roof. Such new structure contains two laminas in the wall and new cavity – gastrocel – inside. Blastopor is the opening which connects the gastrocel with outer space. It has 4 lips: dorsal, ventral, and two lateral ones.
2. Epyboly – the other type of gastrulation – is characteristic feature of amphibian (f.e. frog) embryogenesis. Zygote which is developing from the moderate polylecital ovum has full unequal asynchronic cleavage. There are two different groups of cells in it. The cells of the roof are dividing more quickly then the cells of the bottom which contain much yolk inclusions. That’s why these smaller cells move over the larger ones and cower them producing two germ layers in such way.
3. Migration takes place in periblastula. Some cells of the blastula wall move inside into the blastocell, then they begin to connect and thus produce new germ layer inside (entoderm). This type of gastrulation may be seen in the insects whose oocytes contain much inclusions and zygotes have
4. Delamination means tangential splitting of blastomers into two layers – superficial ectoderm and deeper endoderm. Such gastrulation occurs in birds and higher Vertebrata. In such case the embryo looks like a shield – embryonal disc. In birds it lies on the yolk surface, in human embryo it is a place of amniotic and yolk sac contact.
Gastrulation later stage result in trilaminar embryo disc formation. It is possible due to such three principle mechanisms as enterocoelic, teloblastic and by means of primitive streak. Each of them depends on type of oocyte and its cleavage.
1. Enterocoelic type of later gastrulation occurs in blastula which was formed from the primary isolecital oocyte and undergone the full equal synchronic cleavage and invagination in the early gastrulation.
Mesoderm, the third germ layer, has no proper laminar structure. It consists of such principle components as somits, nephrogonotom and splanchnotom with parietal and visceral sheet.
2. Teloblastic type of later gastrulation takes place in amphiblastula which was formed from the moderate telolecital oocyte. After the epyboly group of cells from the lateral lips of blastopore are growing between the ectoderm and endoderm near the notochord. These cells are known as “teloblasts” and their reproduction result in the mesoderm development.
3. Migration with primitive streak formation is the most difficult process of mesoderm appearance. The cell of the germ disc are moving from cranial to the caudal part of an embryo and then return back together making the primitive streak and primitive node with a small pit (similar to blastopore). Through this pit some cells migrate between the ectoderm and entoderm. Thus notochord appears from the anterior lip and mesoderm components from posterior and primitive streak cells.Human embryo gastrulation has some peculiarities. During the second week of development, the human blastocyst becomes furmly embedded in the uterine mucosa, and the trophoblast and embryoblast begin their specific development. The trophoblast penetrates continuously deeper into the endometrium, thereby differentiating into the syncytiotrophoblast and the cytotrophoblast; the cells of the embryoblast form the ectodermal and endodermal germ layers, the two layers which constitute the bilaminar germ disc. Such structure consists of epyblast or primary ectoderm and hypoblast or primary entoderm. The cells of ectodermal layers are initially firmly attached to the cytotrophoblast, but with further development small clefts appear between the two layers. These clefts subsiquently coalesce, thus forming a cavity known as amniotic cavity. The hypoblast cells are dividing and form the yolk sac.
The place of both sac contact has a discoid shape. Thus the proper germ disk consists of amniotic sac bottom cells and yolk sac roof ones. All the other cells are so called extraembryonic material.
At the 8th day of embryogenesis extraembryonic mesoderm appears and it has parietal sheet which underlies the trophoblast and visceral one covering amniotic and yolk sac. Till 13-14th day the embryo looks like two vesicles inside the exocoelomic cavity connecting with trophoblast by “amniotic foot” (connecting stalk). Next three days trilaminar embryo is developed in germ disc by means of primitive streak.
The period from 17th till 20th day is presomit period, when embryo body is separating from extraembryonic organs. The embryonic disc begins to bulge up into the amniotic cavity and shows a marked folding in cephalo-caudal direction. This folding is most pronounced in the regions of head and tail, where so-called head fold and tail fold are formed.
At the beginning of the third week of development, the ectodermal germ layer has the shape of a flat disc which in cephalic region is somewhat broader than caudally. Simultaniously with the formation of the notochord, and in all probability under its inductive influence, the ectodermal disc changes in form and gives rise to the central nervous system.
Initially the nervous system appears as a round to oval thickening of ectoderm in the cephalic region of the embryo. By the end of the third week, however, it has an elongated, slipper-shaped form, the neural plate, which gradually expands in the direction of the primitive streak. During the next few days the lateral edges of the neural plate become more elevated to form the neural folds. As a result of their fusion a tube-like structure, the neural tube, is formed.
Human embryo early gastrulation peculiarities.
1. Full subequal asynchronic cleavage.
2. Forestall development of extraembryonic organs.
3. Embryo implantation into endometrium and placenta formation.
4. All three germ layers are formed from primary ectoderm.
As were told before, in the course of human embryogenesis early stages some temporary organs appear to supply the normal life, growth and development of proper embryo. They are so-called extraembryonic or provisional organs.
At first yolk sac like a provisional organ arises in fishes. Then amnion, serous tunic, alllantois, chorion, placenta and umbilical cord were developed in animals.
There are such human extraembryonic provisional organs as: amnion alllantois, chorion, placenta and umbilical cord.
Amnion is a fetal membrane which begin its development at the third week of embryogenesis when embryoblast delamination produce epy- and hypoblast formation. Amniotic cavity appear in the epyblast, then extraembryonic mesoderm visceral sheet cover this structure. Thus amnion consists of epithelium of ectodermal origin and mesodermal connective tissue. Till 3rd month it is flat epithelium, later it perform into cuboidal and over placenta it is tall columnar epithelium. Amnion has protective function, homeostatic function and produces amniotic fluid.
At about the same time with amnion (3-8 week) yolk sac appear from the hypoblast. Primary entoderm gives rise to flat epitelium and extraembryonic mesoderm visceral sheet lines it outside being transform into connective tissue.
In human embryogenesis yolk sac had lost its first and principal function – to store the nutritive substances – but it has some more important functions, for example, it perform hemopoietic function, it is the source of gonocytoblast and primary vessels appear in its wall.
Allantois like a fingerlike process of primary intestine ventral wall appears at about 16th day of development. It grow up from the caudal portion of intestine into the connecting stalk (amniotic foot) and may be observed there till 2 month. Allantois consists of cuboidal epithelium surrounded with connective tissue. It has excretory, breathing function, it is the guide of blood vessels in umbilical cord and, at last, it perform immune function (primary B-lymphocytes appear in its wall).
Umbilical cord is a tube which arise of connecting stalk (amniotic foot) when the amnion is growing up. That’s why, it is covered with cuboidal epithelium of amniotic origin and it is full up with Wharton’s jelly. It is connective tissue with a special properties, which contains much glucosaminoglycans and numerous macrophages. There are two arteries, one vein and the remnants of yolk sac and allantois in umbilical cord. It perform functions of excretion, breathing and nutrition.
Chorion is developed from trophoblast when it is differentiating into cytotrophoblast and syncytiotrophoblast and underlying with extraembryonic mesoderm. There are two periods of chorion happening: 1) previllious –till 7-8th day and period of villi formation –9-50th day. Primary villi appear when the cells of the cytotrophoblast proliferate locally and penetrate into the syncytium, thus forming cellular columns surrounded be syncytium. During further development (12-13th day) mesodermal cells penetrate the core of primary villi and grow in the direction of uterine mucosa. The newly formed structures, the secondary stem villi, are composed of a loose connective tissue core covered by a layer of cytotrophoblastic cells, which in turn is covered by a thin layer of syncytium.
By the end of the third week the mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, thus forming the villous capillary system. The villi are now known as tertiary stem villi, and during the fourth week of development this type of villi cam be found over the entire surface of the chorion. Such chorion with villi is so-called bushy chorion (chorion frondosum). But later (by the third month) branched villi may be observed only in the placental portion of chorion, the other surface will lost the villi and form chorion laeve with smooth surface.
By the 11th to 12th day of development the blastocyst is completely embedded in the endometrial stroma, and the uterine surface epithelium covers almost entirely the original defect in the epithelial lining of the mucosa. In the latter, embryo is developed inside in the endometrium which produce decidual tunics. The decidua over the chorion frondosum, the decidua basalis (maternal placenta), consists of a compact endometrial layer of large cells with abundant amounts of lipids and glycogen. This layer is tightly connected to the chorion. The decidual layer over the abembryonic pole is known as decidua capsularis. At first it is similar to decidua basalis but with increase in the size of the chorionic vesicle, this layer becomes stretched, and begins to degenerate. Subsequently, the chorion laeve comes into contact with the epithelium of the decidua parietalis on the opposite side of the uterus and the two fuse. The lumen of the uterus is then obliterated. Hence, the only functional portion of the chorion is the chorion frondosum and together with the decidua basalis, the two make up the placenta. Thus, placenta is a unic organ which consists of two principally different structures of both – mammy and baby.
There may be recognized a few type of placenta due to their type of nutrition and structure. In opposite to placentas of first type the second type placenta are producing the proteins which are typical to embryo. There are diffuse and numerous placentas in the first group and tape and diskoidal in the second one.
Chorionic villi may contact with different maternal tissue of endometrium that’s why there are 4 types of placentas due to their structure: 1) epitheliochorial, 2) desmochorial, 3) endotheliochorial and 4) hemochorial placenta.
Human placenta is of nutrition secondary type and hemochorial.
By the beginning of the fourth month, the placenta has two components: 1) a fetal portion formed by the chorion frondosum and, 2) a maternal portion formed by the decidua basalis. On the fetal side the placenta is bordered by the chorionic plate; on its maternal side by the decidua basalis, of which the compact layer or decidual plate is most intimately incorporated into the placenta. In the so-called junctional zone the trophoblast and decidua cells intermingle.this zone, which represents the zone of invasion of the trophoblastic cells into the uterine tissues, is characterized by decidual and syncytial giant cells and rich in amorphous mucopolysaccharide material. Between the chorionic and decidual plates are the intervillous spaces which are filled with maternal blood. They are derived from the lacunae in the syncytiotrophoblast and are at all times lined with syncytium of a fetal origin. The villous trees grow into the intervillous blood lakes.
During the 4th and 5th months the decidua forms a number of septa, the decidual septa, which project into the intervillous spaces but do not reach the chorionic plate. As the result of this septum formation, the placenta is divided into a number of compartments or cotyledons. Since the decidual septa do not reach the chorionic plate, contact between the intervillous space in the various cotyledons is maintained. In the second half of pregnancy lacuna surface is covered by special membrane – Ror fibrinod which appears from syncytiotrophoblast. Similar fibrinoid of Langhans covers chorionic villi in the third trimester of pregnancy.
At full term, the placenta has discoid shape, a diameter of 15 to 25 cm, is approximately 3 cm thick, and has a weight at about 500-600 gm. At birth it is torn from the uterine wall and, approximately 30 minutes after child birth, expelled from the uterine cavity. Under the examination from the maternal side, 15 to 20 slightly bulging areas, the cotyledons, covered by a thin layer of decidua basalis and cytotrophoblastic shell, are clearly recognizable. The grooves between the cotyledons are formed by the decidual septa. Cotyledone is the structural unit of placenta, it is adequate to steam chorionic villus with surrounding endomethrium.
The fetal portion of placenta does not show a cotyledon structure, and is covered entirely by the chorionic plate. A number of large arteries and veins, the chorionic vessels, are seen to converge toward the umbilical cord. The chorion in turn is covered by the amnion. The attachment of the umbilical cord is usually eccentric and occasionally even marginal. Rarely, however, does it insert into the chorionic membrane outside the placenta (velamentous insertion).
Uterine decidual tunics. By the 11th to 12th day of development the blastocyst is completely embedded in the endometrial stroma, and the uterine surface epithelium covers almost entirely the original defect in the epithelial lining of the mucosa. In the latter, embryo is developed inside in the endometrium which produce decidual tunics. The decidua over the chorion frondosum, the decidua basalis (maternal placenta), consists of a compact endometrial layer of large cells with abundant amounts of lipids and glycogen (decidual cells). This layer is tightly connected to the chorion. The decidual layer over the abembryonic pole is known as decidua capsularis. At first it is similar to decidua basalis but with increase in the size of the chorionic vesicle, this layer becomes stretched, and begins to degenerate. Subsequently, the chorion laeve comes into contact with the epithelium of the decidua parietalis on the opposite side of the uterus and the two fuse. The lumen of the uterus is then obliterated. Hence, the only functional portion of the chorion is the chorion frondosum and together with the decidua basalis, the two make up the placenta.
Placental blood circulation has two relevant characteristics: 1 – the fetal blood circulation is closed (within blood vessels), 2 – the maternal blood circulation is open (not bound by blood vessels). Maternal blood enters the intervillous space under reduced pressure, regulated by the cytotrophoblastic cell plugs, and leaves through the uterine veins after exchanges occur with the fetal blood in the terminal branched villi.
The umbilical vein has a subendothelial elastic lamina; the two umbilical arteries lack an elastic lamina. The umbilical vein carries 80% oxygenated fetal blood. Although the partial pressure of oxygen in fetal blood is low (20 to 25 mm Hg), the higher cardiac output in organ blood flow, higher Hb concentration (by the way HbF) in fetal red blood cells, and higher oxygen saturation provide adequate oxygenation to the fetus. The umbilical arteries return deoxygenated fetal blood to the placenta.
Hemochorial (placental) barrier is a dividing membrane between the maternal and fetal circulations. In the early stages it consists of four layers: 1) the endothelial lining of the fetal vessels, 2) the connective tissue in the core of villus, reach in macrophages, 3) the cytotrophoblastic layer, and 4) the covering syncytium.
From the fourth month on, however, the placental barrier becomes much thinner, since most of the villi lose their cytotrophoblastic layer as well as the connective tissue surrounding the fetal capillaries. The endothelial lining of the vessels comes then in intimate contact with the syncytial membrane, thus greatly increasing the rate of exchange. In the final stages of pregnancy, the small villi show an extremely thin, double-layered membrane separating the maternal and fetal circulations. These layers, however, persist at all times. It is necessary to underline that placental barrier is very strong and safely protect an embryo from different negative influence. For example, AID virus can pass through this barrier only in 25 % cases. It is unpenetratable to bacteria.
Main functions of the placenta are the next: 1) nutritive, 2) excretory, 3) protective, 4) immune, 5) respiratory, 6) endocrine.
For example endocrine function of placenta includes synthesis of hormones likes progesterone, estrogens, insulin like hormone etc.
Crucial periods of human ontogenesis. In the human ontogenesis there are some periods of especially high sensibility to different influences. At first Australian scientist Norman Gregg had told about this periods in 1944. Later, in 1960, Russian morphologist P. Svetlov had grounded a theory of critical periods of embryogenesis. He thought, that in the course of ontogenesis there are some periods of an important quantitive changes. Some negative influences from outside may damage human organism at that moment and even interrupt the pregnancy or cause the death.
Such important events are:
1. Progenesis or gametogenesis which is characterized by specific changes of chromosomes amount in mejosis.
2. Fertilization – merging the gametes and restorations of the chromosomes amount.
3. Implantation – introducing of the embryo into the endometrium (7-8 day).
4. Placentation (3-8th week).
5. Growth of the brain (15-20th week).
6. Organo- and systemogenesis (formation of the vitally important system 20-24th week).
7. The birth.
8. Neonatal period and first year of life.
9. Pubertation (11-16th years).
10.Menopause.
During last period of time in medical practice of many countries of the world so-called “artifitial fertilization” (correct term “extracorporal fertilization”) is used very often in the case of male or female infertility.
It was in 1976 in Great Britain, Luisa Brawn was born thanks to the efforts of embryologists Edwards and Stantow.
What is extracorporal fertilization.
1. Special surgical manipulation allows to take few oocytes before ovulation.
2. Fertilization occurs “in vitro” in special lab tube.
3. Incubation lasts for 3-4 days (period of cleavage)
4. Blastocysts which consist of 18-32 blastomeres – “free blastocyst” is introduced in uterus. Usually more then one blasocyst begin to implant, so pregnancy is multiple.
5. 15 % of implantation are successful.
There are some advances of such extracorporal fertilization:
1. It is possible to choice the future sex of baby (this allow to shun some hereditary diseases).
2. The sperm may be reached with spermatozoa, abnormal ones may be picked out.
3. Abnormal sites of implantation are almost impossible in such case, i.e. tubal or abdominal pregnancy.
References:
1. Medical Embryology. Langman J. Baltimore. Wilkins Co. 1969. P. 3-78.
2. L. Carlos Junqueira, Jose Carneiro, Robert O. Kelley. Basic Histology. – 7th ed. Appleton and Lange. Norwalk, Connecticut, 1992.
3. Inderbir Singh. Human Embryology. – 4th ed. Jaypee Brothers Medical Publishers (P) LTD, 2002, pp. 3- 67.
4. Victor P. Eroschenko. Atlas of Histology with functional correlations. – 9th ed. Lippincott Williams and Wilkins, 2000.
5. Tissues and organs: a text atlas of scanning electron microscopy. – Kessel RG, Kardon RH. – Freeman Co. – 1979.
6. Charts:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#n15
http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
http://en.wikipedia.org/wiki/Histology
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.udel.edu/biology/Wags/histopage/histopage.htm
