Endocrine Histology
A high-yield Step 1 walkthrough of endocrine histology — pituitary, thyroid, parathyroid, adrenal, and pancreatic islet cell types with their hormones, embryologic origins, and the classic tumors and vignettes that test them.
How the boards test endocrine histology
Endocrine glands are ductless: they release hormones directly into fenestrated capillary networks rather than onto an epithelial surface. Step 1 tests endocrine histology as a four-step chain — identify the cell → name its secretory product → recall its embryologic origin → link it to disease. Two motifs recur: (1) two-part glands whose halves have different origins and functions (pituitary, adrenal), and (2) spatial cell arrangements you can read straight off a slide (central vs peripheral islet cells; the layered adrenal cortex). Lock in the cell-to-hormone map below and most 'identify the labeled arrow' vignettes become automatic.
- Anterior pituitary (adenohypophysis) = Rathke pouch → oral (surface) ectoderm; cells classified by cytoplasmic staining
- Acidophils (pink): somatotrophs → GH; lactotrophs → prolactin
- Basophils (blue): corticotrophs (ACTH), thyrotrophs (TSH), gonadotrophs (FSH/LH)
- Chromophobes: pale, degranulated, typically non-functioning
- Posterior pituitary (neurohypophysis) = neuroectoderm; it stores but does NOT synthesize hormone
- Herring bodies = dilated axon terminals storing ADH + oxytocin, made in the hypothalamus (predominantly ADH → supraoptic, oxytocin → paraventricular nucleus)
- Pituicytes = glial support cells of the posterior lobe
Adrenal cortex + medulla
| Layer | Product | Main stimulus |
|---|---|---|
| Zona glomerulosa | Aldosterone (mineralocorticoid) | Angiotensin II, ↑K⁺ |
| Zona fasciculata (largest; foamy 'spongiocytes') | Cortisol (glucocorticoid) | ACTH |
| Zona reticularis | Androgens (DHEA) | ACTH |
| Medulla (chromaffin cells) | Epinephrine > norepinephrine | Preganglionic sympathetic |
- Adrenal cortex — 'GFR: Salt, Sugar, Sex': Glomerulosa → aldosterone (salt), Fasciculata → cortisol (sugar), Reticularis → androgens (sex); then the medulla → catecholamines (neural crest).
- Anterior pituitary basophils — 'B-FLAT': Basophils = FSH, LH, ACTH, TSH.
- Acidophils (the rest) = GH + prolactin.
Thyroid
- Follicular cells (endoderm-derived): cuboidal epithelium around colloid (stored thyroglobulin) → T3/T4
- Parafollicular C cells: between follicles, neural-crest (ultimobranchial body) → calcitonin; origin of medullary thyroid carcinoma
Parathyroid
- Chief cells → PTH
- Oxyphil cells: eosinophilic, mitochondria-packed, function unclear, ↑ with age
Endocrine pancreas — islets of Langerhans (pale islands in the exocrine sea)
- β cells: central, ~65% → insulin + amylin (amylin = islet amyloid in T2DM)
- α cells: peripheral → glucagon
- δ cells → somatostatin; PP/F cells → pancreatic polypeptide
Vignette: A 42-year-old has episodic pounding headache, palpitations, and diaphoresis with paroxysmal hypertension. Plasma free metanephrines are markedly elevated.
Histology/origin: Tumor of chromaffin cells of the adrenal medulla (neural-crest derived); nests ('Zellballen') stain for chromogranin A. Recall the rule of 10s (10% bilateral, extra-adrenal, malignant, familial).
Dx: Pheochromocytoma. Next best step: Confirm biochemically (plasma/24-h urine fractionated metanephrines) → then CT/MRI to localize. Management: α-blockade first (phenoxybenzamine), THEN β-blockade, then surgical resection. Never β-block first (unopposed α → hypertensive crisis).
Vignette: A 30-year-old woman reports amenorrhea and galactorrhea; exam shows bitemporal hemianopia. MRI reveals a sellar mass and serum prolactin is very high.
Histology/origin: Prolactinoma — the most common pituitary adenoma — arises from lactotrophs (acidophils).
Dx: Prolactinoma. Next best step: Before calling it a tumor, exclude pregnancy, primary hypothyroidism (↑TRH → ↑prolactin), and dopamine-antagonist drugs, then confirm the prolactin level. Management: a dopamine agonist (cabergoline or bromocriptine) is first-line — medical therapy shrinks the tumor; surgery is reserved for drug failure or apoplexy.
Anterior pituitary cells at a glance
| Cell (stain) | Hormones | Classic tumor |
|---|---|---|
| Acidophil (pink) | GH, prolactin | Prolactinoma; acromegaly (GH) |
| Basophil (blue) | ACTH, TSH, FSH, LH | Cushing disease (ACTH) |
| Chromophobe (pale) | none (degranulated) | Non-functioning adenoma |
Practice Histology now
Board-style questions, spaced-repetition flashcards, and a Socratic AI tutor — free to start.