Endocrine system, hypothalamus.
Pituitary gland. Pineal body. Thyroid
and parathyroid glands
1. The
general morphofunctional characteristic of the endocrine system. Endocrine glands classification.
2. Pituitary gland location and
embryonic origin.
3. 2 major divisions of the
pituitary and their descriptions.
4. Cell types in pituitary
portions and characteristic staining properties.
5. Hormones produced by the
pituitary, indicating for each one the division and cell type responsible for
its production as well as its target site.
6. Hypothalamus nuclei.
Peculiarities of the neurosecretory cells.
7. Description of role of the
hypothalamus in controlling pituitary function.
8. Description of blood supply
to the pituitary and its role in pituitary function.
9. Pineal gland general
structure and functions.
10. Pinealocytes and neuroglial
cells.
11. Pineal gland
interconnections with the other endocrine glands and nervous system.
12. Age changes of the pineal
gland.
13. The general morphofunctional
characteristic of the endocrine system. Endocrine glands classification.
14. Location, shape, and
embryonic origin of the thyroid gland.
15. Role of the pituitary gland
and hypothalamus in the regulation of thyroid follicular activity.
16. Effects of thyroid hormones
on their target tissues.
17. Structure, function,
location, and embryonic origin of the C or parafollicular cells.
18. Description of the thyroid
follicles, follicular cells, basement membrane, colloid, capillaries of the
thyroid gland.
19. Distinguish between an
active and inactive thyroid gland on the basis of follicular morphologic
characteristics.
20. Comparison of parenchymal
cell types of the parathyroid glands (chief cells and oxyphilic cells) in light
and electron microscopes.
21. Role of the parathyroid
hormone in controlling blood calcium and phosphate levels and the specific
effects of this hormone on its target tissues.
22. Factors that controls the
secretory activity of the parathyroid glands.
23. Comparison of the
parathyroid hormone and calcitonin in terms of their source and function.
For a multicellular organism survivement and maintainment its integrity in a varying, often adverse condition, connective tissue, tissue fluid and plasma perform important functions. They are stabilized by the coordinated regulatory activity of the autonomic and endocrine systems, the latter including the diffuse neuro-endocrine system proper. The autonomic nervous system utilizes conduction and neurotransmitter release to transmit information; it is swift and localised in the responses induced. The diffuse neuro-endocrine system uses only secretion: Is slower and the induced responses are less localized because the secreting neurotransmitteres can act on contiguous cells, on group of nearest cells reached by diffusion or on distal cells via blood like hormones. The endocrine system proper comprising clustered cells and discrete ductless glands producting hormones, is even slower and less localised though its effects are specific and often prolonged. There regulatory systemes over-lap is form and function, witn a gradation from the neural autonomic system, through intermediate system proper.
Endocrine cells. The term " endocrine cells " is used in relation to the cells that synthesize and secrete into the internal environment of the body a particular hormone. In practice, the term endocrine cells is used in relation to the secretory cells of the glands of internal secretion, endocrine single cells and small clusters of them (such as neuroendocrine cells in the respiratory system ), often combined in the diffuse endocrine system (eg, enteral endocrine system - a set of all cells of the gastrointestinal tract, producing biologically active substances regulatory nature). Endocrine cells are usually found in close contact with blood capillaries. These capillaries in the endocrine glands have a standard structure: fenestrovannoho type of endothelium and a wide lumen and a continuous basement membrane .
Endocrine cells have a structure which is determined by the chemical nature of a hormone synthesized them. For endocrine cells that synthesize peptides and proteins well developed granular endoplasmic reticulum, Golgi complex and protein secretory granules are typical. Well developed smooth endoplasmic reticulum and numerous mitochondria with tubular and vesicular cristae are typical for cells that synthesize steroid hormones. They also have lipid inclusions.
The close interrelationship of the autonomic and endocrine systems, both structural and functional, is exemplified by the hypothalamus. Though conviniently considered separately, the autonomic, diffuse neuro-endocrine and endocrine systems are really a single neuro endocrine regulator of the metabolic activities and internal environment, providing conditiones in which it can function successfully. There are, in addition to endocrine glands and diffuse-endocrine system, other hormone producing cells which form minor components of other systems and are describe with them.
Endocrine system includes
glands and cells, which have general functional feature – they secrete specific
substances called hormones into the circulation, which can regulate different
functions of organs and systems of the human organism. All endocrine glands
have general structural features: 1. they have no excretory ducts; 2. they have
very well developed blood network (capillaries); 3. cells of the endocrine
glands form special accumulations as follicles, cords, trabeculae or
clusters; 4.in endocrine cells one can find special granules, which contain hormones.
Endocrine system is classifying under following
basic components:
I. Central regulatory
formation of endocrine system,
1. Hypothalamus
2.
Hypophysis
3.
Epiphysis
II. Peripheral endocrine
glands
1. Thyroid gland
2.
Parathyroid gland
3.
Suprarenal gland: a) cortex, b) medulla,
III. Organes, having both ehdocrine and non-endocrine functions.
1.
Gonads: a) testis, b) ovary.
2.
Placenta
3.
Pancreas
IV. Solitary hormone
producing cells.
1. APUD-cells (of nervous origin)
2. Solitary hormone producing cells
(not of nervous origin).
Hormones as integrators
Hormones have been
defined as products of specialized tissues, which are carried by the blood
system to influence other cells, tissues and organs, or the organism as a whole. The integrative action of hormones consists in the
depression, activation or maintenance of cells other than, as well as,
themselves, as, for example, thyrotrophic hormone of the anterior lobe of the
hypophysis and thyroglobin. Some hormones affect certain organs and tissues
almost specifically, under certain conditions, and there are called target organs;
other hormones have a more general effect, which probably influences some basic
cell reactions about which little is known. The hormones may be secreted almost
as rapidly as they are formed (adrenal cortex), or they may be stored
intercellularly or extracellularly in the gland while the blood level is
maintained (insulin in the pancreatic islets, thyroglobulin in the thyroid
gland colloid). Hormones differ greatly in chemical composition, and include
proteins, polypeptides, modified amino acids, and steroids. Like certain drugs,
vitamins and trace elements, they may be effective in minute concentrations.
Tropic hormon - hormone whosetarget cells are other endocrine cells
(eg, endocrine cells of the anterior pituitary synthesizes and secretes into
the blood ACTH (adrenocorticotropic hormone). Targets for ACTH - endocrine
cells of the adrenal glands cortex, which synthesize glucocorticoids.
Releasing hormones ( releasing factors) [from
Liberin - releasing hormone enhances the synthesis and
secretion of hormones related to endocrine cells of the anterior pituitary (e.g.
corticoliberin activates the secretion of ACTH from ACTH- synthesizing
endocrine cells of the anterior pituitary).
Statin - releasing hormone, unlike liberin inhibiting the
synthesis and secretion of hormones in target cells.
There are next main types of hormones:
1) oligopeptides (e.g., neuropeptides
), polypeptides (e.g. , insulin );
2) glycoproteins (e.g. tyreotropin );
3) steroids (such as aldosterone and
cortisol);
4) tyrosine derivatives (e.g.,
iodine-containing thyroid hormones : triiodothyronine - T3 and thyroxine - T4);
5) retinoic acid derivatives (e.g.
vitamin A);
6) eicosanoids ( arachidonic acid
metabolites).
Target cells. The target cell - cell that can record specific
presence of hormone using the receptors and may change mode of operation upon
binding of the hormone (ligand) to its receptor .
Ligand. The term " ligand " means a chemical
compound that binds to another chemical compound, usually with higher molecular
weight. In endocrinology context, the term "ligand" is used in
relation to hormone molecules that bind to specific receptors on their target
cells.
Receptor- high-molecular substance that specifically binds to a
specific ligand , such as a hormone. Receptors liy in glycocalix of target cell
or inside the cell. So, there are two classes of receptors -the nuclear and membrane
.
Membrane. Peptide receptor ligands (e.g., insulin, growth
hormone, different tropic hormones) are usually located in the plasma membrane
of the cell.
Nuclear . Receptors of steroid hormones (eg, glucocorticoids,
testosterone, estrogen ), tyrosine derivatives and retinoic acid have
intracellular localization.
Regulation of target cells
Depending on the distance from the
producer of the hormone to the target cells differentiated endocrine, and paracrine
and autocrine regulation may occur.
Variations of ligands influence on target cells . A - endocrine , B - paracrine , B - autocrine
Endocrine or dystant
regulation. Hormone secretion occurs in
the internal environment, the target cell may be from endocrine cells
arbitrarily far. The most striking example: the secretory cells of the
endocrine glands and hormones of which enter the general circulation system.
Paracrine regulation. Producers of biologically active substances and target
cells located nearby, hormone molecules reach target cell by diffusion in the
intercellular substance. For example, in the parietal cells of the gastric
glands secretion of H stimulates gastrin and histamine, and inhibits
somatostatin and prostaglandins, which are secreted by cells located nearby.
Autocrine regulation. In autocrine regulation cell which is producing the
hormone has receptors for the same hormone (i.e., cell- producing the hormone
at the same time is its own target). As an example of endothelins produced by
endothelial cells and affect the same endothelial cells; T- lymphocytes that
secrete interleukins, have different target cells, including T- lymphocytes.
Mechanisms of hormone action
Hormone molecule that circulates with blood or lymph,
"recognizes" its receptor on the surface of a target cell. Stereochemistry of the highly active center of hormone
molecule and its receptor configuration has decisive role in this recognition.
Here, as in several other vital processes of the body, under the principle of
"key" (hormone ) to " lock " (plasmolemma receptor).
Binding of hormone to the receptor causes a conformational (three- dimensional)
changes in receptor molecules, which, in turn, influences the enzyme system of
cell, including adenilat - cyclase. This enzyme adenylate cyclase leads to
conversion of ATP to cyclic adenosine monophosphate (cAMP). Molecules of the last
one act as a universal stimulator of cells intracytoplasmic enzymes. It is
characteristic that the effect of hormones can manifest itself not only to
strengthen but also in inhibition of cells and systems. Note that for description of the universal role of cAMP in the
mechanism of hormones action american scientist E. Sutherland in 1971 was
awarded the Nobel Prize.
Interaction between the individual
links of the endocrine system, as well as between endocrinocytes and target cells is based on the principle of feedback. Effect
of a hormone in the target cell leads to increased production of certain
chemicals. Increasing the concentration of the latter in the internal
environment is the signal to inhibition of endocrinocytes. Conversely, reducing
the concentration of hormone in the blood or lymph is an stimulus for synthetic
activity of endocrinocytes. The principle of feedback remains in force in the
case of depressing (inhibitory) effect of hormones on the target organ.
All endocrine glands have several
common features of the structure :
1. they are ductless (have no
excretory ducts).
2. They have well-developed vascular
network, especially microcirculatory bed (capillaries of visceral type with
fenestrated endothelium and continuous basement membrane ).
3. Endocrine cells form
characteristic clusters in the form of follicles (vesicles), or trabeculae
(cords), or lobules. So, parenchyma of the gland has sufficient structural or
morphological units - follicles, trabeculae, lobules.
4. In endocrine cells (cells producing
hormones) you can usually identify the specific granules , which accumulates
biologically active substance.
Hypothalmus
Hypothalmus is the
highest centre of control and coordination of the endocrine system. It controls
and interprates all the visceral functions of the organism and unites the
endocrine mechanism of regulation with the nervous regulation and especially
with the sympathetic and parasympathetic parts of the vegetative nervous
system. The hypothalamus contains special neurosecretory cells which are
aggregated in nuclei (30 pairs of nuclei) which are grouped into
anterior, middle
and posterior
groups.
Endocrine
function is related to the activities of specific neurosecretory cells of the
anterior and middle hypothalamus. Neurocytes of posterior nuclei, to a lesser
extent, of middle and anterior hypothalamus send their processes in the
sympathetic and parasympathetic nerve trunks to the relevant objectives
(targets), so, they provide nervous regulation of their activities.
In the
hypothalmus region an eminentia medial is present which forms the neurogemal
centre of the hypothalamo-hypophysial system. It
is formed of ependyma (of individual specialized cells which
differentiate to from Tanicytes). The tanicytes are characterized by branched
processes which contact with the primary capillaries of the hypophysial portal
system. The hypothalamo-adenohypophysial system
accumulates neurohormones called adeno-hypotrophin released by the hypothalamus
and which pass into the portal system of the hypophysis.
The hypothalamo-neurohypophysial system accumulates
nonapeptides which are released into the blood. The
anterior group of nuclei of the hypothalamus contains two main nuclei: 1) supraoptical nuclei- are formed by large cholinergic neurosecretory cells which contain
secretory granules both in the perlkaryon and in the processes. The axons of these cells pass
through the medial eminentia and the infundibulum of hypothalamus into the
posterior lobe of the hypophysis. Here they form the terminal buds of Herring
on the wall of blood capillaries. These cells produce neurohormon-vasopressin
also called antidiuretic hormone which controls reabsorption of water in renal
tubules. In recent years it was also proved the important role of vasopressin
in the regulation of body temperature, the cardiovascular system activity, this
hormone is necessary for normal brain development.
2). Paraventricular nuclei- are composed of a central and peripheral portion,
while the central part is formed of large cholinergic neurosecretory cells
whose axons pass into the posterior lobe of the hypophysis, the peripheral part is made of
small adrenergic neurosecretory cells whose axons pass into the medial
eminence. The cells of the central part release hormone oxytocin which
regulates contraction of the uterus and mammary gland smooth muscles.
In the medial group of nuclei small adrenergic cells are present which
produce adenohypophysotropic neurohormones by means of which the hypothalamus
regulates the activities of the adenohypophysis. These hormones are low molecular oligopeptides
which are divided into liberins (releasing factors) which stimulate the activity of
the anterior and medial lobes of the pitutary, and statins (inhibitory
factors) which inhibit their activity. Some of the
important nuclei lie in the region of the tuber cinereum (nucleus
arcuatus, nucleus ventromedialis and nucleus dorsomedialis). The
principle areas of production of liberin and statin includes both ventromedial
and arcuate nuclei, small peptidoadrenergic cells of the paraventricular nuclei
and analogous cells of the grey periventricular matter, preoptic zone of the
hypothalamus and suprachiasmatic nuclei.
Common name of
liberins and statins is releasing factors. Liberins are physiological
antagonists of statins: the first stimulate and
the latest depress production and the output of pituitary hormones into
the bloodstream. Liberins and statins pass to the pituitary through portal vein:
axovasal synapses lie in the medial eminence (primary mesh) from which blood is
collected into the portal vein, which then forms secondary capillary net in the
adenohypophysis. Next liberins are known: folliliberin, lyuliberin,
somatoliberin, prolactoliberin, tyroliberin, melanoliberin, cortycoliberin, a
group of statins include somatostatin, and prolactostatin melanostatin. Names
of hormones of middle hypothalamic nuclei are formed of two parts: the first
part corresponds to the name of pituitary hormone that produces target cell (e.g.
folitropin, lutropin, somatropin), the second part includes the word liberin,
statin or - depending on the physiological action of hormones. American
scientists Tiymen R. and E. Shelley in 1977 were awarded by he Nobel Prize for the
discovery of hypothalamic liberins and statins.
The effects
of various hypophyseal hormones on target organs and the feedback mechanisms
that control their secretion. For definitions of abbreviations
The
hypothalamus begins to develop in the fourth or fifth week of embryogenesis in
the basement of the intermediate brain vesicle.
Hypothalamus
controls the visceral activites of organs by two mechanisms:
1) through
its regulation of hypohysial activity, it is called transadenohypophysial
regulation,
2) by
sending efferent impulses to control sympathetic and parasympathetic nervous
system. It is called parahypophysial regulation
Hypophysis
Hypophysis is also known as pituitary
gland. The central endocrine organ whose function is the regulation of a number
of peripheral parts of the endocrine system (the so -called pituitarydependent)
as well as in the implementation of a direct impact on a number of cells of nonendocrine
nature. Pituitarydependent elements of endocrine system are the thyroid,
adrenal cortex, endocrine cells of gonads. Among nonendocrine cells pituitary
gland regulates lactocytes of mammary gland, melanocytes, adipocytes ,
chondrocytes, testis spermatogonia and others. Deposited in pituitary oxytocin
and vasopressin - are hormoness that cause contraction of the smooth muscle
cells of the uterus and vascular wall.
There may be distinguished 3 lobes: anterior, medial and posterior. But hystologically it is more
convinient to divide it into adenohypophysis and neurohypophysis due to the
origin and structure. Such
structures originate due to the specific development of pituitary gland in
embryogenesis. The pituitary gland begins to develop in the fourth week of
embryogenesis of epithelial and neural primordia.
The epithelium
of the upper part of oral cavity forms a pouch-like structure which deepens in direction
to future brain and gives rise to adenohypophysis structures. The distal
portion is formed as a result of epithelium growth of the anterior wall of
pituitary pouch, intermediate lobe - from its posterior wall. Towards the
pituitary pouch intermediate brain vesicle produces outgrowth , which is
converted in the future into the funnel of the third ventricle of the brain. Glia
of the distal end of the funnel, growing, forms the neurohypophysis, proximal
part of the funnel becomes a pituitary stem. Adrenocorticotropocytes in the
pituitary gland appears on the fifth week of embryogenesis, cell producers of
other pituitary hormones later - at 13th weeks. By the time the baby is born pituitary
gland differentiation in general is completed. In the postnatal period phasing
of pituitary endocrinocytes activation is observed: in the early postnatal
period mainly somatotropocytes and tyrotropocytes cells are activated, in
puberty activation of gonadotropin-releasing adenocytes is predominant.
Development of the adenohypophysis and the neurohypophysis
from the ectodermal epithelium of the oral cavity roof and from nerve tissue of
the diencephalon floor.
Adenohypophys
is covered with fibrous capsule, parenchyma is represented by cords of
endocrine cells, surrounded by a network of reticular fibers, such fibers also
surround the capillaries with fenestrated endothelium and wide lumen of secondary
capillary network.
Endocrine cells
of the anterior lobe that synthesize peptide hormones contain elements of
granular endoplasmic reticulum, Golgi apparatus, numerous mitochondria and
secretory granules of different diameters. Cells are placed between blood
capillaries and secrete hormones here. They also receive some liberins and
statins from blood.
Specimen
of cat pituitary stained with hematoxylin and eosin. In the left part
edenopituitary has typical trabecular arrangement of adenocytes. Anterior and
intermediate pars are separated by split.
1). Adenohypophysis includes the
pars anterior (distalis), intermedia and tuberalis.
2).
Neurohypophysis includes the pars posterior (nervosa).
Sometimes the pars
intermedia is cystic, as it is here. In this particular instance, the pars
intermedia is fused with the pars distalis to the left, and the lumen of
Rathke's pouch has disappeared. The split seen at the right is a fixation
artifact; pars nervosa is at the lower right
Adenohypophysis is highly vascularized
and consists of epithelial cells of varying size and shape arranged in cords
supported by a delicate skeleton of connective tissue. Each trabecule (cord) is formed of gland cells (adenocytes) of 2 types. One of them, arranged in perifery of
trabecula and stained intensively with special dies due to presence of
secretory granules in it (chromophills). And
ofher type of cells arranged in middle part of trabecule, does not stains
intenslvely, without granules, so called chromophobic
cells. The latter
ones include:
1)
undifferentiated cambial cells, which is a reserve for the replacement of endocrinocytes
who completed their life cycle;
2) cells that
have entered the stage of differentiation, but have not yet accumulated in the
cytoplasm special hormone granules;
3) cells which at
the time of the pituitary gland taking for histological examination threw their
secretory granules outside the cytoplasm;
4) follicular -
stellate cells, whose function is still not clear. Clusters of follicular -
stellate cells can form microfollicular structure with deposition of secretory
products in the lumen of the follicles.
Some stains allow the recognition of
cell types of the pars distalis: chromophils (acidophils and basophils) and
chromophobes. Gomori’s trichrome stain. High magnification.
Pars distalis stained to show the 3 major types of secretory epithelial
cells there:
- a = pale
chromophobes
- b = blue
basophil
- c = red
acidophil
Notice that these cells are
characteristically in cord-like clumps. The clumps are separated by a fine reticular
fiber stroma (thin blue lines), where the blood capillaries lie. All endocrine
glands are highly vascularized, since secretions go directly into capillaries
(depending on the organ.) In endocrine glands the endothelium of these vessel s
is typically continuous and fenestrated.
Chromophilic cells are
divided into I) acidophils ahd 2) basophils.
I. Acidophils cells: or L-cells:
a). Somatotrophes - These
are ovoid and usually grouped alongcapillaries: they are largest and most
abundant class of adenohypophysial chromophils, secreting somatotropin.
Ultrastruturally are seen to contain numerous electron dense, spherical
secretory phase, relatively small amount of granular endoplasmic reticulum; the
nucleus lies in central part of the cell. Cells of similar fine structure
characterize human eosinophilic adenoma associated with acromegaly or
gigantism.
Pale capillaries between clumps of
acidophils of pars distalis. At the left, on the lower edge of the sinusoidal
wall, is an elongate nucleus of an endothelial cell. EM would show that the
endothelium here is continuous and fenestrated.
Somatotropic cell from the anterior pituitary. Notice
the numerous, middle-sized osmiophilic spherical secretory granules.
Electron micrograph of a somatotroph (growth hormone–secreting
cell) of a cat anterior hypophysis. Note the numerous secretory granules, long
mitochondria, cisternae of rough endoplasmic reticulum, and Golgi complex.
x10,270.
b). Mammotrophes-secreting
the polypeptide hormone prolactin are dominant in pregnancy and hypertrophy
during lactation. They are distinguished by their affinlty to dry erytrosinc
and azacarmine. Their granules are largest in any hypophysial cells (about 500-6OO
nm in diameter). Their size is bigger in pregnant and lactating females than
that in non-pregnant females and males. The granules are evenly dense, ovoid or
fuse with lysosomes to form autophagic vacuoles, which degrade unused granules.
In active cells granular endoplasmic reticulum and a Golgi complex are
prominent.
II. Basophils or B-cells:
a). Thyrotrophes-secrete
TSH. They are also called B-basophils. They are elongate, polygonal and lie in
clusters towards the adenohypophysial centre. They usually form cellular cords,
which has no direct contact with capillaries. They are stained selectively with
aldehyde fuscin. Their small granules, peripheral and irregular, are less
electron dense than in other basophils: being
100-150 nm in diameter
and among the smallest granules in adenohypophysial cells.
Mammotroph
from the anterior pituitary. The secretory granules are sparse and eliptical.
b). Honadotropes-also
known as basophils are larger than thyrotrophs, are rounded and usually
situated next to capillaries. They have secretory granules with an affinity for
the periodic acid. In some cells, usually peripheral in lobe, granules are
stained in purple white, in others, more central, they are red, suggesting that
the former secrete FSH and latter LH. It is shown by latest researches that FSH
and LH may coincide in the same cell within the same secretory granules.
Gonadotropes has plevarphic nucleus and spherical granules about 200 nm in
diameter, along with this are present vesicular endoplasmilc reticulum and a
well developed Golgi complex.
Corticotropes are the
fifth group of chromophills. They are an intermediate cells due to their
possibility for staining. The identification of the cells which secrete adrenocortlcotropin
(ACTH) was difficult to achieve until it was realised that a precursor
molecule, pro-oplo-melanocortlcotropin, is cleared into a number of different
molecules including ACTH, B-lipotropIn and B-endorphin. The function of latter
two is unknown, although the opio-gelanocortlcotropin coaplex is also
synthesized in neurons of the central neurvous system and has neuromodulator
function. In human this precursor is glycosylated. Making the granules
PAS-positive. They are also basophilic. These cells are irregular in shape and
size and have short dendritic processes which are inserted along other
neighbouring cells. Their granules are also small (about 200 nm) and
difficult to defect under light microscope.
Chromatophobic cells are predominat cells in anterior
pituitary. They constitute the majority of the cells of the adenohypophisis
(principal cells). They appear to consist of a number of cells of different
types, including degranulated secretory cells of the types, stem cells capable
of giving rise to chromatophils and follicular cells containing numbers of lysosomes and forming cells
clusters around cystes of various sizes: cystes are often present in the
junctional areas with the neurohypophysis and are filled with a PAS-positive substance
of unkrown significance.
Pars intermedia is
a constituent of the anterior pitutary, being derived embryologically from the
cells lining the cavity of Rathkes porch. Pars intermedia has many B-cells and
follicles of chromatophobic cells surrounding PAS-positive colloidal material.
Adenocytes of these pars has capacity to produce proteins or mucous secretion,
which may be stored within adjoining cells. Secreting cells of pars intermedia
have granules containing either -endorphin of -endorphin scattered uniforalyz
these cells contain peptide hormone including ATCH, melanocyte-stimulating
hormone, lipocytestimulating hormone, which assigned to the APUD series, as are
other adenohypophysial secretory cells.
Pars tuberalis. This part
is remarkable for its large number of blood vessels, between which cords of
undifferentiated cells are admixed with some and cells. From pars tuberalis penetrates trabecules in anterior lobe. In some cells of trabecules are found some
basophilic granules, but secretion from there cells only starts after signals
from neurons.
Posterior lobe of hypophysis or
neurohypophysis. Neurohypophysis
comprises the posterior lobe of the pituitary and pituitary part of stalk.
Neurohypophysis is mainly made up of
ependymocytes (glial cells”) – pituicytes (the origin of which is
related cells which line the wall of the 3rd ventricle of the brain).
It contains also blood vessels, axons of the hypothalamic- pituitary tract and
their endings on blood capillaries (axo-vasal synapses). Proper endocrine
function pf piruicytes is yet unknown, they contain numerous intermediate
filaments, pigment granules and lipid inclusions. In
contrast to the anterior pituitary, posterior lobe (neurohypophysi ) is part of
the brain. Neurohypophysis
contains axons and their endings, which belong to neurons with large
perikaryons. These neurons are located in the paraventricular and supraoptical nuclei
of the hypothalamus.
Pituicytes have
spinous processes and send them to adventocytes of blood vessels or basal
membranes of capillaries. Hormones vasopressin and oxytocin secreted by big
peptidocholinergic neurosecretory cells of anterior lobe of hypothalamus are
stored in neurohypophysis. Axons of this neurosecretory cells form the
hypothalamo-neurohypophysial tracts. ln posterior lobe of hypophysis, these
ends are called Herring’s bodies.
Section of the pars nervosa. Most of the tissue is formed by
axons. Herring bodies and nuclei of pituicytes can be seen as well as
erythrocytes (in yellow) within blood capillaries. Mallory’s trichrome stain.
High magnification.
Two pale pink Herring bodies,
collections of secretion in the pars nervosa of the pituitary. They represent
accumulations of neurosecretion within the axons of neurons whose cell bodies
lie in the hypothalamus of the brain.
Pineal gland
Epyphysis cerebri participates
in regulation of processes, which are characterized by rythmizm and periodism e.g.
ovario-menstrual cycle. Such cyclic functions which keep on charping in their intensity
depending upon day or night are called Circa diem. The capability of epyphysis
and regulate rhytmic functions is assentuated by its capacity and release
hormones under the stimulation of light. The organ is covered with a thin
connective tissue capsule from which arise septas dividing the whole organ into
lobules. There are two types of cells arranged in lobules in the parenchyma:
1). pinealocytes
(endocrinocytus pinealis)
2). gliocytes
(gliocytus centrales)
Pinealocytes
are found towards the centre of the lobules. They are large neurosecretory
cells, containing a large nucleus and large nucleoli. From their bodies arise
long processes simular to dendrites, which undergo branching and intermediate
with the processes of gliocytes. The processes usually are directed towards
capillaries and undergo contact with them. The cells contain both granular and
agranular endoplasmic reticulum, mitochondria and Golgi complex. An organelles
of unusual structure made up of groups of microfibrils and perforated lamellae
may be present called "Canaliculate lamellar bodies". The terminal
buds of the processes of these cells contain aonaalnes and polypeptide
hormones, along with a neurotransmitter gama-amino-bityric acid.
There are two types of pinealocytes: 1) light ones, which contain light homogenous cytoplasm
and 2) dark ones, which contains acidophilic
inclusions. These
two cells are the functional diversity of a single cell. The cell also contains
well developed ribosomes and polysomes. The pinealocytes are separated from one
another by neurogial cells that reseable astrocytes in structure. Their
processes are directed towards the interlobular connective tissue septa.
The pinealocytes
produce a number of hormones. These hormones have chiefly inhibitory actions on
the endocrine system including pitutary, thyroid, parathyroid, gonads,
suprarenalis and pancreas. The hormones reach the hypophysis through blood or
cerebrospinal fluid.
Parenchyma
of epiphysis with brain sends.
The
epiphysis regulates the function of the gonads by releasing serotonin. The
pinealocytes also produce a class of hormones called antigonadotropin, which
decreases the secretion of luteinazing hormone from the
adenohypophysis. It also produces hormonal factors which increase the amount of
pottasium in blood (mineral metabolism). They produce as many as 40 types
of regulatory peptides-thyroliberin, lueliberin, tyrotropin and vasotocin etc.
Glandula thyroidea
Thyroid gland is peripheral pituitarydependent gland, which differs from all
other endocrine glands in that hormone storage is developed to the highest
degree and reflected morphologically most markedly.
The thyroid gland,
located in the cervical region anterior to the larynx, consists of two lobes
united by an isthmus. Thyroid gland is composed of numerous spherical
follicles. Each follicle consists of a simple epithelium of follicular cells
enclosing a central lumen filled with colloid, a gelatinous substance. In
typical sections, follicular cells range from squamous to low columnar; the
follicles have a variable diameter. It depends on thyroid gland stimulation.
A loose connective tissue
capsule that sends septa into the parenchyma covers the gland. These septa
gradually become thinner; they reach all the follicles, separated from one
another by fine, irregular connective tissue composed mainly of reticular
fibers. The thyroid is an extremely vascularized organ, with an extensive blood
and lymphatic capillary network surrounding the follicles. Endothelial cells of
these capillaries are fenestrated, as they are in other endocrine glands. This
configuration facilitates the passage of the hormones into the blood
capillaries.
Its function is to
synthesize the hormones thyroxine (T4) and triiodothyronine (T3),
which stimulate the rate of metabolism.
The major
regulator of the anatomic and functional state of the thyroid gland is
thyroid-stimulating hormone (thyrotropin), which is secreted by the anterior
pituitary.
Relationship between the
hypothalamus, the hypophysis, and the thyroid. Thyrotropin-releasing hormone
(TRH) promotes secretion of thyrotropin (TSH), which regulates the synthesis
and secretion of the hormones T3 and T4. In addition to
their effect on target tissues and organs, these hormones regulate TSH and TRH
secretion from the pars distalis and the hypothalamus by a negative-feedback.
Solid arrows indicate stimulation; dashed arrows, inhibition.
The morphologic appearance of thyroid
follicles varies according to the region of the gland and its functional
activity. Normally in man there is a preponderance of smaller over larger
follicles. But in certain conditions may increase in size, and the external
surface may be markedly irregular. In man the size the follicles varies considerably
from region to region, with corresponding differences in the follicular cells
and colloid. This has been attributed to cyclic states of activity, which take
place regionally rather than uniformly. In the thyroid of animals other than
man, the folicles are more uniform. In the rat and guinea pigs the follicles
on the periphery of the gland are larger than the more central ones, and the
colloid of the former is more basophile
Low power view of thyroid
gland with its characteristic colloid-filled follicles. This is the only
endocrine gland that typically stores its hormonal secretion extracellularly
before releasing it into the bloodstream.
The gland has a
thin capsule of connective tissue whose extension divide it into masses of
irregular form and size. The
structural-functional units of the gland are follicles, which are spherical
vesicles of varying size and filled with a cavity in the centre. If the
thyroid gland is highly active then the walls of the follicles forms many
branched infoldings and the contour of the follicle becomes stellated. In the
cavity of the gland is present colloid which has a protein called
thyroglobulin.
Photomicrograph of a
section of thyroid and parathyroid glands. The thyroid is formed by thousands
of spheres called thyroid follicles. They are filled with a glycoprotein, the
colloid, which appears fragmented here because of an artifact. The parathyroid
is separated from the thyroid by a thin connective tissue capsule
nad consists of cords.
H&E stain. Low magnification.
The space
between the follicles is fullfilled by a stroma made up of delicate connective
tissue in which there are numerous cappillaries and lymphatics, along with
large number of nerve fibers. In these septas are found compactly arranged
groups of epitheliocytes along with lymphocytes and labrocytes (must cells).
Section of a thyroid
showing the follicles formed by a simple epithelium, containing colloid.
H&E stain. Medium magnification
Thyroid gland cells (thyrocytes). The thyroid epithelium rests on a basal lamina. The
follicular epithelium exhibits all the characteristics of a cell that
simultaneously synthesizes, secretes, absorbs, and digests proteins.
Normally, each follicle of the human
thyroid consists of an outer shell of gland cells thyrocytes that enclosed the
colloid. These cells are commonly low cuboidal,
and are in close relation with the connective tissue and its network of blood
and lymph capillaries. The epithelium of the gland varies in size and
arrangement, depending on age, sex, and season of the year, diet and certain
pathological processes. In general, it is believed that the epithelium is squamous when the gland is underactive
(hypo function), and columnar and folded when it is overactive (hyper function).
There are, however, so many exceptions that it is impossible to determine the
functional state of the gland in all cases through histological examination
along.
The processes of synthesis and iodination
of thyroglobulin and its absorption and digestion. These events may occur
simultaneously in the same cell.
The basal part of these cells is rich
in rough endoplasmic reticulum. The nucleus is generally round and situated in
the center of the cell. It is commonly spheroidal and poor in chromatin, and
contains one or more nucleoli. After stimulation of the gland cells, the
nucleus enlarges and stains more lightly. The apical pole has a discrete Golgi
complex and small secretory granules with the morphologic characteristics of
follicular colloid. Abundant lysosomes and some phagosomes are found in this
region. The cell membrane of the apical pole has microvilli to be seen only in
electron microscope. During functional activity of the thyroid gland the
number and length of this microvilli increase. Cisternae of rough endoplasmic
reticulum are dispersed throughout the cytoplasm. Mitochondria, which are
usually short, thin and rod-like in the human gland, are more numerous in the
apical portion of the cytoplasm.
The gland cells of any one follicle
are more or less uniform, though occasionally some columnar cells may be
present when most cells are cuboidal. Rarely, there may be “colloid cells” of
Langendorff with a dark picnotic nucleus and dense-staining, osmiophilic
cytoplasm. They are probably dead or dying cells.
EM of thyroid
cell showing sparse microvilli on the apical surface that lies next to the
stored colloid in the follicle.
Follicular cells or tyrocytes are found around the follicles and also in
the extrafollicular epithelium. In the follicles these cells form a single layer lying on a basal
membrane forming its outer boundary. Normally the cells are cuboidal and the
colloid in the follicles are distended with abundant colloid. When
highly active - the cells are columnar and colloid scanty. On the apical part
of the cell towards the colloid there is a layer of microvilli. Between neighboring
cells there are well developed polydesmosomal contacts and in mature follicles
are found lateral interdigitation amongst cells of the follicles. The cell
shows well developed organelles especially those concerned with the synthesis
of protein, granular endoplasmic reticulum, Goldgi apparatus, lisosomes,
microtubules, microfilaments etc.
Higher power of thyroid,
showing follicular epithelium, which varies from simple squamous to simple
cuboidal in height, depending on how distended the follicles are. Notice
distended capillaries between follicles.
Colloid. The lumen of the
follicles is normally filled with the characteristic material called colloid.
This is clear, viscous fluid whose consistency varies when the gland is in
different states of activity.
It is optically and probably chemically homogenous, except for some desquamated
cells and, under certain conditions, some macrophages. The colloid is thus an active reservoir, which is in a continual state
of flux rather than an inert storage center. During hypo function
colloid becomes watery (liquid), foamy (frothy)
and bad staining. During hyper
function colloid becomes very dense and solid,
well staining.
Histophysiology
The thyroid is the
only endocrine gland whose secretory product is stored in great quantity. This
accumulation is also unusual in that it occurs in the extracellular colloid. In
humans, there is sufficient hormone within the follicles to supply the organism
for up to 3 months. Thyroid colloid is composed of a glycoprotein (thyroglobulin)
of high molecular mass (660 kDa)
This mechanism maintains
an adequate quantity of T4 and T3 within the organism.
Secretion of thyrotropin is also increased by exposure to cold and decreased by
heat and stressful stimuli.
Synthesis & Accumulation of Hormones by Follicular
Cells
Synthesis and accumulation of
hormones take place in four stages: uptake of iodide from the blood, synthesis
of thyroglobulin, release of thyroglobulin, activation
of iodide and iodination of the tyrosine residues of myroglobulin.
1. The uptake of circulating
iodide is accomplished in the
thyroid by a mechanism of active transport, using die iodide pump. This pump,
located within the cytoplasmic membrane of the basal region of the follicular
cells, is readily stimulated by thyrotropin. The uptake of iodide can be
inhibited by such drugs as perchlorate and thiocyanate, which compete with
iodide.
2. The synthesis of
thyroglobulin is similar to
that in other protein-exporting cells. Briefly, the secretory pathway consists
of the synthesis of protein in the rough endoplasmic reticulum, the addition of
carbohydrate in the endoplasmic reticulum and the Golgi complex.
3. The release of
thyroglobulin from formed
vesicles at the apical surface of the cell into the lumen of the follicle.
4. During the activation of
iodide, iodide is oxidized by
thyroid peroxidase to an intermediate, which in turn combines the colloid with
the tyrosine residues of thyroglobulin. In contrast to the first three
processes, iodination of tyrosine residues bound to thyroglobulin takes place,
not inside the follicular cells, but in the colloid, in contact with the
membrane of the apical region of the cells.
5. Hormone exfusion to the
bloodstream.
It is postulated that the
union of the iodinated tyrosine is catalyzed by an enzymatic mechanism.
Thyroglobulin must have the correct spatial configuration for this process to
occur normally. When disease causes the production of abnormal amounts of
thyroglobulin, this process is blocked, resulting in deficient synthesis of
thyroid hormone. The process can also be blocked by drugs (e.g.,
propylthiouracil, carbamazole) that inhibit the peroxidase-catalyzed iodination
of thyroglobulin. Some forms of thyroid dysfunction are related to a genetic
deficiency of peroxidase or the iodide pump.
Liberation of T3 & T4
When stimulated by
thyrotropin, thyroid follicular cells take up colloid by a form of pinocytosis.
Folds of apical cytoplasm (lamellipodia) encircle a portion of colloid and
bring it into the follicular cell. The pinocytotic vesicles then fuse with
lysosomes. The peptide bonds between the iodinated residues and the
thyroglobulin molecule are broken by proteases in lysosomes, and T4,
T3, diiodotyrosine (DIT), and monoiodotyrosine (MIT) are liberated
into the cytoplasm. The free T4 and T3 then cross the
cell membrane and are discharged into the capillaries. MIT and DIT are not
secreted into the blood, because their iodine is removed as a result of the
intracellular action of iodotyrosine dehalogenase. The products of this
enzymatic reaction, iodine and tyrosine, are reused by the follicular cells. T4
is the more abundant compound, constituting 90% of the circulating thyroid
hormone, although T3 acts more rapidly and is more potent.
Thyroxine has a gradual effect, stimulating mitochondrial
respiration and oxidative phosphorylation; this effect is dependent on mRNA
synthesis. T3 andT4 increase the numbers of both
mitochondria and their cristae. Synthesis of mitochondrial proteins is increased,
and degradation of the proteins is decreased.
Most of the effects of
thyroid hormones are the result of their action on the basal metabolic rate;
they increase the absorption of carbohydrates from the intestine and regulate
lipid metabolism. Thyroid hormones also influence body growth and the development
of the nervous system during fetal life.
Thyroid Disorders & Hormone Synthesis
A diet
low in iodine hinders the synthesis of thyroid hormones, causing hypothyroidism. Thyroid hypertrophy as a result of
increased thyrotropin secretion causes the disorder known as iodine deficiency
goiter, which occurs widely in some regions of the world.
The syndrome of adult hypothyroidism, myxedema, may be the result of a number of diseases of the
thyroid gland, or it may be secondary to pituitary or hypothalamic failure. Autoimmune
diseases of this gland impair its function, with consequent hypothyroidism. In Hashimoto thyroiditis it is possible to detect antibodies against thyroid
tissue in the patient's blood. As with other autoimmune malfunctions, Hashimoto
disease is more common in women.
Children
who are hypothyroid from birth are
called cretins; cretinism is characterized by arrested physical and
mental development.
Hyperthyroidism, or thyrotoxicosis, may be caused by a
variety of thyroid diseases, of which the most common form is
Parafollicular
cells
Another type of cell, the
parafollicular,
or C (clear) cell, is found as part
of the follicular epithelium. They are located in the wall of the
follicles, lying between basal parts of the two thyrocytes, but their apical
portions do not touch the lumen of the follicle. Parafollicular cells are
somewhat larger than thyroid follicular cells and stain less intensely. They
have a small amount of rough endoplasmic reticulum, long mitochondria, and a
large Golgi complex. The most striking feature of these cells is their numerous
small (100-180 nm in diameter) granules containing hormone. These cells are
responsible for the synthesis and secretion of calcitonin, a hormone whose main effect is to lower blood calcium levels by
inhibiting bone resorption. Secretion of calcitonin is triggered by an
elevation in blood calcium concentration.
Parafollicular
cells called calcitoninocytes. The cells are polyhedral, with oval excentric nuclei. They lie between
the follicular cells and their basement membrane. They may, however, lie
between adjoining follicular cells, but do not reach the lumen. They may also
be arranged in groups within the connective tissue septa.In size they are very
large cells, do not absorb iodine but synthesize neuroamin noradrenalin and
serotonin by means of decarboxylation of tyrosine and 5-hydroxytryptophan with
formation of hormone called calcitonin and somatostatin. The cytoplasm is
filled with oxyphilic and acidophilic secretory granules. The cell shows well
developed endoplasmic reticulum (granular), Golgi complex, mitochondria. They
contain small but strongly osmiophilic granules-release calcitonin. Those which
contain large but weakly osmiophilic granules-release somatostatin.
Calcitonin believes as an anta-gonist to the hormone of parathyroid gland and
decreases calcium ions in the blood.
High
magnification of a section of a thyroid. Calcitonin-producing parafollicular
cells can be distinguished from the follicular epithelial cells because they
are larger and their nuclei stain lighter. H&E stain. High magnification.
Electron
micrograph of thyroid showing 2 calcitonin-producing parafollicular cells and
part of a thyroid follicle. Note 2 blood capillaries at both sides of the
parafollicular cells.
Electron
micrograph of a calcitonin-producing cell. Note the small secretory granules
(SG) and the scarcity of rough endoplasmic reticulum (RER). G, Golgi region.
x5000.
Parathyroid glands
The parathyroids
(peripheral pituitary independent glands) are four small yellow-brown, oval glands—3 x 6 mm—with a total weight
of about 0.4 g. They are located behind the thyroid gland, one at each end of
the upper and lower poles, usually in the capsule that covers the lobes of the
thyroid. Sometimes they are embedded in the thyroid gland. The parathyroid
glands are derived from the pharyngeal pouches—the superior glands from the
fourth pouch and the inferior glands from the third pouch. They can be found in the mediastinum, lying beside the
thymus, which originates from the, same pharyngeal pouches.
The human parathyroid glands, viewed
from behind.
Each parathyroid gland is
contained within a connective tissue capsule. These capsules send septa into
the gland, where they merge with the reticular fibers that support elongated cord-like clusters of secretory
cells.
The parenchyma of the
parathyroid glands consists of densely packed groups of two types of cells: the
chief, or principal, cells and the oxyphilic cells, which may form a continuous mass of cells or may be
arranged as anastomosing cords, or less commonly as follicles with a colloidal
material in their lumen.
Low power of parathyroid, showing a
somewhat lobulated appearance and considerable adipose tissue intermingled with
secretory portions.
The chief (principal) cells are the more numerous and probably
the more important. They are small polygonal cells with a vesicular centrally
placed nucleus and a pale-staining, slightly acidophilic cytoplasm. The mitochondria are filamentous to
granular and sometimes are close to the nucleus. The Golgi apparatus is short
and situated close to the nucleus, with no evidence of vascular polarity. The
cytoplasm is rich in glicogen and contains numerous fat droplets. Electron
microscopy shows irregularly shaped granules (200-400 nm in diameter) in their
cytoplasm. They are the secretory granules containing parathyroid hormone,
which is a polypeptide in its active form.
Oxyphilic cells are larger polygonal cells; their
cytoplasm contains many acidophilic mitochondria with abundant cristae. The nucleus is smaller and
darker-staining, embedded in a deeply staining cytoplasm crowded with granular
mitochondria. There is a little or no demonstrable glycogen or fat in most
oxyphile cells. The function of the oxyphil cells is not known.
With increasing age,
secretory cells are replaced with adipocytes. Adipose cells constitute more
than 50% of the gland in older people.
The parathyroid glands
show certain changes with increasing age:
1.
Increase
in amount of connective tissue, including increased number of fat cells as well
as mast cells;
2.
The
oxyphilic cells are said to appear at four and one half to seven years, and to
increase in number especially after puberty;
3.
In
the closely packed masses of gland cells, some cords and follicles appear in
the year-old infant and increase thereafter; colloidal accumulation in the
lumen of the follicles shows the same tendency.
Histophysiology
Parathyroid
hormone binds to receptors in osteoclasts. This is a signal for these cells to produce an
osteoclast-stimulating factor, which increases the number and activity of
osteoclasts and thus promotes the absorption of the calcified bone matrix and
the release of Ca2+ into the blood. The resulting increase in the
concentration of Ca2+ in the blood suppresses the production of
parathyroid hormone. Calcitonin from the thyroid gland also influences
osteoclasts by inhibiting both their resorptive action on bone and the
liberation of Ca2+. Calcitonin thus lowers blood Ca2+
concentration and increases osteogenesis; its effect is opposite to that of
parathyroid hormone. These hormones constitute a dual mechanism to regulate
blood levels of Ca2+, an important factor in homeostasis.
In addition to increasing
the concentration of Ca2+, parathyroid hormone reduces the
concentration of phosphate in the blood. This effect is a result of the
activity of parathyroid hormone on kidney tubule cells, diminishing the absorption of phosphate and causing an increase of
phosphate excretion in urine. Parathyroid hormone indirectly increases the absorption of Ca2+ from the
gastrointestinal tract by stimulating the synthesis of vitamin D, which
is necessary for this absorption. The secretion of parathyroid cells is
regulated by blood Ca2+ levels.
In hyperparathyroidism, concentrations of blood phosphate
are decreased and concentrations of blood Ca2+ are increased. This condition frequently produces
pathologic deposits of calcium in several organs, such as the kidneys and
arteries. The bone disease caused by hyperparathyroidism, which is
characterized by an increased number of osteoclasts and multiple bone cavities,
is known as osteitis fibrosa
cystica. Bones from patients with osteitis fibrosa cystica are less resistant
and prone to fractures.
In hypoparathyroidism, concentrations of blood phosphate
are increased and concentrations of blood Ca2+ are decreased. The bones become denser and more
mineralized. This condition causes spastic contractions of, the skeletal
muscles and generalized convulsions called tetany.
These symptoms are caused by the exaggerated excitability of the nervous
system, which is due to the lack of Ca2+ in the blood. Patients with
hypoparathyroidism are treated with calcium salts and vitamin D.
The
functional significance of this gland lies in the regulation of calcium
metabolism. It produce a hormone called parathyrohormone which causes the
release of calcium from bones into the blood. I.e, hypercalcification and
demineralization of bones. The parathormone increases serum calcium by three
mechanisms:
1).
Increases bone resorption through stimulation of osteoclastic activity,
2).
Insreases calcium reabsorption from the renal tubules (inhibiting phosphate
resorption).
3),
Enchancing calcium absorption from the gut.
There are four
parathyroid glands as a rull. Each gland is covered with a connective tissue
capsule from which some septa extend into the gland substance.
Within the gland a network of
reticular fibres support specialized cells called endocrinocytus
parathyroideus. The parenchyma of the gland is made up of cells that are arranged
in cords, wlth numerous fenestrated capillaries lying in close relationship to
them. Between neighbouring cells there are well developed desmosomal contacts
and interdigitation.
There are two types of cells:
1) Endocrinocytus principalis (chief cells) - cytoplasm basophllic, densly collected ribosomes
toward the periphery, which shows a very high activity of the cell (protein
synthesis). Golgl complex highly developed with flattened vesicles and
cisternaes, giving rise to secretory granules 150-200 nm.
Mitochondria abundant and of elongated form with transversaly arranged cristae.
The chief cells are divided into light and dark cells depending upon their
state of activity and glycogen contect,
2) Endocrinocytus oxyphilicus are found in lesser number and appear only a little before puberty.
Their cytoplasm is densely packed with mitochondria. True secretory granules
are absent. The division of the cells is not based on any morphological
difference but on maturation or functional stapes. Both active and inactive
cells contain glycogen, however the number of inactive cells is more in normal
conditions then active cells. This gland is not under any regulation of
hypophysis for its secretions but works on the basic of concentration of
calcium in blood. Its activity increses under hypocalcification and decreases
with hypercalcification. Parathyrocytes carry receptors which can directly
measure the amount of calcium in the blood.
Section of a parathyroid gland showing the
chief cells of the gland arranged as cords separated by blood capillaries.
H&E stain. Medium magnification.
Parathyroid
at higher power, showing that the cells are actually tightly packed epithelial
cords. The larger, pale pink cells in the middle and to the right are oxyphil
cells; they have a smaller, darker nucleus and relatively larger amount of
cytoplasm than the majority of cells, which are called chief cells (to the left
in the photo). The chie f cells secrete parathormone; the significance of the
oxyphil cells is not clear.
Photomicrograph
of a section of a parathyroid gland. Note a group of large, acidophilic oxyphil
cells at the right. Medium magnification.
Students’ Practical Activities:
Students
must know and illustrate such histologic specimens:
Specimen 1.
Pituitary gland.
Stained with haematoxylin
and eosin.
This speciment
from a mid-line section through the brain and cranial floor illustrates the
pituitary gland in situ. The pituitary is almost completely enclosed in a bony
depression in the sphenoid bone, called the sella turcica. The three major
components of the gland, the anterior pituitary, pars intermedia and the
posterior pituitary, are easily seen at this magnification. The posterior
pituitary is connected to the hypothalamus by a short stalk, the pituitary
stalk. H & E section of hypophysis, with the darker pars distalis on the
left and the paler pars nervosa on the right. The slit between the two parts is
the remnant of Rathke's pouch. The very narrow band of darker-staining cells
along the right-hand margin of the slit is all there is of the pars intermedia.
Illustrate and indicate: 1.Anterior pituitary
(adenohypophysis). 2.Pars intermedia. 3.Posterior pituitary (neurohypophysis).
Specimen 2. Pituitary
gland.
Paraldegid
and
Cells of the anterior pituitary,
chromophils and chromophobes, form cords of secretory cells, which are
surrounded by a rich network of sinusoidal capillaries. The cromophils are
subdivided into two groups, acidophils and basophils because of their staining
properties with a variety of histological methods. For example in speciment,
acidophils are stained orange and basophils are stained blue. The somatotrophs
and mammotrophs represent the acidophils of traditional light microscopy, the
basophils being the thyrotrophs, gonadotrophs and probably the corticotrophs
(which were formerly thought to be chromophobic). The chromaphobes are the
smallest cell in the anterior pituitary and contain few cytoplasmic granules;
they have little affinity for either acidic or basic dyes and probably
represent resting forms of chromophil cells.The cells of pars intermedia are
basophilic, form irregular clumps lying between the pars anterior and pars
posterior but tending to spill out into the neural tissue of the pars
posterior.
The posterior pituitary contains the
non-myelinated axons of neurosecretory cells, the cell bodies of which are
located in the hypothalamus. Cells called pituicytes similar in structure and
function to the neuroglial cells of the central nervous system support the neurosecretory
cells axons. Most of the nuclei seen in this micrograph are those of
pituicytes. A rich network of fine, fenestrated capillaries pervades the
posterior pituitary.
Pars distalis stained with H & E. There are several
purple basophils in the center and a good clump of pink acidophils above them.
Most of the rest of the cells, rather pale ones, are the chromophobes.
Detail of the last
picture with purple basophils toward the bottom center, pink acidophils at top
center, and pale chromophobes scattered randomly. Very pale capillaries can be
seen to the immediate left and right of the central clump of cells, lying
within the delicate c.t. framework (stroma). Some secretion has collected in
the center of the p icture. Be sure to consult your handout chart from class
for hormones secreted and target organs affected.
Illustrate and indicate: 1.Adenohypophysis: a)
acidophils; b) basophils; c) sinusoidal capillaries. 2.Cells of the pars
intermedia. 3. Neurohypophysis: a) pituicytes; b) capillaries.
Specimen 3.
Pineal gland.
Stained
with haematoxylin and eosin.
At a low magnification it is seen that pineal body is cowered by the
connective tissue capsule, from which the septas appear and divide the
parenchyma onto the lobules. At a high magnification let you find irregular
shaped light cells with round shaped nuclei – pinealocytes. They occupy the
middle part of lobules. Glial cells are smaller and have dark nuclei.
Illustrate
and indicate: 1. Capsule. 2. Connective tissue septum. 3.
Lobules. 4. Pinealocytes. 5.Neuroglial cells. 6. Pineal send.
Specimen 4. Thyroid
gland.
Stained
with haematoxylin and eosin.
The thyroid gland consists of two
lobes, connected by the isthmus and is lying in the neck in front of the upper
part of the trachea. The functional units of the thyroid gland are the thyroid
follicles, irregular, spheroidal structures. The follicles are variable in size
and contain a homogeneous, colloid material which is stained pink in this
preparation. The thyroid gland is enveloped in an outer capsule of loose
connective tissue. From the capsule, connective tissue septa extend into the
gland dividing the gland into lobules, and conveying a rich blood supply,
together with lymphatics and nerves.
Thyroid follicles are lined by a simple cuboidal epithelium which is
responsible for the synthesis and secretion of the iodine-containing hormones T3
and T4. Thyroid follicles are filled with a glycoprotein complex
called thyroglobulin or thyroid colloid, which stores the thyroid hormones
prior to secretion. In actively secreting thyroid glands, as in these
spacement, the follicles tend to be small and the amount of colloid diminished;
the cuboidal lining cells are relatively tall reflecting active hormone synthesis
and secretion. Conversely, the follicles of the less active thyroid are
distended by stored colloid and the lining cells appear flattened against the
follicular basement membrane.
A second secretory cell type is found in the thyroid gland either as single
cells among the follicular cells or as small clumps in the interfollicular
spaces. These are so-called parafollicular cells. Parafollicular cells
synthesize and secrete the hormone calcitonin in direct response to raised
blood calcium levels.
Illustrate and indicate: 1.Capsule. 2.Connective tissue septa.
3.Thyroid follicles: a) simple cuboidal epithelium (thyrocytes); b) thyroid
colloid. 4.Parafollicular cells. 5.Capillaries.
Specimen 5. Parathyroid gland.
Stained with haematoxylin and eosin.
This micrograph shows a parathyroid gland characteristically embedded in
capsule of a thyroid gland. The thin capsule of the parathyroid gland gives
rise to delicate connective tissue septa, which divide the parenchyma into
dense, cord-like masses of secretory cells. The septa carry blood vessels,
lymphatics and nerves.
The parathyroid gland contains secretory cells with two types of
morphological characteristics:
1). Chief or principal cells: these are the most abundant cells and are
responsible for the secretion of parathyroid hormone. Chief cells have a
prominent nucleus and relatively little cytoplasm which varies in staining
intensity according to the degree of secretory activity of the cell. Actively
secreting cells contain much rough endoplasmic reticulum and stain strongly; in
contrast, inactive cells contain little rough endoplasmic reticulum and stain
poorly.
2). Oxyphilic cells: these
are larger and much less numerous than chief cells and tend to occur in clumps.
They have smaller, densely stained nuclei and strongly eosinophilic (oxyphilic)
cytoplasm containing fine granules. Few oxyphil cells are found in the human
parathyroid gland until puberty, after which they increase in number with age.
Oxyphilic cells do not secrete hormones except in certain pathological
conditions and their function is poorly understood.
Illustrate and indicate: 1.Connective tissue septa. 2. Parathyrocytes.
4. Blood vessels.
References:
A-Basic:
1. Practical classes materials
http://intranet.tdmu.edu.ua/data/kafedra/internal/histolog/classes_stud/English/medical/III%20term/16%20Endocrine%20%20system.htm
http://intranet.tdmu.edu.ua/ukr/kafedra/index.php?kafid=hist&lengid=eng&fakultid=m&kurs=2&discid=Histology,
cytology and embryology
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby,
2000. – P. 251-264
4. Wheter’s
Functional Histology : A Text and
Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 328-337
5. Inderbir
Singh Textbook of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers
Medical Publishers (P) LTD, 2002. – P. 299-309
6. Ross M. Histology : A Text and Atlas /
M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 740-780
B - Additional:
1. Eroschenko
V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. – Lippincott Williams
and Wilkins, 2008. – P. 383-401
2. Junqueira L. Basic Histology / L. Junqueira, J. Carneiro, R.
Kelley. – [seventh edition]. –
3. Charts:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#inf3
4. Disk:
http://intranet.tdmu.edu.ua/data/teacher/video/hist/
5. Volkov
K. S. Ultrastructure of the main components of body / K. S. Volkov. – Ternopil : Ukrmedknyha, 1999. – P. 24-36
http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
http://en.wikipedia.org/wiki/Circulatory
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.udel.edu/biology/Wags/histopage/histopage.htm