ENDOCRINE ORGANS.
I. General background on endocrine organs.
A. Endocrine glands are a group of organs that
synthesize and release hormones into
capillaries. These hormones affect the function
of other target organs or tissues in the body.
B. Often there are interactions between the
hormone(s) and the nervous system.
1. So we can say that the nervous system and
the endocrine system are each affected by and
modulate the activities of the other.
2. Thus, the nervous and endocrine elements of
control are often lumped together under the
heading neuroendocrine system.
II. Pituitary gland = Hypophysis - an endocrine organ with obvious
neural and non-neural endocrine components
A. Embryological development
1. Neural hypophyseal
tissue (derived from a
portion of the forming
brain called the
infundibulum of the
diencephalon)
2. Non- neural
hypophyseal tissue
(derived from the
external epithelial lining
of a structure called
Rathke’s pocket in birds
and mammals)
3. Together these tissues form the
hypophysis = pituitary gland
a. Diencephalic infundibulum forms
neurohypophysis
* pars nervosa = neurohypophysis =
posterior lobe
* Continuous with hypothalamus of brain
at the median eminence.
median eminence
b. The epithelial lining of Rathke's
pocket forms the adenohypophysis
= non-neural hypophysis = anterior
lobe
* This is composed of 3 major parts.
** pars distalis (anterior lobe) - largest
component of adenohypophysis
median eminence
** pars intermedia - lies between
neurohypophysis and pars distalis.
** pars tuberalis - adenohypophyseal
tissues that surround the stalk of
the pars nervosa.
B. Cardiovascular circulation to the pituitary gland
1.
Since these tissues secrete hormonal
substances, we would expect a well
developed vascular system that would
provide a means of circulating these
hormones to other parts of the body.
2.
Five arteries provide blood to the pituitary
gland. these are,
a. 2 superior hypophyseal arteries
b. 2 inferior hypophyseal arteries.
c. Trabecular artery
3. These arteries enter the pars nervosa, the pars
tuberalis, and the median eminence region
of the hypothalamus where they form a
capillary network.
4. The capillaries merge to form venules and veins
B. Cardiovascular circulation to the pituitary gland
5. In the case of the pars nervosa, the blood exits
these tissues via neurohypophyseal veins.
6. Venules that form from capillaries in the
median eminence extend through the pars
tuberalis, enter the pars distalis, and form a
second capillary network (a portal system)
within its tissue = hypophyseal portal system.
7. These capillaries coalesce to form venules and
veins that then leave the pars distalis via
adenohypophyseal veins
8. Why might this be? The hypophyseal portal
system carries hormones from the median
eminence of the basal hypothalamus to the
pars distalis where they have an effect on the
secretion of hormones by cells in that part of
the pituitary gland.
Median eminence
III. Adenohypophysis (pars distalis) Tissue structure and cell types.
A. Has typical appearance of endocrine
tissue, that is groups of cells
organized into cords or follicles
B. Through this tissue run fenestrated
capillaries that are part of
hypophyseal portal system.
http://www.vh.org/Providers/Textbooks/
MicroscopicAnatomy/Section15/Plate15279.html
C. Two major classes of cells are present in the pars
distalis. These are the chromophobes and
chromophils. Classification is based on staining
characteristics.
1. Chromophobes
a. cytoplasm doesn’t pick up stain
b. cytoplasm appears white or clear in stained
sectioned material
c. This group of cells includes the
* Cells that form a supporting meshwork for the
tissues of the adenohypophysis.
* Secretory cells that are thought to secrete hormones,
but that are not well understood at present.
** One type of chromophobe in rats secretes
adrenocorticotropic hormone that stimulates the
adrenal gland. (However, in humans this hormone is
produced by a basophilic chromophil - see below.)
http://www.vh.org/Providers/Textbooks/
MicroscopicAnatomy/Section15/Plate15279.html
http://www.lab.anhb.uwa.edu.au/mb140/
2. Chromophils - these cells pick up basic or acidic stains basophils and acidophils
a. Basophils
* Gonadotropic cells
** Secrete hormones that affect reproductive organs (follicle
stimulating hormone, FSH; leuteinizing hormone, LH;
interstitial cell stimulating hormone, ICSH).
*** FSH - glycoprotein that stimulates and supports early
growth of follicles in ovary and spermatogenesis in testis.
*** LH - glycoprotein that reaches peak during menstrual
cycle. 24 hr after peak, ovulation occurs.
*** ICSH = LH in males - stimulates interstitial cells of
Leydig in testes to secrete testosterone
http://www.lab.anhb.uwa.edu.au/mb140/
2. Chromophils
a. Basophils
* Gonadotropic cells (cont.)
** Small round cells with dense, basophilic, secretory
granuals
** Distributed throughout pars distalis.
** FSH and LH secreting cells test positive with the
periodic acid schiff (PAS) stain because the hormones
are glycoproteins.
* Thyrotropic cells
** Secrete thyroid stimulating hormone =thyrotropin
(TSH)- stimulates synthesis of thyroid hormones.
** Positive PAS because hormone is a glycoprotein.
http://www.lab.anhb.uwa.edu.au/mb140/
b. Acidophils
* Mammotropic cells - secrete prolactin
** Peptide hormone that triggers secretion of milk by
mammary glands.
* Somatotropic cells
** Secrete Growth hormone = somatotropin (STH)- protein
hormone.
*** Most marked affect is on epiphyseal cartilage of bone,
hormone acts on liver to cause production of a peptide
called somatomedin. This peptide stimulates growth of
epiphyseal plate. Lack of this hormone results in
hypopituitary dwarfism which can be treated in some
cases with hormone injections.
Overproduction of STH can cause gigantism and may
eventually result in acromegaly.
http://www.lab.anhb.uwa.edu.au/mb140/
D. As discussed earlier, secretion of hormones by cells of the pars distalis is
caused by the action of hormonal factors synthesized in the hypothalamus and
secreted in the median eminence. These are called releasing factors, e.g.
growth hormone releasing factor (GHRF), gonadotrophic releasing hormone
(GnRH).
E. In many cases the hypothalamus, hypophysis, and affected organ work in
concert with positive or negative feedback occurring between all three.
F. READ ABOUT OTHER PARTS OF THE ADENOHYPOPHYSIS (pars
intermedia, pars tuberalis) IN TEXT.
IV. Neurohypophysis -Tissue structure and cell types of the
pars nervosa
A. This is the posterior lobe of the pituitary gland that is
formed from the tissues of the diencephalic
infundibulum. Recall that these tissues remain attached
to the part of brain that is the hypothalamus.
B. The body and stem of the pars nervosa are mainly
composed of unmyelinated axonal processes and
associated glial cells.
1.
The axons arise from neuron perikarya that are located
in the supraoptic and paraventricular nuclei of the
hypothalamus.
•
Some axons from these neurons end blindly (no
synapse) in the median eminence of the hypothalamus
•
Other axons extend into the lower regions of the
posterior lobe of the pituitary where they also end
blindly.
Median eminence
2. These hypothalamic cells are neurosecretory neurons that synthesize
hormones such as oxytocin, vasopressin (lower posterior lobe), and the
various releasing factors (median eminence) mentioned above. These
substances accumulate in the blind endings of these axons that are
located in the pars nervosa or median eminence).
In the case of vasopressin and oxytocin.
a. Major effect of vasopressin is to increase permiability to water of the
tubules of the kidney. Causes a higher rate of reabsorption of water by
these tubules and thus concentrates the urine.
b. Oxytocin promotes contraction of the smooth muscle of the uterine
wall during copulation and parturition. Also, contraction of myoepithelial
cells that surround ducts of the mammary glands.
* In the case of child birth and intercourse, distension of the vagina
excites stretch receptors in the vaginal wall. These cause action
potentials to be sent to the CNS where the appropriate neurosecretory
cells of the hypothalamus receive nervous stimulation and release
oxytocin in the pars nervosa. This hormone enters the circulatory
system and is carried to the muscles (myometrium) of the uterine wall
where it causes these muscles to contract.
Median eminence
3. So the the hypothalamus is
actually a component of the
posterior pituitary's endocrine
function since the actual hormones
are synthsized there and then
released into or stored in the
median eminence and pars
nervosa respectively.
C. Accumulations of oxytocin or
vasopressin in the blind endings of axons
in the pars nervosa form the Herring
corpuscles (bodies) seen in the lower
posterior lobe with the light microscope.
http://www.lab.anhb.uwa.edu.au/mb140/
D. To sum it up - When appropriate neural
stimulation of the neurosecretory cells occurs, the
blind endings of the axons release their secretory
products which enter capillaries in the pars
nervosa (oxytocin, vasopressin) or median
eminence region (releasing hormones). The
hormones are then carried to their point of action
in other parts of the body (oxytocin, vasopressin)
or to the adenohypophysis (releasing hormones).
E. In addition to axons, you will also find the nuclei
of pituicytes in the tissues of the pars nervosa.
1. These cells are of irregular shape and often
have numerous cytoplasmic processes.
2. They are considered a type of glial cell, but their
function is not well understood.
http://www.lab.anhb.uwa.edu.au/mb140/
V. Adrenal glands -Paired organs that
usually lie on the superior poles of the
kidneys, embedded in adipose tissue.
A.
General structure
1. Covered by capsule of dense collagenous
connective tissue.
2. Thin septa or trabeculae extend from the
connective tissue capsule into the
interior of the gland.
3. Internally there are two major regions
called the adrenal cortex and adrenal
medulla.
4. The supporting framework of cortex and
the medulla is collectively called the
stroma. This framework is mainly
composed of reticular fibers.
http://www.ucs.mun.ca/~fking/adrenal/slide3.html
http://education.vetmed.vt.edu/Curriculum/
VM8054/Labs/Lab24/Examples/exadrenl.htm
B. Embryological development
1. In a sense, the adrenal cortex and medulla
may be considered as 2 morphologically distinct
endocrine organs.
2. Similar to what we saw in the hypophysis, the
cortical and medullary portions of the adrenal
glands have different embryonic origins. That is
to say, they are derived from different basic
tissue types in the embryo.
3. Again, we have a situation where neural and
non-neural tissues become associated to form
an organ (neural ectoderm and mesoderm).
a. As an embryo develops, the cortex of the
adrenal gland is derived from mesodermal cells
in the region of the kidney.
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/
Lab24/Examples/exadrenl.htm
b. The cells of the adrenal medulla are derived
from a specialized group of neural ectoderm
cells that are called the neural crest.
* The neural crest cells separate from the
developing CNS and migrate through the
body's tissues giving rise to a number of
different cell types including many of the
peripheral ganglion neurons.
* Some of the neural crest cells migrate to the
developing adrenal cortex, penetrate this
mesodermal tissue and lodge themselves
centrally within it to form the adrenal medulla.
* So, the secretory cells of the adrenal medulla
can be thought of as postganglionic neurons
that have lost their axons and dendrites and
have become secretory cells.
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/
Lab24/Examples/exadrenl.htm
C. Caridovascular circulation of the adrenyl gland.
1. Blood is supplied to the adrenal glands by a number of
arteries.
2. These vessels enter through the capsule tissue and
then branch out into a sub-capsular plexus of arterioles
that give rise to capillaries that extend throughout the
cortex.
3. The capillaries supply blood to a network of sinusoids in
the cortex.
4. Some of the arterial branches do not form capillaries in
the cortex, but rather run through it to the medulla.
a. These are the medullary arteries.
b. They form a dense capillary network around the cells of
the medulla.
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/
Lab24/Examples/exadrenl.htm
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/adre
nal/histo_overview.html
5. If we look at the endothelium of the capillaries,
we find it is fenestrated with the fenestrations being
occluded by a thin membrane (diaphragm). In the
cortex, the basal lamina of the endothelium is not
continuous - set-up for the exchange of materials
between the blood and surrounding cells or visa
versa.
6. Capillaries of medulla and cortex coalesce to
form the adrenal veins that exit the adrenal glands.
http://education.vetmed.vt.edu/Curriculum/
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/
VM8054/Labs/Lab24/Examples/exadrenl.htm
endocrine/adrenal/histo_overview.html
D. Adrenal cortex
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/
Lab24/Examples/exadrenl.htm
1. Composed of 3 layers
a. zona glomerulosa
b. zona fasciculata
c. zona reticularis
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/adrenal/histo_overview.html
Adrenal
http://www.lab.anhb.uwa.edu.au/mb140/
Corticosteroid
Folded
cords
mineralocorticoids
cortisol
Straight
cords
cortisol, androgens,
progesterone, estrogen
irregular
cords
______
2a. Zona glomerulosa - structure
a. Columnar to pyramidal cells that are
arranged in folded cords surrounded by
capillaries/sinusoids.
b. Cells have spherical nuclei and
basophilic granules in cytoplasm.
c. Where cells are adjacent to the
endothelium of the capillaries and
sinusoids, there is a space between the
cell plasmalemma and the endothelium.
d. The plasmalemma of these cells is
thrown into folds (microvilli) that extend
into the space.
http://www.medinfo.ufl.edu/year1/histo/images/f24b.jpg
2b. Zona glomerulosa - function
a. The zona glomerulosa cells secrete
mineralcorticoids, mainly aldosterone
which is important in maintaining water
balance.
b. These and other corticoid hormones
are steroid hormones that are lipid
molecules derived from cholesterol.
http://www.finchcms.edu/anatomy/histology/organology/endocrine/images/ff954.jpg
http://www.leeds.ac.uk/bms/teaching/histology/pcd1855/img0014.jpg
c. Ultrastructure is typical for cells involved in lipid synthesis (see 3. below).
* Lots of smooth endoplasmic reticulum with a few short segments of RER.
* Well developed Golgi body.
* Mitochondria are spherical or oval and have tubular cristae.
* Lipid droplets may be present in cytoplasm associated with smooth endoplasmic reticulum.
3a. Zona fasciculata - structure
a. Cells are polyhedral and are arranged in straight cords
(columns), 1-2 cells thick with capillaries running between.
b. Cells have central spherical nucleus with basophilic
cytoplasm. Microvilli in plasmalemma are present within the subendothelial space (next to the capillaries).
c. Many lipid droplets present in cytoplasm. These are extracted
during most embedding procedures, so the cells often appear
highly vaculated.
b. Ultrastructure is typical for cells involved in lipid synthesis and
secretion (as discussed earlier)
* However, these cells have more RER than those of the zona
glomerulosa
* This, in addition to the presence of many lipid droplets, is why
the cytoplasm has a weak overall basophilic affinity - but usually
does not stain heavily.
http://www.finchcms.edu/anatomy/histology/organology/endocrine/images/ff958.jpg
http://www.med.uiuc.edu/histo/small/atlas/image/tem8/21700a1.htm
3b. Zona fasciculata - function
a.
Zona fasciculata cells secrete glucocorticoids,
mainly cortisol (important in regulation of lipid,
protein and carbohydrate metabolism). Another
type of steroid. Also sex hormones (e.g.
androgens, estrogen, progesterone)
b.
Also may secrete a weak androgen dehydroepiandrosterone (DHEA).
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab24/Examples/exadcort.htm
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab24/Examples/exadcort.htm
4a. Zona reticularis - structure
a. Cells are polyhedral with spherical
nucleus and are arranged in irregular cords
with capillaries and sinusoids between.
b. Lipofuscin pigment granules in cells.
c. Cytoplasm acidophilic.
http://www.leeds.ac.uk/bms/teaching/histology/pcd1855/img0013.jpg
5. Secretion of steroids by adrenal cortex is good example of a feedback system
between target organ and pituitary gland.
a. A psychological stimulus such as stress, exercise, causes neurosecretory
neurons in the hypothalamus to secrete adrenocorticotropic hormone
releasing factor into the capillaries in the median eminence.
b. The releasing factor is carried by the hypophyseal portal system from the
median eminence to the pars distalis.
c. This causes corticotrophic cells in the pars distalis to secrete
adrenocorticotropic hormone (ACTH - also called adrenocorticotropin) into
surrounding capillaries.
d. The ACTH is carried to the adrenal glands where it causes an increase in
cortisol secretion by adrenal cortex cells of the zona fasciculata and zona
reticularis.
e. As levels of cortisol increase in blood, these inhibit both the secretion of
releasing factors by the hypothalamus and secretion of ACTH by pituitary.
http://www.finchcms.edu/anatomy/histology/organology/end
ocrine/o_e_4.html
E. Adrenal medulla
1. Composed of irregular cords of
polyhedral, epithelioid, secretory
cells that form a compact irregular
network surrounded by capillaries,
venules, and a few sympathetic
ganglion cells/neurons (blue
arrows).
2. The epithelioid secretory cells
are considered to be modified
postganglionic sympathetic
neurons.
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab
24/Examples/exadrmed.htm
http://www.medinfo.ufl.edu/year1/histo/images/g4b.jpg
3. Axons from preganglionic sympathetic
neurons synapse on the surface of the
secretory cells.
a. When stimulated , these axons release
acetylcholine (cholinergic synapses) that
causes the exocytosis of secretory granules at
the apical end of the secretory cells.
b. The secretory product consists of the
catecholamines epinephrine (adrenaline),
norepinephrine (noradrenaline) or dopamine.
c. Three different types of cells, one for each
catecholamine.
http://webpages.ull.es/users/isccb12/ChromaffinCell/Primer.html
d. Secretory granules stain a brown color with
chromium salts - called chromaffin reaction. As
a result, these cells are called chromaffin cells.
http://www.finchcms.edu/anatomy/histology/organology/endocrine/o_e_4.html
4. Adrenal medulla secretes continuously into blood stream.
a. The adrenal medulla cells secrete only small amounts of epinephrine and
norepinephrine unless stimulated by nervous activity related to emotional
reactions.
b. Secretion of these substances prepares body to react to stressful
situations. Blood vessels constrict, blood pressure rises, etc.
VI. Thyroid
A. Located below the larynx, partially encircling the esophagus.
http://www.endocrineweb.com/thyroid.html
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrin
e/thyroid/anatomy.html
B. Thyroid is covered by layer of loose
connective tissue.
interfollicular
1. Surrounded by a connective tissue
capsule that extends septa into the interior
separating lobes (lobules) of follicles
(interlobular C.T.)
a. This C.T. also extends between the
follicles separating them from each other
(interfollicular C.T.).
b. Septa that separate follicles are
composed mainly of reticular fibers.
2. Arterioles and venules may be found
within the C.T. These are connected by
fenestrated capillaries amongst the
follicles.
http://www.lab.anhb.uwa.edu.au/mb140/
http://www.finchcms.edu/anatomy/histology/organology/endocrine/images/ff934.jpg
C. The thyroid tissue of the lobes is
composed of follicles, each containing
a lumen filled with a gelatinous
substance - colloid.
1. Each follicle consists of a simple
cuboidal epithelium surrounding the
lumen
a. The epithelium changes to simple
squamous if the follicle is inactive.
http://www.vh.org/Providers/Textbooks/MicroscopicAnatomy/
Section15/Plate15287.html
2. Cells of follicles are responsible for synthesis of thyroid
hormones, the most abundant of which is thyroxin.
a. Colloid consists of mainly iodinated thyroglobulin.
b. When hormones required, iodinated thyroglobulin in colloid is
endocytosed.
c. Endocytotic vesicles merge with lysosomes.
d. Lysosomal enzymes break iodinated residues off and process
thyroglobulin => thyroid hormones
e. This liberates the thyroid hormones into the cytoplasm.
f. These diffuse across the cell membrane into capillaries where
blood carries them to target organs.
http://arbl.cvmbs.colostate.edu/hbooks/
pathphys/endocrine/thyroid/anatomy.html
g. Thyroxine stimulates mitochondrial respiration and oxadative
phosphorylation. So, more ATP produced faster.
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/synthesis.html
3. In addition to follicle cells, parafollicular cells (C cells, clear cells) are found
between the follicles in humans and other mammals.
a. These cells secrete the polypeptide calcitonin that causes a reduction of calcium
in the blood, in part by inhibiting the activities of osteoclasts.
http://www.vh.org/Providers/Textbooks/MicroscopicAnatomy/S
ection15/Plate15287.html
4. Control of thyroid hormone secretion:
a. Stimuli provided by axons from parasympathetic and sympathetic ganglia can
influence metabolism of thyroid cells.
b. However, thyrotropin (thyroid stimulating hormone, TSH) is the major controlling
factor.
c. Thyrotropin is synthesized and secreted by cells of the adenohypophysis (pars
distalis).
d. Thyroid has feedback system with pituitary similar to adrenal cortex.
* Releasing factors from the neurohypophysis cause the adenohypophysis to secrete
thyrotropin.
* This causes thyroid hormone production by the follicle cells of the thyroid.
* As thyroxin level rises in the blood, it causes an inhibition of the secretion of releasing
factors by the neurohypophysis and, thus, less thyroxin is secreted.
Hyperthyroidism
Excess of thyroid hormone resulting from an overactive thyroid gland (or taking too
much thyroid hormone). Symptoms can include increased heart rate, weight loss,
depression, and cognitive slowing. Histologically, the follicles are small and there is
considerable scalloping around the edges of the colloid due to overproduction of thyroid
hormones. Lymphatic nodules may also be present in the thyroid tissue.
http://mspis.medsci.indiana.edu/c602web/602/C602web/endo/slide12.htm
VII. Parathyroid glands
http://arbl.cvmbs.colostate.edu/hbooks/
http://www.endocrineweb.com/thyroid.html
pathphys/endocrine/thyroid/anatomy.html
A. These are small, but important, organs that are embedded in the wall of the thyroid.
1. History - complete removal of the thyroid gland was noted to cause death because of
spasms of the laryngeal and thoracic muscles that prevented breathing - called tetany
2. In 1892, the French physiologist Gley showed that it was actually the removal of the
parathyroid glands that "rode" along with the thyroid that caused of these titanic seizures. (due
to the lack of parathormone)
3. Thus, when the thyroid is removed, it is critical that the parathyroid glands be separated from
it and left in the body.
B. There are three cell types in the parathryroid glands
1. Adipose cells - increase in number as one grows older.
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrin
e/thyroid/anatomy.html
http://www.lab.anhb.uwa.edu.au/mb140/
2. Chief cells
a. polygonal with vesicular nucleus
b. cytoplasm slightly acidiphilic, pale staining
cytoplasm
c. secrete parathyroid hormone (parathormone).
3. Oxyphil cells
a. Not present at birth
b. start appearing in parathyroid tissues at about
age 7 in humans
http://www.finchcms.edu/anatomy/histology/
organology/endocrine/images/ff941.jpg
c. acidophilic cytoplasm (due to many
mitochondria)
d. no known function
e. Similar to, but larger than chief cells.
http://www.lab.anhb.uwa.edu.au/mb140/
C. Major function of parathyroid glands
1. Parathyroid hormone (parathormone) causes increase in calcium in the blood
by promoting the activities of osteoclasts in the breakdown of calcified bone
matrix.
(Remember c-cells in thyroid that secrete calcitonin?)
D. Calcitonin of thyroid parafollicular cells (C cells) and parathyroid hormone
(parathormone) of chief cells balance and regulate calcium levels in the body.