Histology                                                                                                         Stanley Shostak
BioSc 1450                                                                                                      Spring 04

Lectures 16 and 17. Endocrine (ductless gland) System: Hypophysis, Adrenals, etc.

PARACRINE, JUXTACRINE, AUTOCRINE

Diffuse endocrine system = APUD: amine precursor uptake and decarboxylation (paraneurones): derived from neural crest; cells with little rER, much sER, free ribosomes, sm membrane-bound secretory granules (synaptic vesicle-like structures = dense-core vesicles); high uptake amine precursors, ability to decarboxylate

paracrine (enters circulation but operates over short distance), juxtacrine (diffuses through ecm and operates over short distances), autocrine (cell's product operate on cell)

gastrointestinal glands (enteroendocrinocytes): secrete peptide and amine hormones; regulate and coordinate most aspects of gastrointestinal activity in concert with autonomic nervous system

gastrin secreting cells = gastrin or G cells: (found in pylorus of stomach, duodenum and pancreas)
open type: exposed to intestinal tract lumen; cells pyramidal; apex extending to tract lumen; base resting on basement membrane; secretory granules at base; receptive to gastrointestinal contents; e.g., gastrin secreting cell (large, moderately dense secretory granules); somatostatatin secreting cell (smaller, more dense granules)
closed type: small cells; rounded; lack polarity;deep to mucosal surface; receptive to changes in local tissue environment; enterochromaffin cells: stained with chromium salts (fixed with); currently: cells throughout intestinal tract producing multiple amines including serotonin (5-hydroxytryptamine: potent local muscle constrictor) argentaffin cells (silver reducing); argyrophil cells  (silver absorbing)
Respiratory (lower tract) Endocrine Glands scattered (in epithelium or clumps protruding into airways) peptide and amine-secreting endocrine cells; involved in local and autonomically-mediated regulation of respiratory tract function; serotonin, calcitonin, bombesin & lucine-encaphalin
produce amines or peptides with hormone-like activity or suspected neurotransmitters adrenal medullary cells
thryoid C cells
pancreatic endocrine cells
gastrointestinal hormone-secreting cells
enterochromaffin (serotonin) cells
ACTH and MSH cells of pituitary
renin-secreting juxtaglomerular cells of kidney
chemoreceptors of carotid body
mast cells

also neurotransmitters in brain: gastrin, CCK, VIP, substance P, bombesin & serotonin

secretin, CCK, serotonin, enteroglucagon, somatostatin (inhibition insulin and glucagon secretion and secretion of many gastrointestinal hormones), substance P, vasoactive intestinal peptide (VIP), bombesin, gastric inhibitory polypeptide (GIP), motilin & pancreatic polypeptide.


ENDOCRINE GLANDS: source of hormone and target organ at distance; reached through circulation

composed of islands (knots) of secretory (epithelial) cells (parenchyma) with intervening supporting tissue (stroma) rich in blood and lymphatic capillaries; prominent nuclei; prolific cytoplasmic organelles (mitochondria, ER, golgi bodies & secretory vesicles)

Hypophysis = pituitary gland 

mediates non-neural mechanisms; 1 cm in diameter, immediately beneath third ventricle; anterior and posterior parts

pituitary-dependent endocrine glands: thyroid, adrenal cortex and gonads

hypothalamus: controls secretion of all pituitary hormones

neurohypophysis: 2 parts: pars nervosa; infundibulum or neural stalk
pars nervosa = posterior lobe +  pituitary stalk or neural stalk (stem and mediuan eminence) connected to hypothalamus non-myelinated axons of neurosecretory cells supported by pituicytes similar to neuroglial cells (= most nuclei) rich network of small capillaries hormones synthesized by neurons in hypothalamus pass down axons of hypothalamo-pituitary tract through pitutary stalk to posterior pitutary; stored in distended terminal parts of axons; released under nervous control: neurosecretion.
 antidiuretic homrone (ADH) = vasopressin: synthesized mainly by neurons of hypothalamic supraoptic nucleus

oxytocin

synthesized mainly by neurons of paraventricular nucleus of hypothalamus
adenohypophysis: 3 parts: pars distalis; pars intermedium; pars tuberalis anterior pituitary: outgrowth of hypophyseal (Rathke's) pouch (may be present as cleft or cyst-like spaces; hypothalamic control mediated by releasing hormones conducted from median hypothamic eminence by hypothalamic portal veins; except prolactin (under inhibitory control of dopamine)

pars distalis
branching cords of secretory cells with basement membane surrounded by rich network of sinusoidal capillaries supported by delicate stroma containing reticulin and fine collagen fibers. Endothelial lining characteristically fenestrated

chromophils acidophils somatotrophs: most numerous; almost half

mammotrophs (lactotrophs): 2% to 20%; increase in number during pregnancy

 basophils corticotrophs: about 20%; ACTH is polypeptide is split from pro-opimelanocortin (also lipotropins (involved in regulation lipid metabolis), endorphins (endogenous opioids) and various species of MSH

thrytrophs: much less numerous; 5%

gonadotrophs: remaining 5%; probably two distinct types

 chromophobes smallest cell type; contain few cytoplasmic granules; have little affinity for either acidic or basic dyes; probably resting or degranulated chromophils

  pars intermedia (thin zone tissue lying against posterior lobe) and pars tuberalis (surrounds stlk) may synthesizes and secretes melanocyte stimulating hormone (MSH) cells basophilic; form irregular clumps lying btwn pars anterior and pars posterior; tend to spill out into pars posterior; contain secretory granules, proopiomelanocortin synthesized; broken into fragments of MSH, endorphins and lipotrophins cystic spaces fillled with eiosinophilic colloid
 hormones acting directly on non-endocrine tissues   growth hormone (GH)

prolactin (may also be trophic for endocrine tissue of ovary)

antidiuretic homrone (ADH) - posterior pituitary

oxytocin- posterior pituitary

melanocyte stimulating homrone (MSH): pars intermedia


hormones modulating secretory activity of other endocrine glands (prophic hormones)
 

thyroid stimulating hormone (TSH)

adrenocorticotrophic hormone (ACTH)

gonadotrophic hormones

 
follicle stimulating hormone (FSH)

leuteinizing hormone (LH)

 Adrenal = supra-renal adrenal cortex

dense fibrous capsule; supports delicate collagenous framework supporting secretory cells; outer cortex and pale staining inner medulla with prominent central vein

steroid secretory cell: mitochondria with unusual tubular cristae.

secretes steroid hormones; structurally related to common precursor cholesterol; regulated by ACTH; 3 functional classes

mineralocorticoids
glucocorticoids
sex hormones Zonae zona glomerulosa: moderately stained appearance;secretory cells in rounded clumps (irregular ovoid clumps), round, strongly stained nuclei, little cytoplasm; sER, sm lipid droplets; mineralocorticoids (aldosterone); regulation body sodium in renal tubules; blood pressure regulation via renin-angiotensis-aldosterone (controlled by juxtaglomerula apparatus); largely independent of ACTH

zona fasiculata: parallel narrow cords of secretory cells at rt angles to capsule; cells large, abundant cytoplasm, poorly stained, sER, lipid droplets; secretes glucocorticoids (cortisol); raises blood glucose levels and increases cellular synthesis of glycogen; increased breakdown of proteins and liberation of lipid from tissue stores; control by adenohypophyseal adrenal corticotrophic hormone ACTH; also small amâts androgenic sex hormones

zona reticularis: small closely-packed cells in irregular network of branching cords and clumps; cells smaller than those of z. fasciculata, eosinophilic cytoplasm; few lipid droplets, cytoplasm stains more strongly; lipofuscin; probably secretes small quantities androgens and glucocorticoids.
 

 adrenal medulla

secretory cells: large, granular nuclei, extensive, strongly basophilic cytoplasm;

secretes catecholamine hormones: adrenaline (epiephrine; with N-methyl group) and noradrenaline (norepinephrine; lacks methyl group) under direct preganglionic sympathetic nervous control; not secreted continuously; stored in cytoplasmic granules and released in response to nervous stimulation; also secretes enkephalins, opioid peptids; after chrome salt fixation, stored catecholamine granules form brown color®chromaffin cells.

vasculature: subcapsular plexus

 Endocrine Pancreas endocrine cells migrate from duct system and aggregate around capillaries to form isolated clumps of cells scattered throughout exocrine galndular tissue; pancreatic islets; most numerous in tail of gland; numerous fenestrated capillaries; delicate capsule; cells small with poorly stained granular cytoplasm; beta (70%) ®insulin: promotes uptake glucose by most cells (liver, skeletal muscle, adipose tissue)
alpha (20%; toward periphery) ® glucagon: opposite effects
delta ® somatostatin
intestinal peptide (VIP)
pancreatic polypeptide (PP)
enterochromaffin (EC) cells? motilin, serotonin & substance P
Thyroid stimulated by thyroid-stimulating hormone (thyrotropin) from adenohypophysis
lobulated; produces 2 types hormones; enveloped by outer capsule of loose supporting tissue and inner capsule of fibro-elastic tissue; gives rise to fine collagenous septa breaking gland into lobules;
Iodine-containing hormones regulated by TSH; tri-iodothyronine (T3), metabolically acitive form

thyroxine = tetra-iodothyronine (T4), which is converted to T3 in general circulation

thyroid follicles: epi. concentrate idodide from blood by an idodide pump in basal plasma membrane; iodide oxidized to iodine and transported into follicular lumen; combines with throxine residues of thyroglobulin which remain bound to glycoprotein;
thyroglobulin-hormone complex engulfed in cytoplasmic vacuoles; vacuoles fuse with lysosomes; hydrolytic enzymes cleave hormone from thyroglobulin; hormones released in basal cytoplasm; diffuse into capillaries
irregular, spheroidal structures; single layer cuboidal epithelial cells + basement membrane; variable in size; contain homogeneous thyroid colloid or thyroglobulin; active gland: follicles small, colloid diminishes, cuboidal lining high; less active gland: follicles distended with stored colloid, lining cells flattened
 

 Polypeptide hormone calcitonin: lowers blood calcium by inhitibing rate of decalcification of bone by osteoclastic resorption and by stimulating osteoblastic activity; regulated by blood calcium levels; parafollicular cells: = C or clear cells: present in follicle walls but not extending to colloid in interfollicular spaces; synthesize and secrete calcitonin; may constitute discrete endocrine organ = ultimo-branchial body of 4th branchial pouch Parathyroid
embedded in capsule of thyroid gland; thin fibrous capsule; delicate septa divide gland into dense, cord-like masses of secretory cells; regulate serum calcium and phosphate levels via parathyroid hormone = parathormone; secretion stimulated by decrease in blood calcium levels;
raises blood calcium level indirectly; increases rate of osteoclastic resorption
increasing renal tubular reabsorption of calcium ions while inhibiting reabsorption of phosphate
promotion of absorption of calcium by small intestine (involves vitamin D)
 

two types secretory cells:

chief (principal) cells: secrete parathyroid hormone; prominent nucleus and relatively little cytoplasm; actively secreting cells represent about 20% of total; stain strongly; inactive cells stain poorly

oxyphil cells: larger, less numerous, tend to clump among chief cells; smaller densely stained nuclei, strongly eosinophilic cytoplasm containing fine granules; increase in number after puberty; function unknown
 

 Pineal Evagination from posterior part of roof of third ventricle in midline; connected to brain via short stalk containing nerve fibers (from hypothalamus); pineal sand (basophilic extracellular bodies) concentric layers of calcium and magnesium phosphate within an organic matrix; pinealocytes (pineal chief cells): highly modified neurons arranged in clumps and cords surrounded by rich network of fenestrated capillaries; round, granular nuclei with prominent nucleoli and poorly stained cytoplasm; highly-branched processes (seen with silver impregnation methods); contain variety of indole compounds including melatonin and its precursor, serotonin. melatonin: secreted as derivative of serotonin; appears to suppress production of gonadotrophin releasing hormone from hypothalamus. neuroglial cells: resemble astrocytes; dispersed btwn clumps of pinealocytes and capillaries;

unmyelinated axons: myelinated sympathetic nerve fibers enter and ramify as unmyelinated axons through gland
 

 last revised: 03-22-04