OBJECTIVES: At the end of this laboratory, you should be able to:

Identify the pars distalis, pars intermedia, and pars nervosa; and recognize the individual cell types in the pars distalis.
Identify the thyroid gland and the specific cell types and their functions.
Identify the parathyroid gland and specific cell types within it and understand their function.
Identify the adrenal gland, distinguish cortex from medulla, distinguish the three zones of the cortex, and identify specific cell types within the adrenal gland and understand their function.
Identify islets of Langerhans in the pancreas and distinguish them from the exocrine portion of the gland, understand the cells types present within each islet and understand their function.
Identify the pineal gland and distinguish between pinealocytes and glial cells.
Recognize each endocrine organ microscopically and distinguish them from each other as well as from all other organs.

Pituitary gland, mammal
Identify the major regions of the pituitary gland: the pars nervosa, pars distalis, pars intermedia, pars tuberalis, and infundibulum.
Pars Distalis: The pars distalis is composed of two general cell types: chromophils (50%) and chromophobes (50%). The cytoplasm of chromophobes does not take up stain because their storage granules have been released. The chromophils can be further subdivided into acidophils (40%) and basophils (10%). The basophils generally are slightly larger cells, with a pale blue cytoplasm. The cytoplasm of acidophils stains red with the H&E stain. Chromophobes are undifferentiated or resting chromophils that appear weakly stained with smaller nuclei and less distinct borders. Observe the numerous sinusoidal capillaries, the delicate connective tissue framework, and the connective tissue capsule.
Pars Nervosa: Nerve fibers fill most of the pars nervosa but they are not easily identifiable without special stains. Note that the main cell type here is a glial or supporting cell called a pituicyte. The bulk of the pars nervosa consists of axons from neurons in the supraoptic and paraventricular nuclei of the hypothalamus. A few Herring bodies are present in the pars nervosa, and are particularly abundant in the infundibulum. These are the storage sites of the secretory granules of the pars nervosa neurons. The Herring bodies are identified as amorphous rounded light brown-stained bodies in this specimen.
Pars Intermedia: The pars intermedia (rudimentary in humans) is located between the pars distalis and pars nervosa. It consists mainly of colloid filled cysts lined by cuboidal epithelium.

Thyroid gland, monkey
The thyroid follicle forms the parenchymal unit of the thyroid gland. Each follicle consists of follicular epithelium (simple cuboidal) and a central mass, the colloid. Active follicles generally have "high" epithelium. Each follicle is surrounded by a delicate layer of loose connective tissue, containing a capillary network. Thus, the follicle cells are bounded by colloid on one side, and are in association with capillaries on the other. Try to identify parafollicular cells (or “C” cells) which are located within the confines of the follicular basement lamina yet excluded from the follicle lumen by follicular epithelium cells. These cells secrete calcitonin and may be difficult to identify. They are somewhat larger and lighter staining than follicular cells.

Parathyroid gland
The parathyroid gland, like all endocrine glands, has a capsule and is well vascularized. Note the general arrangement of solid cell cords that form the gland. Note the abundant principal (chief) cells which have a slightly eosinophilic cytoplasm containing lipofuscin pigment granules and moderate amounts of glycogen. Principal cells produce parathyroid hormone (PTH). Identify oxyphil cells which are larger and very eosinophilic. The function of oxyphil cells is unknown but they may also secrete PTH and become more numerous with age. Adipose tissue is frequently found in this gland in older individuals.

Adrenal gland
Examine both the cortex and the medulla. The human adrenal gland resembles a flattened triangle in cross section but frequently may be very irregular in shape due to the presence of deep folds involving the entire organ. Identify the thick connective tissue capsule surrounding the cortex.
Adrenal Cortex: The cortex is divided into three zones: the zona glomerulosa (outermost layer), the zona fasciculata (middle layer), and the zona reticularis (innermost layer). Study the cell arrangement in the various zones. The zona glomerulosa is composed of cells grouped in an ovoid configuration. The zona fasciculata is composed of lipid-droplet laden cells arranged in radial columns. Finally, the zona reticularis is composed of a loose meshwork configuration. Note the cortical sinusoids passing between the cords of cells in all layers. Identify medullary arterioles (note the smooth muscle in the arteriole wall, which distinguishes the arterioles from sinusoids) as they pass through the zona fasciculata. Know the major classes of secretions produced by each morphological area.

Adrenal Medulla: The medulla is entirely surrounded by the cortex. The principal cells are the chromaffin cells which can be thought of as modified post-ganglionic sympathetic neurons that lack dendrites and axons. Identify the central vein, noting particularly the abundance and arrangement of smooth muscle in the wall.

Pineal gland
The pineal is attached to the posterior aspect of the diencephalon. It consists of connective tissue, blood vessels, glial cells, and pinealocytes (which secrete melatonin). Pinealocytes have larger, lighter staining round nuclei and glial cells have small darker staining nuclei. With age, calcified formations appear in the pineal gland (corpora aranacea or “brain sand”). These are visible in X-rays and CAT scans and allow the pineal to serve as a landmark when reading films.

Pancreas
Within the parenchyma of the pancreas you will note small islets or clusters of lighter staining cells; these are the islets of Langerhans (endocrine part of the pancreas). Note the number of islets on your sections and the variation in their size. The islets are composed of three major cell types: alpha cells, beta cells, and delta cells. These cells cannot be readily distinguished without special stains. Alpha cells (15-20%) are found at the periphery and secrete glucagon. Beta cells (70%) are found more centrally and secrete insulin. Delta cells (5-10%) are found throughout the islet and secrete somatostatin.

Endocrine System