OBJECTIVES: At the end of this laboratory, you should be able to:
1. Identify the pars distalis, pars intermedia, and pars nervosa; and recognize the individual cell types in the pars distalis.
2. Identify the thyroid gland and the specific cell types and their functions.
3. Identify the parathyroid gland and the specific cell types and their functions.
4. 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 give their function.
5. Identify islets of Langerhans in the pancreas and distinguish them from the exocrine portion of the gland, name the cells types present within each islet and give their function.
6. Identify the pineal gland and distinguish between pinealocytes and glial cells.
7. Recognize each endocrine organ microscopically and distinguish them from each other as well as from all other organs.
SLIDES FOR THIS LABORATORY: 38, 39, 40, 41, 42, and 80
Slide 38 Pituitary gland.
PARS DISTALIS . The pars distalis is composed of two general cell types: chromophils (50%) and chromophobes (50%). The chromophils can be further subdivided into acidophils (40%) and basophils (10%). The acidophils secrete GH (somatotropes) and prolactin (mammotropes). Basophils secrete TSH (thyrotropes), LH (gonadotropes), FSH (gonadotropes), and ACTH (corticotropes). The different acidophils and basophils cannot be distinguished in the light microscope. Chromophobes are undifferentiated or resting chromophils that appear weakly stained with smaller nuclei and less distinct borders. Observe the numerous blood vessels , the delicate connective tissue framework , and the connective tissue capsule . Recall that the hypophyseal portal circulation carries releasing hormones from the hypothalamus to the adenohypophysis targeting the acidophils and basophils and causing release of hormones into the blood stream.
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. These are the storage sites of the neurosecretory material of the pars nervosa neurons. The Herring Bodies contain many greyish-brown storage vesicles.
PARS INTERMEDIA . This structure (rudimentary in humans) lies between the pars distalis and pars nervosa. It consists mainly of colloid filled cysts lined by cuboidal epithelium. Note: a lumen may be present between the pars intermedia and pars distalis.
Slide 42 Thyroid gland.
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.
Slide 40 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 clear-cut cell outlines of 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. Adipose tissue is frequently found in this gland in older individuals.
Slide 39 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.
ADRENAL CORTEX . The cortex is divided into three zones (starting from the capsule): the zona glomerulosa, the zona fasciculata, and the zona reticularis. Identify the capsule and 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. Know the major classes of secretions produced by each morphological area.
Weak androgens (DHEA)
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.
Slide 41 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 nuclei and glial cells have small darker staining nuclei. With age, calcified formations appear in the pineal gland (brain sand or corpora aranacea ). These are visible in X-rays and CAT scans and allow the pineal to serve as a landmark when reading films.
ISLETS OF LANGERHANS
Slide 80 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 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.