The Endocrine System

BIO 408

Histology

Dr. D. L. Daley

The Endocrine System

n Endocrine glands lack ducts that are present in exocrine glands

n The products of endocrine glands are hormones that are secreted directly into the blood

n Many endocrine glands are discrete organs others like the pancreas have endocrine and exocrine function

Function of Hormones

n Hormones are regulatory molecules and include such chemicals as amines, steroids, polypeptides and glycoproteins

n Interactions between various hormones produce effects that may be synergistic, permissive or antagonistic

Pituitary Gland

n Also called the hypophysis

n Two subdivisions

n Adenohypophysis or anterior pituitary

n Pars distalis (pars anterior)

n Pars intermedia

n Pars tuberalis

n Neurohypophysis or posterior pituitary

n Median eminence

n Infundibulum

n Pars nervosa

Pituitary Gland

Blood Supply of the Pituitary

n Right and left superior hypophyseal arteries - supply the pars tuberalis & infundibulum

n Also form a capillary network, primary capillary plexus in the median eminence

n Right and left inferior hypophyseal arteries - primarily supply the posterior lobe & a few branches to the anterior lobe

Blood Supply of the Pituitary

n Hypophyseal portal veins drain the the primary capillary plexus of the median eminence - which delivers blood too the secondary capillary plexus located in the pars distalis

n The capillaries of both plexuses are fenestrated

n Hypothalamic neurosecretory hormones, manufactured in the hypothalamus & stored in the median eminence - enter the primary capillary plexus - drained by the hypophyseal veins

Blood Supply of the Pituitary

n The hypophyseal portal veins connect to the secondary capillary plexus in the anterior lobe

n Here the neurosecretory hormones are released to stimulate or inhibit the parenchymal cells

n Thus the hypothalamic hormones control the pars distalis

Adenophysis

n Pars Distalis (anterior lobe) - covered by a fibrous capsule

n Composed of cords of parenchymal cells that are surrounded by reticular fibers

n These are surrounded by large sinusoidal capillaries of the secondary capillary plexus

n Parenchymal cells of the pars distalis

n Chromophils - acidophils or basophils

n Chromophobes - no affinity for dyes

Chromophils

n Acidophils - most abundant cell type in the pars distalis - large secretory granules - stain orange to red with eosin

n Somatotrophs - secrete somatotropin or growth hormone

n Mammotrophs - arranged as individual cells rather than clusters - produce prolactin (promotes lactation after birth)

Chromophils

n Basophils - stain blue with basic dyes - located in the periphery of the pars distalis

n Cortiotrophs - round to ovoid - secrete ACTH & lipotrophic hormone

n Thyrotrophs - deeply embedded within in cords of the parenchymal cells - produce TSH

n Gonadotrophs - round cells - secrete FSH & LH (was called ICSH in males)

Chromophobes

n Groups of small weakly staining cells

n May be degranulated chromophils

Pars Intermedia & Tuberalis

n Pars Intermedia - between the pars distalis and the pars nervosa

n Remnants of Rathke’s pouch (an invagination of the oral ectoderm during development - gave rise to the ant. pituitary

n Pars Tuberalis - surrounds the hypophyseal stalk and is composed of cuboidal to low-columnar basophilic cells

Neurohypophysis

n Posterior pituitary

n Unmyelinated axons of neurosecretory cells from the supraoptic and paraventricular nuclei of the hypothalamus enter the posterior pituitary to terminate in the vicinity of the capillaries

n Form the hypothalamohypophyseal tract

n Produce ADH (vasopressin) and oxytocin

Pars Nervosa

n Receives the terminals of the neurosecretory of the hypothalamohypophyseal tract

n The axons are supported by glia-like pituicytes (25% of the volume of the PN)

n EM shows one population of axons containing granules with vasopressin and a second population of axons that contain oxytocin

n Chrome-alum hematoxylin staining reveals blue-black distensions of the axons - Herring bodies (accumulation of granules)

Thyroid Gland

n Located in the anterior portion of the neck - just inferior to the larynx, anterior to the junction of the thyroid and cricoid cartilages

n Composed of a right lobe and left lobe - connected across the midline by an isthmus

n Mat have an additional pyramidal lobe that ascends from the left side of the isthmus

n Secretes thyroxine (T4), triiodothyronine (T3) and calcitonin

n Surrounded by slender capsule of dense connective tissue

Thyroid: Cellular Organization

n The hormones are stored in the lumina of follicles - cyst-like structures ranging from 0.2 to 0.9 mm in diameter

n Follicles are composed of cuboidal epithelium surrounding a central colloid-filled lumen

n Each follicle can store several weeks’ supply of hormone within the colloid

n T3 and T4 are stored in the colloid bound to a large glycoprotein called thyroglobulin

n Hormones released by exocytosis

Thyroid: Cellular Organization

n Follicular cells (principal cells)

n Squamous to low columnar in shape

n Ovoid nucleus two two nucleoli

n Numerous short villi extend into the colloid

n Numerous small vesicles dispersed throughout the cytoplasm - probably contain thyroglobulin

n Iodination of the thyrosine residues of T3 and T4 occurs in the follicles at the colloid-follicular cell interface

Thyroid: Cellular Organization

n Parafollicular Cells (Clear cells, C cells)

n Pale staining and lie singly or in clusters among the follicular cells

n Larger that follicular cells

n EM reveals dense secretory granules - contain calcitonin (thyrocalcitonin)

Parathyroid Glands

n Usually four in number - located on the posterior surface of the thyroid gland

n Each is enveloped by a thin collagenous connective tissue capsule

n They produce PTH - parathyroid hormone

n Acts on the bone, kidneys, and the intestines in maintaining the optimal concentrations of calcium within the blood and interstitial tissue fluid

Parathyroid Cellular Organization

n Chief cells - major functional cell

n Secrete PTH

n Oxyphil cells - function unknown

n Probably an inactive phase of a chief cells

n Also intermediate cells - also probably an inactive phase of a chief cell

Suprarenal (Adrenal) Glands

n Located at the superior poles of the kidneys and are embedded in adipose - rich blood supply

n Right and left are nor mirror images

n Right is pyramid-shaped and sits directly on top of the right kidney

n Left is more crescent-shaped and lies along the medial border of the left kidney

n Each gland has a suprarenal cortex (outer yellowish portion- glucocorticoids, mineralcorticoids and androgens) and an inner dark suprarenal medulla (epinephrine & norepinephrine)

Suprarenal Cortex

n Three zones - three classes of steroids

n Zona Glomerulosa - outer ring of capsular parenchymal cells - 13% by volume

n Small columnar cells compose the zone - arranged in cords and clusters

n Angiotensin II and ACTH stimulate these cells to produce aldosterone and deoxycorticisterone - both mineral corticoids

Suprarenal Cortex

n Zona Fasiculata - intermediate layer of cells - largest layer - 80% by volume

n Contains sinusoidal capillaries - arranged longitudinally between columns of parenchymal cells (polyhedral cell shape)

n Cell contain large lipid droplets - lipid loss in fixation - see large vacuoles - cells called spongiocytes

n Secrete glucoccorticoids - cortisol and corticosterone

n These function in the control of carbohydrate, fat and protein metabolism

Suprarenal Cortex

n Zona Reticularis - innermost layer - 7% of gland volume

n Cells are similar to those of the Zona fasciculata (spongiocytes) - but smaller with fewer lipid droplets

n Secrete androgens - principally dehydroepiandrosterone and some androstenedione - weak masculinzing hormones - negligible effects under normal conditions

n Also secrete small amounts of gulcocorticoids

Suprarenal Medulla

n Two populations of cells - chromaffin cells and sympathetic ganglion cells (scattered throughout the connective tissue)

n Chromaffin cells - large epithelioidal cells - arranged in clusters or short cords

n Contain intensely staining granules

n Synthesize - epinephrine and norepinephrine

n Postganglionic sympathetic fibers lack dendrites and axons

Pineal Gland

n Cone shaped, midline projection of the roof of the diencephalon, with a recess of the third ventricle extending into the stalk that is attached to it

n Parenchymal cells - primarily pinealocytes and interstitial cells

n Secretions influenced by the light and dark periods of a day

Cells of the Pineal Gland

n Pinealocytes - parenchymal cells responsible for the secretion of serotonin (during the day) and melatonin (at night)

n Interstitial cells (astroglia-like cells)

n Increase during short photoperiods and are reduced during when the gland is actively secreting