The term "endocrine" implies secretion into the internal milieu of a multicellular organism. In contrast to exocrine tissues, where the secretory products are discharged into the external space - the outer surface of the body, mucosal surfaces, duct systems - the endocrine organs and cells secrete their products into the vascular system. The blood vessels then serve as conduits for these secretions to travel to their target tissues.

Endocrine tissues function according to a basic model: individual cell types respond, via surface receptors, to different signals. They produce a very limited spectrum of secretory products, called hormones, that themselves function as signaling molecules for specific cell populations. These hormones are effective at exceedingly low concentrations. Because of this, relatively few cells are sufficient for the integrative function of large, multicellular organisms. Therefore, endocrine tissues are characteristically present in small quantities.

The focus of this laboratory will be on five endocrine tissues: the pituitary gland, the thyroid, the parathyroid, the adrenal gland, and the endocrine pancreas.

The pituitary gland, or hypophysis, is a collection of different cell types that control the activity of other endocrine organs. It is governed by the hypothalamus, which sends both electrical and hormonal signals to the pituitary, and by feedback regulation (both positive and negative) through the secretions of its target glands. The pituitary gland sits within the sella turcica of the sphenoid bone and is traditionally divided into two sections that can be distinguished structurally, functionally, and embryologically.

The anterior pituitary is also known as the adenohypophysis or pars distalis and is derived from Rathke's pouch, a growth of the primitive oral cavity. It contains cells that, when viewed under the light microscope, appear as acidophils, basophils, or chromophobes.

The posterior pituitary is also known as the neurohypophysis or pars nervosa and is derived from the hypothalamus. It is mostly composed of unmyelinated axonal processes and terminals of the supraoptic and paraventricular nuclei of the hypothalamus. The pituitary stalk connects the two glands. The posterior pituitary has characteristic Herring bodies, focal axonal swellings that are packed with secretory granules. Pituicytes are the glial cells of the pituitary gland; the nuclei that are visible in sections of posterior pituitary belong to these cells, as well as cells of the vasculature.

A third section of the pituitary, the intermediate lobe, is vestigial in man.

The pituitary gland has a unique vascular system. Cells from the hypothalamus secrete releasing hormone into a hypophysisal portal system that flows from the hypothalamus to the anterior pituitary. This system supplies the anterior pituitary and drains its products, which include the stimulating hormones produced there. It then enters the systematic circulation, which delivers those hormones to target organs throughout the body. The posterior pituitary has a separate system that carries away its products, as it does not receive hormonal stimulation from the hypothalamus.

The adrenal gland has two distinct parts, the cortex and medulla, which differ in structure, function, and embryological origin.

The adrenal cortex is organized into cords that are two cells thick and are arranged radially around the medulla. The cortex is similar in embryology to the gonads; this can be easily recalled because both glands secrete steroid hormones synthesized from cholesterol. Three cortical zones can be distinguished on the basis of differences in the arrangement and cytology of the cells.

The adrenal medulla is the innermost portion of the gland and shares an embryological origin with the sympathetic nervous system. Its cells possess abundant cytoplasmic granules that contain stored peptide hormones and catecholamines. These cells are frequently called chromaffin cells because they can be stained with chromium salts. Preganglionic sympathetic fibers traverse the adrenal cortex and synapse directly on chromaffin cells, where they release acetylcholine to stimulate the exocytosis of catecholamine granules during a sympathetic response.

It is important to understand the blood supply of the adrenal gland. The suprarenal arteries form a plexus beneath the capsule of the gland. These arteries give rise to two sets of downstream arteries:

The thyroid is located in the neck and is surrounded by a collagenous capsule continuous with the middle cervical fascia. It is unique because it stores large amounts of inactive hormone within extracellular compartments; most glands store small amounts of hormone intracellularly.

The thyroid consists of structural units called follicles, which are composed of secretory epithelial cells called principal cells that are adjoined by junctional complexes and surrounded by a basement membrane and reticular connective tissue. Follicles vary in size, but each displays a central lumen containing colloid. Colloid consists of the glycoprotein thyroglobulin, which is secreted by the principal cells and serves as a precursor to thyroid hormone. The height of the principal cells varies according to their level of secretory activity; in hypothyroidism, the cells are squamous or cuboidal, whereas in hyperthyroidism, they are columnar.

Parafollicular cells (C-cells) lie adjacent to the principal cells in the parenchyma of the organ and do not extend into the follicular lumen. They secrete the hormone calcitonin, which is involved in calcium regulation.

The thyroid is a highly vascular gland, and the follicular epithelium is intimately associated with the capillaries.

The parathyroid glands are closely associated with the thyroid and are derived embryologically from the third and fourth branchial arches. They consist of closely packed groups of two cell types:

The endocrine portion of the pancreas is comprised of the islets of Langerhans. During development, the cells of the islets migrate away from the duct system and aggregate around capillaries. The islets contain three important cell types: