Download Histology of Endocrine system II

Histology of Endocrine system
II
Adrenal gland
• Paired organs that lie on top of the kidneys, embedded
in adipose tissue.
• General structure
– Covered by capsule of dense collagenous connective
tissue.
– Thin septa or trabeculae extend from the connective
tissue capsule into the interior of the gland.
– Internally there are two major layers called the adrenal
cortex and adrenal medulla.
– The supporting framework of cortex and the medulla are
collectively called the stroma. This framework contains
many reticular fibers.
Adrenal gland- Embryological
development
• In a sense, the adrenal cortex and medulla may be considered as 2
morphologically distinct endocrine organs.
• Similar to what we saw in the hypophysis, the cortical and medullary
layers of the adrenal glands have different embryonic origins. That is to
say, they are derived from different basic tissue types in the embryo.
• As was the case for the hypophysis, we have a situation where neural and
non-neural tissues become associated to form an organ.
• As an embryo develops, the cortex of the adrenal gland is derived from
mesodermal cells in the region of the kidney.
• The cells of the adrenal medulla are derived from a specialized group of
neural ectoderm cells that are called the neural crest. These cells are
initially part of the larger group of ectodermal cells that will form the
central nervous system (CNS).
– The neural crest cells separate from the developing CNS and migrate through
the body's tissues giving rise to many different cell types, tissues, and organs.
– Some of the neural crest cells migrate to the developing adrenal cortex,
penetrate this tissue and lodge themselves centrally within it to form the
adrenal medulla.
– So, the cells of the adrenal medulla are sometimes referred to as
postganglionic neurons that have lost their axons and dendrites and have
become secretory cells.
Cardiovascular circulation to the
adrenal gland
• Blood is supplied to the adrenal glands by a number of arteries.
• 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.
• The capillaries supply blood to a network of sinusoids in the cortex.
• Some of the arterial branches do not form capillaries in the cortex,
but rather run through it to the medulla.
– These are the medullary arteries.
– These arteries form a dense capillary network around the cells of the
medulla.
• If we look at the fenestrated endothelium of the capillaries, we find
pores that are occluded by a thin membrane. In the cortex, the
basal lamina is not continuous. So again, as we might suspect, the
circulatory components of this gland are set-up for the exchange of
materials between the blood and surrounding cells or visa versa.
• Capillaries of medulla and cortex coalesce to form the adrenal veins
that exit the adrenal glands.
Adrenal cortex
• Composed of 3 layers
– zona glomerulosa
– zona fasciculata
– zona reticularis
Zona glomerulosa - structure
• Columnar to pyramidal cells that are arranged in folded
cords surrounded by capillaries and sinusoids.
• Cells have spherical nucleus and basophilic granules in
cytoplasm.
• Adjacent to the endothelium of the capillaries and
sinusoids there is a space between the endothelium and
surrounding cells.
• The cell surface of the columnar or pyramidal secretory
cells adjacent to this space are thrown into folds in this area
forming microvilli.
• Ultrastructure is typical for cells involved in lipid synthesis
– 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.
– Small lipid droplets may be present in cytoplasm associated with
smooth endoplasmic reticulum; however, these are not
particularly evident in histological sections for light microscopy.
Zona glomerulosa - function
• The zona glomerulosa cells
secrete mineralcorticoids, mainly aldosterone
which is important in maintaining water
balance.
• These are steroid hormones that are lipid
molecules derived from cholesterol.
Zona fasciculata - structure
• Cells are polyhedral and are arranged in straight cords
(columns) that are 1-2 cells thick with capillaries
running between.
• Cells have central nucleus with basophilic cytoplasm.
Microvilli are present within the sub-endothelial space
(next to the capillaries).
• Many lipid droplets present in cytoplasm. These are
extracted during most fixation and embedding
procedures, so the cells often appear highly vacuolated
in histological sections.
• Ultrastructure is typical for cells involved in lipid
synthesis and secretion
– However, these cells have more RER than those of the
zona glomerulosa
– This is why the cytoplasm has an overall basophilic affinity.
Zona fasciculata - function
• Zona fasciculata cells
secrete glucocorticoids (important in lipid,
protein and carbohydrate metabolism) that
are another type of steroid.
Zona reticularis
• Cells are polyhedral and are arranged in
irregular cords with capillaries and sinusoids
between.
• Lipofuscin pigment granules in cells.
• Cytoplasm acidophilic. Few lipid droplets.
• Cells secrete glucocorticoids.
• Secretion of steroids by adrenal cortex is good
example of a feedback system between target
organ and pituitary gland.
– A psychological stimulus such as stress or exercise
causes neurosecretory neurons in the hypothalamus
to secrete adrenocorticotropic hormone releasing
factor into the capilaries in the median eminence.
– the releasing factor is carried by the hypophyseal
portal system from the median eminence to the pars
distalis.
– This causes secretion of adrenocorticotropic hormone
(ACTH) by certain cells in the pars distalis.
– ACTH causes an increase in corticosteroid secretion
(e.g. cortisol) by adrenal cortex cells.
– As levels of corticosteroids increase in blood, these
inhibit the secretion of releasing factors by the
hypothalamus and secretion of ACTH by pituitary.
Adrenal medulla
• Composed of one layer of cords of polyhedral, epithelioid, secretory
cells that form a compact irregular network surrounded by capillaries,
venules and a few sympathetic ganglion cells.
• These epithelioid secretory cells are considered to be modified
postganglionic neurons.
• Nerve fibers (axons) contact the epithelioid cells on the part of their
surface that is adjacent to a capillary.
– When stimulated , these nerve fibers release acetylcholine that causes the
release of the catecholamine secretory product accumulated in these cells.
– The secretory product consists of epinephrine or norepinephrine.
– Two different types of cells, one for each catecholamine.
– The catecholamine secretory granules stain in a specific way when reacted
with oxidizing agents. This reaction is called the chromafin reaction. As a
result, these cells are called chromafin cells.
• Adrenal medulla secretes continuously into blood stream.
– The adrenal medulla cells secrete only small amounts of epinephrine and
norepinephrine unless stimulated by nervous activity related to emotional
reactions.
– Increased secretion of these substances prepares body to react to stressful
situations. Blood vessels constrict, blood pressure rises, etc.
Thyroid gland
• Located below the larynx, partially encircling the
esophagus.
• Thyroid is covered by thin layer of loose connective
tissue.
• Associated with this loose connective tissue is a dense
network of blood and lymphatic capillaries.
– Capillaries are fenestrated.
• Septa extend into organ from the connective tissue
capsule.
– These septa separate follicles of thyroid tissue from each
other.
– Septa composed mainly of reticular fibers.
• The thyroid tissue within the connective tissue capsule
is composed of follicles, each containing a lumen filled
with a gelatinous substance called colloid.
– Each follicle consists of a simple cuboidal
epithelium surrounding the lumen
• The epithelium changes to simple squamous if the follicle is
inactive.
– Cells of follicles are responsible for synthesis of thyroid
hormones, the most abundant of which is thyroxin.
• These hormones are small molecules consisting of iodinated
amino acids that are all formed from the tyrosyl radicals
of thyroglobulin. (i.e. side chains of thyroglobulin containing
tyrosine are iodinated and separated from the thyroglobulin
molecule and then processed to form the thyroid hormones.)
• Thyroxine stimulates mitochondrial respiration and oxadative
phosphorylation. So, more ATP produced faster.
– In addition to follicle cells, parafollicular cells (C cells) are
found between the follicles.
• These cells secrete the polypeptide calcitonin that causes a
reduction of calcium in the blood by inhibiting the activities of
osteoclasts.
Control of thyroid hormone secretion
• Neurogenic stimuli provided by axons from
parasympathetic and sympathetic ganglia can influence
metabolism of thyroid cells.
• However, thyrotropin (thyroid stimulating hormone,
TSH) is the major controlling factor.
• Thyrotropin is synthesized and secreted by cells in
the adenohypophysis (pars distalis).
• Thyroid has feedback system with pituitary similar to
adrenal cortex.
– Releasing factors from the median eminence 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 in the median
eminence.
• Thyroid hormones are synthesized from the
colloid.
– Colloid is composed of iodinated thyroglobulin.
– When thyroid hormones are required, iodinated
thyroglobulin in colloid is endocytosed.
– Endocytotic vesicles merge with lysosomes.
– Thyroid hormones are synthesized as a result.
– This liberates the 4 thyroid hormones into the
cytoplasm.
– These diffuse across the cell membrane into
capillaries where blood carries them to target
organs.
Parathyroid glands
• These are small, but important, organs that are
embedded in the wall of the thyroid.
– 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
– In 1892, the French physiologist Gley showed that it
was actually the removal of the parathyroid glands
that "rode" along with the thyroid that was the cause
of these titanic seizures. (Due to the lack of
parathormone)
– Thus, when the thyroid is removed, it is critical that
the parathyroid glands be separated from it and left in
the body.
• There are three cell types in the parathryroid
glands
– Chief cells
• polygonal with vesicular nucleus
• slightly acidiphilic, pale staining cytoplasm
• secrete parathyroid hormone (parathormone).
– Oxyphil cells
• Not present in thyroid at birth
• start appearing in parathyroid tissues at about age 7 in
humans
• no known function
• Similar to, but larger than chief cells.
• Adipose cells - increase in number as one
grows older.
• Major function of parathyroid glands
– Secretion of parathyroid hormone that causes an
increase in calcium in blood by promoting the
activities of osteoclasts in the breakdown of
calcified bone matrix.
• So the calcitonin of thyroid parafollicular cells
and parathyroid hormone of chief cells
balance and regulate calcium levels in the
body.