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Thyroid glands are located in the neck, in close approximation to
the first part of the trachea. In humans, the thyroid gland has a
"butterfly" shape, with two lateral lobes that are connected by a
narrow section called the isthmus. Most animals, however, have
two separate glands on either side of the trachea. Thyroid glands
are brownish-red in color.
Close examination of a thyroid gland will
reveal one or more small, light-colored
nodules on or protruding from its surface these are parathyroid glands (meaning
"beside the thyroid"). The image to the
right shows a canine thyroid gland and
one attached parathyroid gland.
Histology of the Thyroid and Parathyroid gland
Histology of the Thyroid and Parathyroid gland
The microscopic structure of the thyroid is quite distinctive. Thyroid epithelial cells the cells responsible for synthesis of thyroid hormones - are arranged in spheres
called thyroid follicles. Follicles are filled with colloid, a proteinaceous depot of
thyroid hormone precursor. In the low (left) and high-magnification (right) images of a
cat thyroid below, follicles are cut in cross section at different levels, appearing as
roughly circular forms of varying size. In standard histologic preparations such as
these, colloid stains pink.
In addition to thyroid epithelial cells, the thyroid gland houses one other important
endocrine cell. Nestled in spaces between thyroid follicles are parafollicular or C
cells, which secrete the hormone calcitonin
The structure of a parathyroid gland is distinctly different from a thyroid gland. The
cells that synthesize and secrete parathyroid hormone are arranged in rather
dense cords or nests around abundant capillaries. The image below shows a
section of a feline parathyroid gland on the left, associated with thyroid gland
(note the follicles) on the right.
Histology of the Parathyroid gland
Histology of the Parathyroid gland
Thyroid Hormones
Thyroid hormones are derivatives of the the amino acid tyrosine bound
covalently to iodine. The two principal thyroid hormones are:
thyroxine (also known as T4 or L-3,5,3',5'-tetraiodothyronine)
triiodotyronine (T3 or L-3,5,3'-triiodothyronine)
As shown in the following diagram, the thyroid hormones are basically two
tyrosines linked together with the critical addition of iodine at three or four
positions on the aromatic rings. The number and position of the iodines is
important. Several other iodinated molecules are generated that have little
or no biological activity; so called "reverse T3" (3,3',5'-T3) is such an
example.
Thyroid hormones are poorly soluble in water, and more than 99% of the T3
and T4 circulating in blood is bound to carrier proteins. The principle carrier
of thyroid hormones is thyroxine-binding globulin
Thyroid and Parathyroid Hormone
The gland
Hormone
Physiologic Effects
Thyroid
Thyroxine
T3 /T4
Metabolic rate
homeostasis and many
others
Calcitonin
CT
Ca++ / PO-homeostasis
Parathyroid
Ca++ / PO-homeostasis
parathyroid
Anatomy of the Adrenal Gland

The two adrenal glands are located
immediately anterior to the kidneys,
encased in a connective tissue
capsule and usually partially buried in
an island of fat. Like the kidneys, the
adrenal glands lie beneath the
peritoneum .The exact location
relative to the kidney and the shape of
the adrenal gland vary among
species. Inspection of a mammalian
adrenal gland that has been sectioned
reveals two distinct regions.
• An inner medulla, which is a source of the catecholamines epinephrine
and norepinephrine. The chromaffin cell is the principle cell type. The
medulla is richly innervated by preganglionic sympathetic fibers and is, in
essence, an extension of the sympathetic nervous system.
• An outer cortex, which secretes several classes of steroid hormones
(glucocorticoids and mineralocorticoids, plus a few others).
Cortex
medulla
Adrenal Medullary Hormones
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

Cells in the adrenal medulla synthesize and secrete epinephrine and norepinephrine.
The ratio of these two catecholamines differs considerably among species: in humans, cats
and chickens, roughly 80, 60 and 30% of the catecholamine output is epinephrine.
Following release into blood, these hormones bind adrenergic receptors on target cells,
where they induce essentially the same effects as direct sympathetic nervous stimulation.
Synthesis and Secretion of Catecholamines
Synthesis of catecholamines begins with the amino acid tyrosine, which is taken up by
chromaffin cells in the medulla and converted to norepinephrine and epinephrine through
the following steps:
Adrenal Cortex Hormones
The adrenal cortex is a factory for steroid hormones. In total, at least two
to three dozen different steroids are synthesized and secreted from this
tissue
Class of Steroid
Major Representative
Physiologic Effects
Mineralocorticoids
Aldosterone
Na+, K+ and water
homeostasis
Glucocorticoids
Cortisol
Glucose homeostasis
and many others
Gonadocorticoids
Androgens/Estrogens
(Testosterone)
Sex behavior
Cortex
Histology of the Adrenal Cortex
Cells in the adrenal cortex are arranged into three concentric
zones.
1) The outermost zone is the zona glomerulosa. Cells within this zone
tend to be columnar in shape and are arranged in irregular cords.
2) The zona fasiculata is the middle and largest of the three zones in
the cortex. Cells in the fasiculata are polyhedral and usually have a
foamy appearance due to abundant lipid droplets. They also are
arranged in distinctively straight cords that radiate toward the
medulla.
3) The innermost zone of the cortex is the zona reticularis. Cells within
this zone are arranged in cords that project in many different
directions and anastomose with one another.
Medulla
Histology of the Adrenal Medulla
The most abundant cell in the adrenal medulla is the chromaffin cell. Chromaffin
cells are columnar in shape and rather basophilic. At higher magnification, they are
seen to have a granular cytoplasm due to hormone-containing granules. They are
arranged in clusters, usually around medullary veins, as seen the right image of
rabbit adrenal (H&E stain).
The adrenal medulla is richly innervated by preganglionic sympathetic fibers.
Additionally, small numbers of sympathetic ganglion cells are commonly
observed in the medulla. Ganglion cells are round or polygonal with prominent
nuclei. A cluster of ganglion cells is seen in the lift image
medullary
veins