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The Endocrine Glands

Objectives

  1. Recognize and distinguish the various endocrine organs (thyroid, parathyroid, adrenal and pituitary).
  2. Understand the relationship between structure and function especially with respect to the pituitary.

The endocrine glands are aggregations of epithelial cells embedded within connective tissue and surrounded by rich vascular networks. They may constitute separate organs, as the thyroid or adrenal glands, or they may occur as collections of cells embedded amid the tissues of another organ, such as the pancreatic islets and the interstitial cells of the testis.

Endocrine glands differ from exocrine glands in that they possess no excretory ducts, and their secretions are usually released into interstitial tissue, to be picked up by blood and lymph capillaries. These secretions, the hormones, are characterized by being effective in minute amounts and often exert their influence on target organs or tissues at some distance from the gland. Hormones may be stored within the glandular cells (pituitary), or by a special means of extracellular storage (the thyroid), or they may be released into the blood as fast as they are produced (the adrenal cortex). In some glands, the hormones, their precursors, and even the products associated with hormone formation may be visible within the individual cells as granules or droplets, particularly if special staining techniques are applied.

In observing the endocrine glands, the student should pay particular attention to the relationships of the major structural and functional components, the epithelial cells and their vascular supply. The particular cytological features within the epithelial cells which aid in identification of the various glands should be noted.

Thyroid

Examine the human thyroid (slide 67) under low power. The glandular epithelium is arranged in follicles lying within a connective tissue stroma, in which blood vessels and lymphatics form rich plexuses around each follicle. The follicles are filled with colloid which contains a high M.W. protein called thyroglobulin. Its staining properties can vary considerably, even within the same follicle. The apparent follicular size varies greatly due to differences in the amount of colloid stored and the plane of section.

   

Under high magnification, observe the follicular epithelial cells which may be low cuboidal to low columnar. Note that in some follicles the edge of the colloid mass is scalloped or vesicular suggesting that some of the colloid was lost during processing. This is usually attributed to lowered viscosity adjacent to the cells because of hydrolytic enzyme activity. Note C.T. and vascular elements found in the interfollicular stroma.

Parathyroid

These glands consist of four structures attached to, or embedded within, the posterior surface of each thyroid lobe. The parathyroid secretion, parathormone, is a polypeptide important in the regulation of calcium and phosphorus metabolism. The parathyroid glands consist of closely packed masses and cords of epithelial cells within a stromal meshwork of collagenous and reticular fibers. Oxyphil cells are interspersed singly or in clumps among the predominant cellular elements, the chief cells.

Study the parathyroids (slide 68) in both even and odd boxes. The chief cells are most numerous. They are the relatively small cells, therefore, their nuclei appeared to be more closely packed. Look for patches of eosinophilic cells with more widely spaced nuclei and distinct cell borders. These are oxyphils which may be found in small to large groups scattered among the chief cells. The oxyphils are larger cells with a smaller and more densely stained nucleus. The eosin staining of oxyphils may be faded on some slides, but they may usually be recognized by their nuclei and distinct cell borders. Some slides have relatively few oxyphils.

Adrenal

The adrenal gland is composed of two embryologically and functionally distinct glands. The cortex is of mesodermal origin and secretes several steroids. Hormones which exhibit at lease three types of activity; an effect on mineral metabolism (mineralocorticoids), on carbohydrate metabolism (glucocorticoids) and on androgenic activity. The activity of the cells in the zona fasciculata and zona reticularis of the cortex is controlled humorally by the anterior pituitary. The cells of the adrenal medulla are of neural ectoderm origin, migrating out from the neural crest in a manner similar to autonomic ganglion cells. They become typical epithelioid secretory cells arranged in irregular rows with arterioles and capillaries on their basement membrane side and large venous capillaries at the opposite pole. Their nuclei lie toward their basement membrane, and the cells are oriented to secrete into venous capillaries. The cells are under autonomic control, receiving preganglionic (acetylcholine) fibers; they release their hormones, epinephrine and norepinephrine in response to stimulation by the preganglionic fiber. There are ganglion cells in the medulla, among the secretory cells.

Start with slide 69, odd (adrenal gland). This well-preserved tissue (from monkey) clearly demonstrates the major structural features of the gland. With reversed ocular observe the central basophilic medulla (containing medium to large veins), surrounded by the eosinophilic cortex. Using the scanning lens, identify the capsule, cortex, and medulla. With the 10x lens, begin at the capsule and examine the three zones of the cortex; the glomerulosa, the fasciculata, and the reticularis. Cells of the zona glomerulosa are smaller and darker than the next layer of cells and are arranged in spherical groups or arcades lying just inside the capsule. Their nuclei stain darkly.

   

Internal to it is the zona fasciculata. No sharp dividing line separates the two zones. Cells of the fasciculata are arranged in cords with large capillary vessels between adjacent cords. In odd slides, the capillaries are collapsed but are recognizable by the flat nuclei of endothelial cells (40x). Cells of this zone are the largest of the cortical zones and light staining due to large empty spaces in the cytoplasm. These spaces originally contained lipid, but solvents used in histological preparations extracted it. These cells produce glucocorticoids and some androgen.

   

The zona reticularis contains cells which are smaller and darker than those of the fasciculata and are arranged in anastomosing cords. From E.M. it is known that cytoplasmic constituents are abundant and closely packed. These cells produce glucocorticoids and androgen. (In the even numbered boxes, much of the reticularis was very poorly fixed. Look at E.M.s of steroid producing cells in your textbook.

   

The medulla constitutes about 10% of the gland, and, in the human, is asymmetrically placed in the gland. The head region (odd number) has a substantial amount, whereas, some of the body and all of the tail (even numbers) have little or none. The cells of the medulla are large, arranged in ovoid clusters, and are polarized with respect to their blood supply. The cells are called chromaffin cells because of their ability to stain with chromium salts. Specific cells secrete epinephrine; others secrete norepinephrine. Histochemical stains are required to demonstrate the different cell types.

   

Slide 69, even is from the body or tail of a human adrenal and, thus, has little or no medulla. The description of the cortex above also applies, but note that the reticularis is poorly preserved. With reversed ocular, locate the central vein, the largest vessel. Observe the central vein with the scanning lens; it is surrounded over much of its surface by cortical tissue although located in the medulla (the cortical cuff is unique to humans). Starting from the capsule, work across the slide observing the cortical zones, medulla, cortical material, central vein, etc.

Pituitary (Hypophysis)

This gland is composed of two tissue types, an adenohypophysis (anterior lobe) derived from buccal ectoderm and a neurohypophysis (posterior lobe) also called pars nervosa derived from neural ectoderm. The adenohypophysis contains mainly cords of glandular epithelium surrounded by blood vessels and some C.T. The major anterior portion of this lobe is called pars distalis. A smaller portion, pars intermedia, lies between the pars distalis and the posterior lobe neurohypophysis. The posterior lobe contains mainly nerve processes and endings, (the terminal portion of the hypothalamo-hypophyseal tract) along with glial cells, blood vessels and some C.T.

The entire pituitary gland (slide 65), acid fuchsin stain (in odd-numbered boxes) and Gomori's chrome-alum-hematoxylin-phloxin (in even-numbered boxes) is enclosed in a C.T. capsule containing collagen, elastic and reticular fibers. From the capsule, trabeculae project into the anterior lobe, and together with a meshwork of reticular tissue, support the epithelial cords and vascular supply. The capillaries are abundant and have a very wide lumen. With the E.M., it is seen that their endothelial cells are fenestrated and have a thick basement membrane. Although traditionally referred to as "sinusoids" their structure is that of typical fenestrated capillaries.

The endocrine cell types of the anterior lobe have been classified for light microscopy (L.M.) by their staining as basophilic, acidophilic or chromophobic. Reddish acidophils and dark blue basophils are clearly discernible and have numerous granules. The chromophobe has a slightly basophilic cytoplasm, and few or no granules. Some, but not all "chromophobes" are degranulated acidophils or basophils. By E.M. and immunohistochemical techniques, at least six types of secretory cells are distinguishable according to their fine structure and hormone content. Such precise discrimination is impossible in sections stained routinely for L.M. Acidophiles produce somatotropin (growth hormone) and prolactin. Basophils produce ACTH, FSH, LH, and TSH. Close examination of the arrangement of the cells in the anterior lobe shows them to be in irregular cords between relatively large blood capillaries. Pick out examples of capillaries in the connective tissue. The pars intermedia at the posterior end of the anterior lobe has dark basophils (presumably ACTH-MSH secreting cells). The intermediate zone also has follicles containing a colloid material.

   

On slide 65, even, locate the posterior lobe by visual inspection and under low power as a light-staining area resembling CNS tissue. Slides in odd-numbered boxes have little or no posterior lobe tissue. The posterior lobe is often separated from the anterior lobe by the intermediate lobe, but in this specimen, the intermediate lobe basophils are significant in number and are interposed between the follicles and neurohypophyseal elements.

At 10x, small, dark blue sites are clear, as are nuclei of pituicytes, a neuroglial "supportive" cell. The dark blue regions are Herring bodies. They are the axon terminals of the hypothalamo-hypophyseal tract, and the material being stained are the hormones vasopressin (anti-diuretic hormone) and oxytocin complexed to carrier molecules (neurophysins). These neurosecretory droplets are synthesized in the hypothalamus and transported down the fibers of the tract and accumulate here.

Pituicytes of various shapes may be seen around nerve fibers and Herring bodies. The nuclei are basically rounded to oval in shape; it will not be possible to see cytoplasmic processes. Other nuclei belong to fibroblasts or endothelial cells.

Other Endocrine Glands

The Leydig or interstitial cells of the testis produce androgens under pituitary gonadotropin control. The interstitial follicular and thecal cells of the ovary contribute to the production of estrogens while the cells of the corpus luteum form progesterone, all under pituitary gonadotropin control. The placenta has extensive endocrine functions involving both steroid and polypeptide hormones, insulin and glucagon. These endocrine tissues are studied when the student examines the organs in which they are embedded.