Endocrine organs are organs whose cells secrete
their products, i.e., hormones, into the bloodstream whereas exocrine organs
such as sweat glands, salivary glands and sebaceous glands secrete their
products into a duct system. These hormones travel via the blood
circulation and when they reach their "target organ" they exert their
specific effect. Some organs are primarily exocrine while some
are primarily endocrine and some contain elements of both. The ovary and
testes are both exocrine organs, "secreting" ova and spermatozoa,
respectively yet they are endocrine organs as well secreting hormones such as
estrogen, progesterone and testosterone. In the pancreas, part of the
organ is an exocrine gland (the acini) secreting and part is endocrine ,
i.e., the islets of Langerhans which secrete various hormones such as
insulin and glucagon. In the digestive system, some cells are endocrine cells, i.e., the enteroendocrine cells.
Other endocrine organs include the thyroid, parathyroid, adrenal, and
pituitary gland.
THYROID
I. Gross
Anatomy
The thyroid gland is located dorsolateral to the trachea,
close to the larynx. It has two lobes that are connected by a narrow isthmus.
II. Histology
| The
thyroid gland is composed of follicles and interfollicular
connective tissue. The capsule, classified as loose
areolar connective tissue, surrounds the mass of thyroid follicles and
sends smaller pieces of connective tissue into the gland to surround the
individual thyroid follicles.
Near the thyroid gland and embedded in
the same connective tissue capsule is the parathyroid gland.
Sometimes patches of lymphocytes can be
observed in the thyroid/parathyroid glands.
|
Micrographs of thyroid gland (Lab slide 95). |
| Thyroid
follicles
consist of a layer of simple epithelium surrounding a
gel-like pinkish material called colloid.
The principal cell is the
most numerous cell present in the simple epithelial layer and is
responsible for secreting the thyroid hormones as
well as thyroglobulin, a glycoprotein.
Thyroid
hormones are stored extracellularly as part of the thyroglobulin which is the main component of the
colloid.
The size of follicles and the height of principal
cells varies even within one section of the gland. Squamous
principal cells indicate a relatively inactive gland whereas cuboidal to
columnar cells indicate more activity in removing the hormone from the
stored form.
In addition to principal cells there is
another type of functional cell in the thyroid gland. This is the
parafollicular cell which may be found as single
cells in the epithelial lining of the follicle or in groups
in the connective tissue between follicles. They usually appear as large, clear cells since they do not stain well with hematoxylin and
eosin. They are sometimes called parafollicular cells
based on their location and clear cells (C cells) based on their appearance
of their cytoplasm.
Parafollicular cells secrete calcitonin,
a hormone that lowers the level of calcium in the blood.
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III. Function
- secretes the thyroid hormones tri-iodothyronine
(T3
) and tetra-iodothyronine (T4 or thyroxin) and that
help to regulate the metabolic rate
- also secretes calcitonin that helps
control blood calcium concentration
IV. Mechanism of Secretion of T3 and T4
(thyroxin).
- Under the influence of increased TSH from the
pituitary gland, principal cells concentrate iodine by active
transport. At the same time they synthesize thyroglobulin and
secrete it into the lumen of the thyroid follicle
- The iodination reaction, catalyzed by the enzyme
peroxidase, is
carried out on the large thyroglobulin molecule at the luminal surface of
the principal cell. Various combinations of iodinated and
non-iodinated tyrosine are possible. If the two molecules of tyrosine
are both fully iodinated, the hormone resulting upon cleavage is T4
but if one of the tyrosines has only one iodine, then the hormone that
results is T3. In the circulation T4
is converted to T3 which appears to be the active form of the
hormone.
- Under the influence of rising TSH levels, the
principal cells take up colloid by pinocytosis, the vesicles fuse with
lysosomes which hydrolyze throglobulin releasing T3 and T4
(thyroxin) which diffuse into the blood and lymph
V. Parafollicular Cells
- Secrete calcitonin which inhibits
osteoclasts from resorbing bone resulting in decrease in calcium in the
blood
- Controlled by the level of calcium in the
blood
PARATHYROID GLAND
I. Gross
Anatomy The parathyroid gland is
difficult to see at the gross level. It is very close to and usually
embedded within the capsule of the thryroid gland.
II. Histology
|
There are three types of cells in the
parathyroid gland: adipocytes, chief cells and oxyphil
cells.
A reticular connective tissue framwork surrounds and
supports these cells.
|
Micrograph of parathyroid gland (Lab slide 95).
|
| The
main secretory cell is the chief cell. These cells secrete
parathyroid hormone. Unfortunately these cells have no
distinguishing features.
Another cell type present is the
oxyphilic cell which occur in the human, ox and horse. These are large
cells that contain numerous mitochondria. Their function is unknown.
|
III. Function
The parathryoid gland secretes parathyroid hormone which is essential
for regulating the levels of calcium and phosphate in the blood.
Parathyroid hormone acts
on
the following target organs.
- Bone: increases blood calcium
by inhibiting osteoblast deposition of calcium and stimulating osteoclast removal of calcium.
- Kidney: increases blood
calcium by increasing calcium ion reabsorption by kidney tubular cells;
inhibits reabsorption of phosphate ion from the glomerular filtrat
- Small intestine: increases the
absorption of calcium from the small intestine
IV. Summary of hormonal control of
blood calcium levels through action on bone.
|
Calcitonin As the level of
calcium in the blood rises, the amount of calcitonin secreted by the C
cells of the thyroid increases. Calcitonin stimulates osteoblasts
to form bone taking calcium out of the circulation. At the same
time, calcitonin inhibits the mobilization of bone (and calcium) by
osteoclasts. The end result is a decrease in the level of calcium
in the blood thus helping to maintain proper blood calcium levels. |
 |
| Parathyroid Hormone A decrease in the normal
levels of calcium in the blood causes the chief cells of the parathyroid
gland to secrete more parathyroid hormone which stimulates osteoclasts
to mobilize bone resulting in an increase in the level of calcium in the
blood. Parathyroid hormone also increases Ca ion reabsorption in
the kidney and decreases the reabsorption of phosphate ions. |
Note: Whereas calcitonin is important in
regulating the level of caclium in the blood, parathyroid hormone is essential!!
ADRENAL GLAND
I. Gross Anatomy The adrenal glands are
located at the cranial end of the kidneys. They are flat organs embedded in fat.
Each gland has an outer cortex that appears yellow in fresh tissue and an inner
medulla that appears gray in fresh tissue.
II.
Histology of the Adrenal Gland and Adrenal Cortex
The adrenal gland is surrounded
on the surface by a connective tissue capsule.
This capsule has projections into
the cortex and through the cortex down into the medulla in some species. |
Micrograph of dog adrenal gland (Lab slide 97).
|
In
most
species 4 cortical zones can be identified. From the zone nearest the
capsule these are:
- In ruminants this zone consists of cells in clusters.
- In carnivores, horses and pigs this zone consists of
columnar cells in arches and is sometimes called a zona arcuata.
- Zona intermedia: This zone is relatively thin and contains mostly undifferentiated cells.
- Zona fasciculata
This is the largest zone containing large, round,
foamy-appearing cells arranged in cords that radiate from the zona
glomerulosa down toward the medulla.
- Sinusoidal capillaries are located
between cords of cells that are one cell thick.
- Cells in the zona fasiculata have a
foamy appearance due to the presence of many lipid droplets prior to
processing for microscopy. These lipid droplets represent the precursors for steroid
hormones.
- Cells of the zona reticularis are arranged in a network of
cords no longer arranged in parallel as in the zona fasiculata.
|
III. Function of the Adrenal Cortex
- Zona glomerulosa cells secrete mineralocorticoids
(principal one is aldosterone). These steroid hormones act on the
kidney to:
- Increase Na+ reabsorption
- Increase K+ secretion
- Zona fasciculata and reticularis cells secrete
glucocorticoids (e.g., cortisol, cortisone, corticosterone) which control glucose
metabolism.
IV. Histology of the Adrenal Medulla
- Cells in the medulla are arranged in groups or cords,
clustered around capillaries and venules.
- The cells have secretory granules which contain
either epinephrine or norepinephrine.
- When fixed in potassium bichromate, the medullary cells
become brown. Therefore, they are called chromaffin cells. The color is the result
of a reaction between chromate and epinephrine or norepinephrine. With the
typical H&E stain, the cells appear as shown in the adjacent figure as
somewhat stellate-shaped cells containing a rather prominent, round nucleus.
- Chromaffin cells are derived from neural crest cells. They
are innervated by preganglionic sympathetic fibers. They release hormone by exocytosis
when stimulated by those fibers.
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V. Function of the Adrenal Medulla
- Secretes catecholamines (epinephrine and
norepineprine).
VI. Vasculature of the adrenal gland.
Blood is supplied to the adrenal gland via the suprarenal
arteries. These arteries penetrate the capsule and form a plexus
just beneath it. From this plexus, blood is further supplied via
two different routes.
- Cortical arteries of the subcapsular plexus branch into sinusoids
in the cortex which intimately surround cells in the zona
fasciculata and zona reticularis. These sinusoids drain into
venules that empty into the central vein of the medulla. This
blood supply provides a mechanism by which cells in the cortex can
influence cells in the medulla (see below).
- The second route of blood supply is more direct to the
medulla. In this case, cortical arteries run along trabecular
branches of the connective tissue capsule directly into the medulla
without forming capillaries or sinusoids in the cortex. These
arteries then branch into capillaries in the medulla supplying the
chromaffin cells with oxygenated blood. These capillaries join
the same small medullary venules as the sinusoids of the cortex and
all the blood flows into the central vein of the medulla.
Because of this peculiar blood supply to the adrenal gland, the
cortex has no veins.
|
 |
PITUITARY
GLAND (HYPOPHYSIS)
I. Gross Anatomy and Development
| The pituitary gland or
hypophysis is located at the base of the brain. Its function
reflects its development from two different types of ectoderm, i.e.,
oral ectoderm which forms Rathke's pouch and neural ectoderm from the
base of the diencephalon which forms the infundibulum. As
developments proceeds, the ectoderm of the infundibulum grows downward
and wraps around Rathke's pouch. In the adult the infundibulum
forms the infundibular stalk and the pars nervosa; the ectoderm of
Rathke's pouch forms the pars distalis and the pars intermedia.
Some of the oral ectoderm remains associated with the infundibular stalk
to form in the adult the pars tuberalis. A vestige of
Rathke's pouch often remains visible in the adults of many species and
forms a cleft that serves to distinguish the anterior lobe of the
pituitary from the posterior lobe. The terms anterior lobe and
posterior lobe are useful in humans but not in domestic species; in most
domestic species such as the cow, ventral lobe and dorsal lobe are more
appropriate terms, respectively. |
|
| In the adult, the pituitary
gland remains
"connected" to the brain via both vascular and nervous routes
(see below under Function). These relationships with the
hypothalamus of the brain provide a basis for understanding the
different functions of the pituitary gland and form the basis of the
integration of the nervous system with the endocrine system.
Vascular Connection: The
vascular connection provides the mechanism by which factors secreted by
neuroendocrine cells in the hypothalamus reach and affect secretory
cells in the pars distalis. These secretory cells respond to the
"releasing hormones" brought to them through the portal blood
vessels.
Neural Connection: The
neural connection provides the mechanism by which hormones secreted by
neuroendocrine cells located in specific nuclei in the brain are
actually released into the bloodstream at the level of the pars nervosa
of the pituitary gland.
|
|
II. Histology
|
The pituitary gland can be divided into various regions
based on structure and function.
One way to describe the pituitary gland is by the type of tissue present
in different regions. If this system is used then regions are as
follows:
Adenohypophysis -
based on grouping of all regions composed of glandular
tissue
This includes the...
-
pars distalis
-
pars intermedia
-
pars tuberalis
Neurohypophysis - based on
grouping of all regions composed of neural or neurosecretory
tissue
This includes the . . .
- median eminence (not shown)
- infundibular stalk
- pars nervosa ( infundibular
process)
Another way to describe the pituitary gland is using the
terms anterior lobe and posterior lobe (in domestic species, ventral
lobe and dorsal lobe) as follows . . .
Anterior or Ventral
Lobe
This includes the . . .
-
pars distalis
-
pars intermedia
Posterior or Dorsal
Lobe
This includes the . . .
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 |
|
The Adenohypophysis
Pars distalis: This region of the pituitary gland is organized
as cords or clusters of cells supported by a reticular connective
tissue. With routine staining two types of cells can be observed:
(1) chromophiles which stain readily and are either red (acidophiles),
blue or purple (basophiles) depending on the type of secretory material
present, and (2) chromophobes which do not take up the stain and
thus appear unstained or rather clear. Chromophobes may be
chromophiles that have lost their secretory granules or chromophiles that
have not accumulated large numbers of secretory granules. Use of specific
antibodies against the protein secretory products has allowed the
identification of the different cells. The cells of the par distalis are:
-
Somatotrophs secrete growth hormone which affects many cells
-
Mammotrophs secrete prolactin which controls milk
production during lactation
-
Corticotrophs secrete ACTH which controls secretion of
cortisol by cells in the adrenal cortex
-
Thyrotrophs secrete TSH which controls secretion of thyroid
gland
-
Gonadotrophs secrete FSH and LH which control development
of follicles and ovulation in the ovary.
Pars intermedia: With routine histological
staining, the cells in the pars intermedia stain blue-purple and thus are
basophilic.
Pars tuberalis: This region is an extension of the
glandular pituitary gland and its cells resemble those of the pars
intermedia and pars distalis. The specific function of the cells in
the pars tuberalis, however, is not clear.
The Neurohypophysis
Pars nervosa: This region consists of
unmyelinated nerve axons (cell bodies are in the hypothalamus) and
supportive cells called pituicytes.
-
Secretes ADH (antidiuretic hormone) which is
synthesized by neurons in the supraoptic nucleus of the hypothalamus.
-
Secretes vasopressin which is synthesized by neurons
in the paraventricular nucleus of the hypothalamus.
|
Micrograph of ox pituitary (Lab slide
93).
|
III. Function - Control of Secretion
Adenohypophysis.
The cells in the adenohypophysis secrete
two classes of hormones: (1) direct acting and (2) trophic. Direct
acting hormones include growth hormone (GH) and prolactin from the pars
distalis, and melanocyte stimulating hormone (MSH) from the pars
intermedia. Trophic hormones include adrenocorticotrophic hormone
(ACTH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH)
and luteinizing hormone (LH).
Secretion of these hormones is
controlled by specific releasing hormones in the hypothalamus. Most
of the releasing hormones are stimulatory in their action except for the
one for prolactin which is inhibitory and the one for growth hormone which
has both inhibitory and stimulatory releasing hormones. Releasing
hormones are produced in the median eminence of the hypothalamus and reach
the adenohypophysis via a portal system of veins known as the pituitary
portal system.
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|
|
Neurohypophysis.
The cells in the neurohypophysis
secrete only direct acting hormones : (1) antidiuretic hormone (ADH)
also known as vasopressin secreted by neurons in the supraoptic nucleus
in the hypothalamus and (2) oxytocin secreted by neurons in the
paraventricular nucleus in the hypothalamus. After synthesis in
the hypothalamus, these hormones move down the axons of the
hypothalamohypophyseal tract through the infundibular stalk and
terminate near blood vessels in the pars nervosa. Accumulations of
these hormones bound to specific glycoproteins can be observed along the
axons of the hypothalamophypophyseal tract and in the pars
nervosa. These "accumulations" often called Herring
bodies represent a storage form of the hormone. Release of these
hormone stores is determined by impulses in the axons of the
hypothalamophypophyseal tract originating in the hypothalamus.
Such a mechanism of secretion controlled by nerve impulses is called
"neurosecretion".
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Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index
