Microanatomy of Endocrine Glands I: Intro and Hypophysis PDF

Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 1 of 13 MICROANATOMY OF ENDOCRINE GLANDS I: Introduction and The Hypophysis Learning Objectiv...

Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 1 of 13 MICROANATOMY OF ENDOCRINE GLANDS I: Introduction and The Hypophysis Learning Objectives: 1. Describe the microanatomy and functions of the pituitary gland (hypophysis). List the 2 primary and 5 secondary divisions of the pituitary gland. Describe the developmental origins of the two primary divisions of the pituitary gland. For the pars distalis of the adenohypophysis, describe the anatomical location, the histological organization, cell types encountered and their function. Describe the control of pars distalis secretion by the hypothalamus and the role of the vascular pattern in effecting this control. For the pars intermedia of the adenohypophysis, describe the anatomical location, the characteristic histological organization, the cell types found and their function. For the pars nervosa, describe the anatomical location, histological organization, cell types and cell processes encountered and their function. Compare and contrast hypothalamic control of the pars nervosa with that of the pars distalis. 2. Describe the microanatomy and functions of the pineal gland (epiphysis). Describe the anatomical appearance, location, and histological organization of the pineal gland. Describe the cell types in the pineal gland and their function. Describe the histological diagnostic feature of the pineal gland. Compare and contrast the features of the pineal gland with those of other endocrine glands. 3. Describe the microanatomy and functions of the thyroid gland. Describe the anatomical appearance, location, and histological organization of the thyroid gland. Describe the cell types in the thyroid gland and their function. Describe thyroid follicles: a. cells making up the epithelium, b. content of the lumina, c. relationship with the vasculature and d. histological staining characteristics in LM (H&E, PAS). Describe the process of thyroglobulin synthesis by follicular cells and the reuptake and processing of thyroglobulin into its component products as well as their role and/or fate. Describe the location and appearance of parafollicular ("C cells") cells in LM and TEM. Describe the function of parafollicular cells and the mechanism controlling secretion. 4. Describe the microanatomy and functions of the parathyroid gland. Describe the anatomical appearance, location, and histological organization of the parathyroid gland. Describe the cell types in the parathyroid gland and their function. Describe age-related changes in the parathyroid gland. Describe the mechanism controlling secretion of the glandular products in the parathyroid gland. 5. Describe the microanatomy and functions of the adrenal (suprarenal) gland. Describe the histological organization of the adrenal cortex. Compare and contrast the organization and products of the subdivisions of the adrenal cortex. Describe the course of blood vessels and blood flow within the adrenal gland. Describe the histological organization of the adrenal medulla. Describe the cell types and products within the adrenal medulla.  Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 2 of 13 Describe the control mechanism for hormone secretion within the adrenal medulla. Lecture Content Outline I. Introduction to endocrine system II. Pituitary Gland (Hypophysis) A. Relationships B. Components and Divisions C. Development D. Blood Supply E. Hypothalamo-Hypophyseal Tract F. Adenohypophysis G. Neurohypophysis I. Introduction to Endocrine System The endocrine system produces a variety of secretions called hormones that influence numerous other cells and organ systems. The hormones are typically carried through paracrine fashion (b). They can even act on the same cells that produce them, which is called an the vasculature to a distant site but may also autocrine effect (c). Figure 21.1 in Ross and Pawlina, 6th ed., 2011. diffuse and function locally in a paracrine manner (Figure 1). Hormones include three classes of compounds: (1) Peptides, proteins, and glycoproteins. These substances are produced within the rough endoplasmic reticulum, packaged in the Golgi complex, stored in secretory vesicles, and released at the cell surface. (2) Steroids. These compounds are produced by the cooperative action of enzymes located in smooth endoplasmic reticulum and mitochondria on substrates found in lipid droplets. Their transport in the blood stream requires binding to plasma proteins or specialized carriers. Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 3 of 13 (3) Amino acid analogues and derivatives, including catecholamines. In general, protein hormones act on cell surface receptors and exert their physiological effects through second messenger systems whereas steroid hormones enter the target cell and bind to their DNA, causing the production of new proteins and hormone-specific responses. The organization of endocrine organs facilitates the release of their products into blood vessels. These organs have no duct system. Instead, they usually appear as clumps or cords of cells, surrounded by a dense plexus of fenestrated capillaries. The lecture will include the pituitary (hypophysis), pineal, thyroid, parathyroid and adrenal gland. Other endocrine organs include the endocrine pancreas. There are endocrine components in the ovary and testis as well as a diffuse system of endocrine cells within the lining of the respiratory system and the gastrointestinal system. All of these other endocrine elements have been discussed in earlier lectures. I. Pituitary Gland (Hypophysis) A. Relationships 1. Attached at base of the brain. Attaches to brain by the infundibular stalk. 2. Lies within the sella turcica, a depression of the sphenoid bone. Figure 2. Diagram of the pituitary gland and related regions of the hypothalamus. The anterior lobe of the 3. Covered partly by the pituitary (adenohypophysis) consists of the pars distalis, pars tuberalis and pars intermedia. The diaphragma sellae, posterior lobe of the pituitary (neurohypophysis) consists of the infundibulum and pars nervosa. Figure which is part of the dura 21.3b in Ross and Pawlina, 6th ed., 2011. mater. Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 4 of 13 B. Components and Divisions (Figure 2) 1. Adenohypophysis a. Pars distalis b. Pars tuberalis c. Pars intermedia 2. Neurohypophysis a. Pars nervosa b. Infundibulum C. Development (Figure 3) 1. Rathke’s pouch, an invagination of the oral ectoderm, gives rise to the adenohypophysis. 2. Neurohypophysis is formed by a down growth from the floor of the diencephalon (neural ectoderm). Figure 3. Diagram showing the sequential stages (a to c) in the development of the pituitary gland. Modified from Figure 21.4 in Ross and Pawlina, 6th ed., 2011. D. Blood Supply (Figure 4) Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 5 of 13 1. Inferior hypophyseal arteries, branches of the internal carotids, supply mainly the pars nervosa. They give rise to a set of fenestrated capillaries which drain into hypophyseal veins. Figure 4. Diagram of pituitary gland (hypophysis), showing the cell types and blood supply. Slide F9-B1 from Erlandsen and Magney (1985). 2. Superior hypophyseal arteries, which are branches of the internal carotids and the circle of Willis, are the blood supply for the hypophyseal portal system: a. Supply median eminence and infundibulum, forming a looped capillary plexus which drains into portal veins. b. Portal vessels carry major blood supply to the anterior lobe. Most of the anterior lobe has no direct arterial supply, although the outer shell and posterior part of the pars distalis may receive a few direct branches from superior or inferior hypophyseal arteries. c. Forms secondary capillary plexus in pars distalis consisting of wide sinusoids with fenestrated endothelium. Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 6 of 13 3. Venous drainage of pars distalis and pars nervosa is via hypophyseal veins that empty into the cavernous sinus. E. Hypothalamo-Hypophyseal Tract 1. Supraoptic and paraventricular nuclei of the hypothalamus send fibers that end mostly in the pars nervosa, with some terminations in the infundibular stem. 2. Hypothalamic control of the adenohypophysis is mediated through fibers from specific hypothalamic areas (tuberal nuclei), which terminate in the infundibular stalk. These neurons produce releasing/inhibitory factors that are carried to the adenohypophysis via the hypophyseal portal vessels. F. Adenohypophysis or Anterior Pituitary Gland (Figures 5 and 6) 1. Pars distalis a. Comprises about 75% of the pituitary. Figure 5. Adenohypophysis stained with H&E. Slide F9- b. Anastomosing B3 from Erlandsen and Magney (1985). cords of cells separated by fenestrated capillaries with wide lumen. c. Cell types can be distinguished by staining Figure 6. Adenohypophysis stained with trichrome. Slide F9-B4 from Erlandsen and Magney (1985). Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 7 of 13 characteristics (LM, Figures 5 and 6), and granule morphology (EM, Figure 7). i. Acidophils stain eosinophilic in H&E and red in trichrome. They include (Table 1): α. Somatotropes that produce growth hormone (GH; somatotropin), which stimulates growth through insulin-like growth factor 1 (IGF-1). Figure 7. Transmission electron micrograph (TEM) of the pars distalis showing somatotropes, i.e., cells that synthesize and secrete somatotropin (growth factor). In principle, the distinct types of cells can be recognized by the morphology of their secretory granules and other features seen in the TEM. Slide F9-B6 from Erlandsen and Magney (1985). β. Lactotropes or mammotropes that produce prolactin (PRL), which stimulates mammary gland development and production of milk. ii. Basophils stain basophilic in H&E and blue in trichrome. They include (Table 1): α. Thyrotropes that produce thyrotropic hormone (TSH; thyroid stimulating hormone), which stimulates production of thyroid hormone. β. Gonadotropes that produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH), Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 8 of 13 TABLE 1. ADENOHYPOPHYSIS CELL TYPES / HORMONES PRODUCED CELL HYPOTHALAMIC CELL TYPE HORMONES PRODUCED CLASS REGULATORS Growth hormone (GH; somatotropin) Somatotrope GHRH (+), Acidophil Stimulates growth through insulin-like growth (50% of cells) somatostatin (-) factor 1 (IGF-1) Lactotrope or Prolactin (PRL) TRH (+), Acidophil Mammotrope Stimulates mammary gland development and dopamine (-) (20%) production of milk Thyrotrope Thyrotropic hormone (TSH) Basophil TRH (5%) Stimulates production of thyroid hormone Follicle-stimulating hormone (FSH) and Gonadotrope Luteinizing hormone (LH) Basophil GnRH (10%) Stimulates follicle development in ovary and spermatogenesis in testis Proopiomelanocortin which is cleaved into adrenocorticotropic hormone (ACTH) and Corticotrope Basophil beta-lipotrophic hormone CRH (15%) Stimulates production of glucocorticoids and gonadocorticoids in adrenal cortex which stimulates follicle development in ovary and spermatogenesis in testis. γ. Corticotropes that produce proopiomelanocortin, which is cleaved into adrenocorticotropic hormone (ACTH) and beta- lipotrophic hormone. ACTH stimulates production of glucocorticoids and gonadocorticoids in adrenal cortex. iii. Chromophobes do not pick up much stain and remain relatively unstained. α. They are inactive or depleted cells. β. They are acidophils or basophils that have Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 9 of 13 released their granule contents, i.e., are depleted, and thus cannot be identified. d. Regulation of hormone release in the pars distalis by the hypothalamus (Figure 8, Table 1) i. Regulating factors are synthesized by hypothalamic neurons and released by their axon terminals in the median eminence. ii. The regulating factors enter the capillary plexus in the median eminence then travel through the hypophyseal portal veins to a secondary capillary plexus (large sinusoid-like) in the pars distalis. iii. There the regulating factors bind to receptors on the Figure 8. Diagram of pituitary gland (hypophysis), showing the regulation of adenohypophyseal hormone release by the hypothalamus. Slide F9-B1 from Erlandsen and Magney (1985). Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 10 of 13 appropriate cells, causing the cell to release its hormone, if they are releasing factors, or inhibit hormone release, if they are inhibiting factors (see Table 1 for specific regulating factors). iv. A feedback system provides control based on the level of a circulating hormone or its metabolite. These substances act directly on the anterior pituitary (adenohypophyseal) cells or on the hypothalamic cells which regulate them. 2. Pars intermedia (Figure 9) a. Poorly developed in humans, may show a remnant of Figure 9. Trichrome stained section of pituitary gland, showing pars intermedia with colloid-filled cyst in center. Rathke’s Pars nervosa of neurohypophysis is to the left, and pars distalis of adenohypophysis is to right. Slide F9-C7 from pouch. Erlandsen and Magney (1985).Figure 1. Endocrine glands release hormones that influence cells and organ systems. The hormones can go into the blood and act at a distant site in true b. Contains endocrine fashion (a). They can act on neighboring cells in a colloid-filled cysts, chromophobes, and basophils c. Functional significance in humans is largely unknown. G. Neurohypophysis or Posterior Pituitary Gland (Figures 8 and 10) - a neurosecretory site for neurons whose cell bodies are in the supraoptic and Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 11 of 13 paraventricular nuclei of the hypothalamus. 1. Pituicytes are the major cell type of the neurohypophysis; they resemble astrocytic neuroglia (Figure 11). 2. Nerve fibers and terminals of the hypothalamo-hypophyseal tract contain neurosecretory material that may aggregate in dilations called Herring bodies, which are visible by light Pars nervosa microscopy (Figure 11). 3. The hormones are packaged in Figure 10. Diagram of pituitary gland, showing pars vesicles, which nervosa of neurohypophysis on lower right. also contain ATP and a neurophysin. The neurophysin is synthesized along with the hormone, but it is cleaved en route to the axon terminal release site. 4. Separate Figure 11. Light micrograph of pars nervosa. Most of the nuclei populations of belong to pituicytes (example indicated by arrow). The pink round structure is a Herring body (HB). HT: neurons in the hypothalamohypophyseal tract, Cap: capillary. From Chapter 10 of Ovalle and Nahirney (2008). Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 12 of 13 supraoptic and paraventricular nuclei (Figure 12) Figure 12. Diagram of pituitary gland (hypophysis), identifying the neurons in the supraoptic and paraventricular nuclei that produce oxytocin and antidiuretic hormone and release the hormones in the neurohypophysis. Slide F9-B1 from Erlandsen and Magney (1985). produce the following two hormones (see Table 2): a. Oxytocin, which targets uterine smooth muscle and mammary gland myoepithelial cells, resulting in contraction. b. Antidiuretic hormone (ADH), which targets distal tubules and collecting ducts of kidney, which results in increased water resorption from urine. TABLE 2. NEUROHYPOPHYSIS: HORMONES SECRETED NEUROSECRETION TARGET ORGAN TRIGGERING AGENTS Uterine smooth muscle; Oxytocin mammary gland Neural stimuli to hypothalamus myoepithelial cells Distal tubules and collecting Antidiuretic hormone Increase in plasma osmolality and ducts of kidney for (ADH; vasopressin) decrease in blood volume increased water resorption Microanatomy of Endocrine Glands I: Intro and Hypophysis Dr. Avril Genene Holt Page 13 of 13 References: Erlandsen, S.L. & Magney, J.E. Histology Microfiche Atlas, Univ. of Minnesota Press, Minneapolis, 1985 (fiche 9 & 12). Hammersen, F., (Sobotta/Hammersen) Histology A Color Atlas of Cytology, Histology, and Microscopic Anatomy, Urban & Schwarzenberg, Baltimore-Munich, 1980. [Cited in figures as “Sobotta”] Kessel, R.G. & Kardon, R.H. Tissues and Organs: A Text/Atlas of Scanning Electron Microscopy, W.H. Freeman & Co., San Francisco, 1979. Meyer, D. Unpublished Histology Slides, Wayne State University School of Medicine, 1972. Netter, F.H., Atlas of Human Anatomy (6th ed.), Saunders-Elsevier, Philadelphia, 2014, Chapter 1. Rhodin, J.A.G., Histology A Text and Atlas, Oxford University Press, New York, 1974, Chapter 32. Ross, M.H. & Pawlina, W., Histology (6th ed.), Lippincott, Williams & Wilkins, Baltimore, 2011, Chaps. 20 & 21. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 6th ed., Lippincott Williams & Wilkins: Philadelphia, 2011, Ch. 21. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 7th ed., Lippincott, Williams, & Wilkins, WoltersKluwer Health: Philadelphia, 2016, Ch. 21.