The endocrine system regulates the function of organs by controlling the activity of cells within organs. Cells of the endocrine system secrete hormones into their surrounding tissue. The hormones enter the circulatory system and are distributed throughout the body. Target cells in organs express receptors for specific hormones. Upon binding to its hormone, the receptor activates a signal transduction pathway that alters the activity of the target cell.
This lab will focus on five organs of the endocrine system: pituitary, adrenal gland, thyroid, parathyroid and 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 the glands it targets. 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.
This slide shows a section of the human pituitary. Note the pituitary is divided into the darker staining anterior pituitary and lighter staining posterior pituitary. The posterior pituitary is connected to the hypothalamus by the pituitary stalk. The stalk contains axons of neurons whose cell bodies reside in the hypothalamus and that release their hormones in the posterior pituitary.
The anterior pituitary is also known as the adenohypophysis. It contains cells that, when stained by H&E, appear as acidophils, basophils, or chromophobes.
Acidophils are typically eosin-stained and tend to be located in the center of each half of the pituitary. They secrete protein hormones like growth hormone and prolactin.
Basophils appear purple and are more prominent at the edges of the gland. They secrete glycoprotein hormones such as adrenocorticotrophic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and leutenizing hormone (LH).
Chromophobes have clear nuclei and scant cytoplasm. They may be cells that are non-secretory or exhibit minimal hormone storage.
The pituitary gland has a unique vascular system. Cells from the hypothalamus secrete releasing hormone into a hypophyseal 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 systemic 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.
Cells in the anterior pituitary are stimulated to release their hormones by hormone-releasing hormones that are produced in the hypothalamus. The hormone-releasing hormones enter the vasculature in the hypothalamus and then travel via portal veins to the anterior pituitary. The hormone-releasing hormones can exit capillaries in the anterior pituitary to bind receptors on the surface of cells in the anterior pituitary. Specific hormone-releasing hormones bind to specific receptors on cells in the anterior pituitary to trigger the release of a one type of hormone in the anterior pituitary.
The posterior pituitary is also known as the neurohypophysis and is derived from the hypothalamus. It is mostly composed of unmyelinated axonal processes and terminals of neurons whose cell bodies reside in the hypothalamus. The neurons release the hormones oxytocin and vasopressin. The pituitary stalk connects the hypothalamus and pituitary. The posterior pituitary also has characteristic Herring bodies, which are focal axonal swellings packed with secretory granules. Because the cell bodies of the secretory cells of the posterior pituitary are located in the hypothalamus, most of the nuclei visible in this slide belong to supporting cells known as pituicytes. Pituicytes are the glial cells of the pituitary gland.
The adrenal gland has two distinct parts, the cortex and medulla, which differ in structure, function, and embryological origin. The cortex secretes hormones produced from cholesterol and can be functionally and histologically divided into three zones: zona glomerulosa (mineralocorticoids), zona fasciculata (glucocorticoids) and zona reticularis (sex hormones). The medulla primarily secretes catecholamines, including adrenaline and noradrenaline. The three cortical zones can be distinguished on the basis of differences in the arrangement and cytology of the cells.
The zona glomerulosa is located just beneath the capsule of the adrenal gland and contains cells arranged in clusters separated by trabeculae that are continuous with the capsule. The capsule is a connective tissue layer with collagen fibers and blood vessels. Capillaries branch from these blood vessels and extend into the trabeculae. The cells of the glomerulosa are pale-staining and have distinct round nuclei and a higher nuclear to cytoplasmic ratio than the cells of the adjacent zona fasciculata. The cells are organized in ovoid clusters which are separated by capillaries. The cells produce mineralocorticoid hormones, such as aldosterone, which regulate salt and water balance. The cells are stimulate to release aldosterone in response to increased potassium levels or decreased blood flow to the kidneys. Recall that aldosterone induces the cells of the distal convoluted tubule and collecting ducts to absorb sodium from urine.
The zona fasciculata comprises the thick middle layer of the cortex. Its cells are extensively vacuolated because of the presence of lipid droplets. The cells of this region are organized into parallel cords separated by straight capillaries and produce glucocorticoids like cortisol, which has metabolic effects. These cells secrete cortisol in response to adrenocorticotrophic hormone (ACTH).
The zona reticularis is the innermost layer of the adrenal cortex. The border between it and the zona fasciculata is less distinct than that between the previous two zones. Cells in the zona reticularis stain deeply and are less vacuolated. This region produces androgens, which supplement sex hormones produced by the gonads.
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 (epinephrine and norepinephrine). 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. The cells of the medulla are stimulated during flight-or-fight responses. Note the high vascularity of the medulla is clear on this slide.
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 short cortical arteries supply the cortex by giving rise to an anastomosing network of capillary sinusoids. These sinusoids descend down through the three layers of cortex and drain into venules and the central vein of the medulla.
The long cortical arteries form a large capillary network around the secretory cells of the medulla and also drain into the central vein of the medulla.
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.
Chief (principal) cells, which have prominent central nuclei surrounded by pale cytoplasm. Chief cells produce parathyroid hormone (PTH), which is the most important regulator of calcium metabolism in humans. When serum calcium levels fall, chief cells release PTH which indirectly stimulates the production of osteoclasts in bone.
Oxyphilic cells, which are large and fewer in number, have small, dark nuclei and an acidophilic cytoplasm with many mitochondria. The function of these cells is unknown, but they increase in abundance as a person ages.
The endocrine portion of the pancreas is comprised of the islets of Langerhans. The islets of Langerhans appear as distinct islands in a sea of pancreatic acinar cells, and constitute just a small percentage (2%) of pancreatic tissue. Regardless of the stain used, the islets can be identified because of their distinct geometric cord pattern and high vascularity when compared to the acini. The islets contain three important cell types.