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 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

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

parathyroidParathyroidCa++ / PO-- homeostasis

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).

medulla

Cortex

Adrenal Medullary Hormones 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:

Class of Steroid Major Representative Physiologic Effects

Mineralocorticoids AldosteroneNa+, K+ and water homeostasis

Glucocorticoids CortisolGlucose homeostasis and many others

GonadocorticoidsAndrogens/Estrogens

(Testosterone)

Sex behavior

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

Adrenal Cortex Hormones

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.

Histology of the Adrenal Cortex

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