PITUITARY (Hypophysis):
• Adenohypophysis (anterior): epithelial; pars distalis (chromophobes + acidophils + basophils — secrete various tropic hormones).
• Neurohypophysis (posterior): nervous tissue + axons from hypothalamus storing ADH/oxytocin.

THYROID: Spherical follicles filled with colloid; cuboidal follicular cells (T3/T4) + parafollicular C cells (calcitonin).

PARATHYROID: Tightly packed chief cells (PTH) + oxyphil cells; small.

ADRENAL:
• Cortex (3 zones): glomerulosa (mineralocorticoids), fasciculata (glucocorticoids), reticularis (sex steroids).
• Medulla: chromaffin cells (catecholamines).

PINEAL: Pinealocytes (melatonin) + glia + 'brain sand' (corpora arenacea).

ENDOCRINE GLAND HISTOLOGY

I. PITUITARY (Hypophysis):
Located in sella turcica (pituitary fossa) of basisphenoid; suspended from hypothalamus by infundibulum.

Two distinct origins, two distinct histologies:

A. Adenohypophysis (epithelial, from oral ectoderm/Rathke's pouch):
• Pars distalis (largest, anterior). Cells:
– Chromophobes (~50%) — pale, no granules; resting/depleted cells.
– Acidophils (eosinophilic granules):
• Somatotrophs → Growth hormone (GH).
• Mammotrophs (lactotrophs) → Prolactin (PRL).
– Basophils (basophilic granules):
• Corticotrophs → ACTH.
• Thyrotrophs → TSH.
• Gonadotrophs → FSH + LH.
• Pars intermedia (between distalis + nervosa) — MSH.
• Pars tuberalis — wraps infundibulum.

B. Neurohypophysis (nervous, downgrowth from diencephalon):
• Pars nervosa: NOT a typical gland → it stores ADH (vasopressin) + oxytocin made in hypothalamic supraoptic + paraventricular nuclei. Hormones travel down axons of hypothalamic neurons; Herring bodies = swollen storage axons.
• Pituicytes = supporting glia (modified astrocytes).

II. THYROID GLAND:
• Round/oval follicles (~0.2 mm) filled with eosinophilic colloid (thyroglobulin).
• Lined by follicular cells (cuboidal when active, squamous when inactive) — synthesise T3/T4. Iodine essential.
• Parafollicular (C) cells — pale, larger, between follicles or scattered in follicle epithelium; secrete calcitonin (lowers blood Ca²⁺).

III. PARATHYROID GLANDS:
• Tiny (rice grain).
• Two main cell types:
– Chief (principal) cells: small, round, light-staining; secrete PTH (raises blood Ca²⁺).
– Oxyphil cells: larger, eosinophilic; many mitochondria; appear at puberty; function uncertain.
• Adipose tissue increases with age in some species.

IV. ADRENAL (SUPRARENAL) GLAND:
Two embryologically distinct parts in one capsule:

A. Cortex (mesodermal; 80-90%):
• Zona Glomerulosa (outer; thin): cells in arches/glomeruli; secrete mineralocorticoids (aldosterone).
• Zona Fasciculata (middle; thickest): straight columns of pale 'spongiocyte' cells with abundant lipid droplets (steroid precursors); secrete glucocorticoids (cortisol).
• Zona Reticularis (inner): irregular network of smaller cells; secrete sex steroids (DHEA, androgens).
Mnemonic: 'GFR' from out to in.

B. Medulla (neuroectodermal — modified sympathetic ganglion):
• Chromaffin cells (named for their reaction with chromium salts) — secrete catecholamines; adrenaline:noradrenaline proportions vary by species and physiologic state. Innervated directly by sympathetic preganglionic fibres.
• Scattered ganglion cells.
• Adrenal venous drainage is species-specific and should not be generalized from a human right-versus-left pattern.

V. PINEAL GLAND (Epiphysis):
• Small (~5 mm); attached to roof of 3rd ventricle (epithalamus).
• Pinealocytes: pale, neuron-like, in clusters; secrete MELATONIN (regulates day/night cycle, seasonal reproduction).
• Glial cells (astrocytes) interspersed.
• Corpora arenacea ('brain sand'): calcified concretions; increase with age.

VI. ENDOCRINE PORTIONS WITHIN OTHER ORGANS:
• Pancreatic islets (covered earlier).
• Atrial myocardium (ANP).
• Kidney (renin, EPO).
• GI tract (gastrin, secretin, CCK).
• Gonads (testosterone, oestrogen, progesterone).
• Placenta/endometrial cups produce species- and stage-specific hormones: hCG is primate-derived, eCG is equine, and placental progesterone production varies among domestic species.