Endocrine System Histology PDF

Histology of the Endocrine System I. Introduction A. The endocrine system is composed of “the glands of internal secretion”, meaning these glands do not have ducts, but instead release their secretions into the interstitium (underlying CT). Endocrine glands/organs synthesize and secrete...

Histology of the Endocrine System I. Introduction A. The endocrine system is composed of “the glands of internal secretion”, meaning these glands do not have ducts, but instead release their secretions into the interstitium (underlying CT). Endocrine glands/organs synthesize and secrete specific molecules (hormones) which may affect local cells or may enter the blood or lymph capillaries and bind to target cells located at a greater distance. B. Forms of endocrine tissue 1. discrete individual glands/organs with purely endocrine function 2. scattered masses of endocrine tissue within exocrine glands or other organs 3. isolated cells in lining epithelium of the gastrointestinal tract or respiratory tract II. Pituitary gland (hypophysis cerebri) – the adenohypophysis & the neurohypophysis A. Subdivisions 1. Adenohypophysis a. Pars distalis b. Pars intermedia c. Pars tuberalis 2. Neurohypophysis (hypothalamo-neurohypophyseal tract) a. Pars nervosa b. Infundibulum/infundibular stalk - attaches to the median eminence of the tuber cinereum of the hypothalamus B. Adenohypophysis 1. Pars Distalis - cells are arranged in corded clusters, surrounded by a dense network of congested (filled with blood) sinusoidal capillaries that are the 2nd capillary bed in the hypothalamo-hypophyseal portal system - two subgroups of cells: a. Chromophils - general grouping of cells which have abundant cytoplasm that readily “takes up” (binds) stain 1. Acidophils = affinity for acid dyes (eosin, aniline blue, or biebrich scarlet) a. Somatotrophs - produce and secrete somatotropin (growth hormone (GH)) which targets all cells in the body & also stimulates general body growth b. Lactotrophs (mammotrophs) - produce and secrete prolactin which initiates and maintains lactation 2. Basophils = affinity for basic dyes (hematoxylin) a. Thyrotrophs - secrete thyroid-stimulating hormone (TSH) which both stimulates the synthesis & storage of thyroglobulin and the release of thyroid hormones T3 & T4 by the thyroid gland b. Gonadotrophs - produce and secrete two different hormones: - follicle-stimulating hormone (FSH) binds the follicular epithelial cells/granulosa cells in the female (stimulates the growth of ovarian follicles) & binds the sustentacular/Sertoli cells in the male (stimulates spermatogenesis) - leuteinizing hormone (LH) binds the theca interna cells in the female (stimulates follicle maturation & ovulation and contributes to corpus luteum formation & progesterone secretion) and binds interstitial cells (of Leydig) in the male (promotes secretion of testosterone) c. Corticotrophs - produce and secrete adrenocorticotropic hormone (ACTH) which stimulates steroid hormone biosynthesis in the adrenal gland b. Chromophobes – cytoplasm stains poorly in that it doesn’t uptake the acidic dyes (eosin of the H&E stain, biebrich scarlet & aniline blue of the trichrome) - are thought to be degranulated/resting cells - unfortunately, their cytoplasm does have abundant RNA which will grab the hematoxylin dye and make these cells appear to be basophils; key to ID is the small amount of cytoplasm these cells have compared to the true basophils 2. Pars Intermedia (between pars nervosa and pars distalis) - populated by melanotrophs: somewhat columnar cells with abundant acidophilic, pale-staining cytoplasm and euchromatic nuclei - region has fine CT septa outlining lobules - spherical foci of colloid usually present; purpose unknown - abundant capillaries that are not as engorged as are in the pars distalis - melanotrophs produce POMC (pro-opiomelanocortin), a polypeptide precursor of several hormones, including ACTH, which is converted to alpha-MSH. - melanocyte stimulating hormone (MSH) - induces melanocytes to make melanin pigment - in the healthy equine, ACTH is produced from POMC in the pars distalis; in the unhealthy equine, too much ACTH is produced in the pars intermedia 3. Pars Tuberalis (surrounds infundibulum) - corded clusters of cells with euchromatic nuclei & basophilic cytoplasm; cells have melatonin receptors and are thought to regulate the seasonal reproductive cycle of some of the domestic species - abundant blood vessels as the hypothalamo-hypophyseal portal system passes through the infundibulum - spherical foci of colloid usually present; purpose unknown C. Neurohypophysis (hypothalamo-neurohypophyseal tract) 1) Pars Nervosa - has thick, patchy CT septa, abundant engorged capillaries, neuropil made up of axons of neurons from the hypothalamic nuclei, and abundant glial cells (pituicytes) that are ID’ed by their euchromatic nuclei - origin of the axon terminals are the supraoptic and paraventricular nuclei of the hypothalamus (mid-brain); these neurons synthesize hormones and transport them to these axon terminals, where the hormones are stored as neurosecretory granules - accumulations of these neurosecretory granules in the axon terminals are referred to as neurosecretory bodies (Herring bodies) - hormones stored in the neurosecretory bodies: a) oxytocin - induces contraction of smooth muscle of the uterus during parturition and mammary gland myoepithelium during lactaion, causing “milk letdown” b) vasopressin (antidiuretic hormone (ADH)) - causes vascular smooth muscle to contract and raise systemic blood pressure 2) Infundibulum - narrow "neck" region/connecting stalk which contains the axonal process in transit from the neuronal cell bodies (nuclei) located in the hypothalamus  D. Hypothalamus - ventral portion of the diencephalon (brain) where a number of releasing hormones/ releasing factors are produced and secreted, passing via diffusion into the hypothalamo-hypophyseal portal system to be transported to the pars distalis, where the hormones diffuse out and have an effect upon the activity of the acidophils and basophils E. Establishing cellular identity in the pituitary - routine stains for light microscopy provide limited means of discriminating different cell types - immunostaining for specific cellular products is the only reliable means of delineating individual cell types III. Thyroid – encapsulated, bi-lobed organ A. Stroma – dense irregular CT capsule that gives off septae infiltrated by blood & lymphatic vessels, nerves, and adipose tissue B. Parenchyma – thyroid follicles - follicles are spheres of varying size lined by epithelium and filled with colloid, the gelatinous storage form of this gland’s hormone (thyroglobulin) - the epithelium lining the follicles is simple and ranges from a very low cuboidal to columnar depending upon the activity of the gland 1. Follicular epithelial cells – simple epithelium - in general, low cuboidal when less active and columnar when more active - presence or absence of resorption spaces in colloid adjacent to epithelium is also a good indicator of activity level of the follicular epithelial cells  2. Colloid - generic term for spherical accumulations of a gelatinous material - the colloid of the thyroid follicles is thyroglobulin, an iodinated glycoprotein - in response to TSH, thyroglobulin is BOTH synthesized and secreted into the follicle lumen (to be iodinated) and “scooped up” by pseudopodia on the apical surfaces of the follicular epithelial cells, internalized, and broken down into two thyroid hormones, triiodothyronine or T3 and tetraiodothyronine or T4. T3 (active form) & T4 (called thyroxine) are released across the basement membrane into the underlying interstitium/CT. - T3 affects basal metabolic rate, thermogenesis, & gluconeogenesis C. Parafollicular Cells (C-cells) - their numbers and distribution varies with species - in some species they cannot be ID’ed without special stains because they are mixed in with the follicular epithelial cells - in other species, the cells have abundant basophilic cytoplasm with euchromatic nuclei and are clustered in small groups of cells between thyroid follicles - these cells monitor the concentration of calcium of the interstitial fluids in the thyroid gland & produce calcitonin (thyrocalcitonin) in response to high calcium levels, which acts to decrease blood calcium levels by inhibiting osteoclasts and decreasing resorption by the kidneys IV. Parathyroid Glands - usually occur as two glands per thyroid lobe (so 4 total); the cranial gland is “external” & the caudal gland is “internal” to the capsule of the thyroid gland lobe - encapsulated with abundant septae that contain an extensive capillary network - 2 cell types: 1. Principal cells (chief cells) - make up the bulk of the gland - euchromatic nuclei & small amounts of cytoplasm with varied stain affinity - form anastomotic bi-layered cords that terminate in cell clusters - produce parathyroid hormone (PTH) - acts to increase blood calcium levels by stimulating osteoclastic activity, increasing absorption by the kidneys, and by enhancing intestinal absorption 2. Oxyphil cells - larger cells with bright, acidophilic staining cytoplasm - filled with mitochondria (unknown function) V. Adrenal Glands (suprarenal glands) A. Stroma - thick dense irregular CT capsule with smooth muscle fibers - trabeculae extend from the capsule to the medulla - vasculature & preganglionic sympathetic axons pass through the trabs B. Cortex (outer region; glandular tissue) - cortical cells have euchromatic nuclei, acidophilic cytoplasm (mitochondria), and abundant lipid vacuoles, all indicative of steroid hormone production - three subdivisions, or “zones”: 1. Zona Glomerulosa (ruminants) OR Zona Arcuata (horses, pigs, carnivores) - thin outer zone (24% depth) immediately beneath the CT capsule - ovoid to columnar-shaped cells arranged in tufts (ZG) or arches (ZA) - produce mineralcorticoids (aldosterone & corticosterone) in response to angiotensin II, elevated K+, and ACTH 2. Zona Fasciculata - thickest of the three layers (50% of depth of cortex) - cells arranged in corded columns separated by sinusoids - produce glucocorticoids (cortisol & cortisone) 3. Zona Reticularis - narrow zone (25% depth) adjacent to medulla - irregularly anastomotic cords of cells separated by large sinusoids - both the cell cords & the sinusoids tend to be perpendicular to the cell columns of the ZF - produce glucocorticoids & gonadocorticoids (androgens) C. Medulla (inner region; neuronal tissue) - modified post-ganglionic sympathetic neurons, ie this is a sympathetic ganglion 1. Chromaffin cells – the modified neurons - this is a generic term for cells that belong to the APUD group (amine precursor uptake & decarboxylation) - these chromaffin cells take up & decarboxylate amines to form the biogenic amines epinephrine & norepinephrine 2. Medullary Organization varies with the species: a. carnivores mix their epinephrine & norepinephrine producing cells b. ruminants, pigs, & horses organize the epinephrine producing cells in the outer medulla and the norepinephrine producing cells in the inner medulla VI. Pineal Gland (epiphysis cerebri) A. Pinealocytes - produce serotoninmelatonin which alters the activity of reproductive organs (species dependent) - the gland's activity is regulated by day/night cycles B. Glial cells - typical neuroglial supportive cells VII. Pancreatic Islets (Islets of Langerhan's) - groups of cells scattered throughout the exocrine pancreas - the principal cells of the islets are as follows: A. Alpha cells - synthesize & release glucagon in response to hypoglycemia (low blood sugar) - 15-20% of the islets cells B. Beta cells - synthesize & release insulin in response to hyperglycemia (high blood sugar) - 60-80% of the islets cells C. Delta cells - synthesize & release somatostatin which inhibits glucagon and insulin release - 5-10% of the islets cells