Pituitary gland histology 5m anatomy
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I now have everything needed for a thorough 5-mark anatomy/histology answer. Here is the complete response:
Pituitary Gland - Histology & Anatomy (5-mark)
Gross Anatomy
The pituitary gland (hypophysis cerebri) is an ovoid body measuring approximately 8 mm AP x 12 mm transverse x 4 mm high, weighing ~100-500 mg. It sits in the sella turcica of the sphenoid bone, connected to the hypothalamus via the infundibular stalk.
It has two major divisions:
| Division | Parts |
|---|---|
| Adenohypophysis | Pars distalis (anterior lobe), pars intermedia, pars tuberalis |
| Neurohypophysis | Pars nervosa (posterior lobe), infundibular stem, median eminence |
The adenohypophysis constitutes two-thirds of the total volume. It is derived from Rathke's pouch (oral ectoderm); the neurohypophysis is a downgrowth of the diencephalon.
Low-Power Histology: Overall Organization
Low-power H&E of entire pituitary gland. The adenohypophysis (PD, PI, PT) is deeply basophilic/stained, contrasting with the pale, neural pars nervosa (PN). IS = infundibular stalk. (x30 H&E) - Junqueira's Basic Histology, 17e
Adenohypophysis (Anterior Pituitary)
Pars Distalis (accounts for 75% of the adenohypophysis)
Composed of cords and nests of epithelial cells supported by a delicate reticular connective tissue skeleton, interspersed with wide-lumen fenestrated sinusoidal capillaries (the secondary portal plexus). A thin fibrous capsule encloses it.
Classic staining (H&E / Azan) divides cells into two broad categories:
1. Chromophils - secretory cells with hormone-containing cytoplasmic granules:
- Acidophils (α-cells) - ~40% of cells; stain orange/pink with eosin or Orange G
- Basophils (β-cells) - ~10% of cells; stain blue/purple; PAS-positive (glycoproteins)
2. Chromophobes (γ-cells) - ~50% of cells; sparse cytoplasm, poorly staining; represent either stem cells (precursors) or degranulated secretory cells of any type.
High-power H&E of pars distalis showing acidophils (A), basophils (B), chromophobes (C), and sinusoids (S). - Junqueira's Basic Histology, 17e
Immunohistochemical Cell Classification (modern)
| Cell Type | Staining | Hormone(s) | Location in Pars Distalis |
|---|---|---|---|
| Somatotrophs | Acidophilic | GH (somatotropin) | Lateral parts |
| Mammotrophs (Lactotrophs) | Acidophilic | Prolactin (PRL) | Dominant in pregnancy |
| Mammosomatotrophs | Acidophilic | GH + PRL | Variable |
| Corticotrophs | Basophilic | ACTH, β-LPH, β-endorphin (from POMC) | Central/anterior |
| Thyrotrophs | Basophilic | TSH | Anterior, central |
| Gonadotrophs | Basophilic | FSH + LH | Pars tuberalis-rich |
| Folliculostellate cells | Supporting cells | Growth factors, cytokines | Throughout |
Electron microscopy: cells are identified by membrane-enclosed secretory granules (60-900 nm diameter), the size varies by hormone type; secretion is by exocytosis.
Pars Tuberalis
A funnel-shaped extension surrounding the infundibulum. Rich in blood vessels and gonadotrophs (FSH/LH). Contains melatonin receptors in some species.
Pars Intermedia
A narrow zone between pars distalis and pars nervosa. Contains corticotrophs, chromophobes, and characteristically colloid-filled cysts (remnants of Rathke's pouch lumen). Corticotrophs here cleave POMC differently to produce primarily MSH, γ-LPH, and β-endorphin.
Pars intermedia (PI) between pars distalis (PD) and pars nervosa (PN), with colloid-filled cysts (C) and invading basophils (B). (H&E) - Junqueira's Basic Histology, 17e
Neurohypophysis (Posterior Pituitary - Pars Nervosa)
Unlike the adenohypophysis, the posterior pituitary does not synthesize hormones itself. It consists of:
- ~100,000 unmyelinated axons from hypothalamic supraoptic and paraventricular nuclei
- Pituicytes - modified glial cells (astrocyte-like), the most abundant cell type
- Fenestrated capillaries for hormone uptake
Key histological feature: Neurosecretory (Herring) bodies - faintly eosinophilic swellings/dilatations along axons, visible by LM, containing membrane-bound granules of ADH or oxytocin bound to carrier proteins (neurophysin I and II).
Pars nervosa showing pituicytes (P), Herring bodies / neurosecretory bodies (NB), and capillaries (C). (x400 H&E) - Junqueira's Basic Histology, 17e
| Hormone | Source Nucleus | Carrier Protein | Main Function |
|---|---|---|---|
| ADH (Vasopressin) | Supraoptic | Neurophysin II | Increases renal collecting duct water permeability |
| Oxytocin | Paraventricular | Neurophysin I | Uterine contraction; milk ejection |
Blood Supply
Hypothalamic-hypophyseal tract (a) and portal vascular system (b). - Junqueira's Basic Histology, 17e
- Superior hypophyseal arteries (branches of internal carotid) supply the median eminence and infundibular stalk, forming a primary capillary plexus
- Portal veins carry hypothalamic releasing/inhibiting hormones to the adenohypophysis via a secondary capillary plexus (the portal system)
- Inferior hypophyseal arteries supply the neurohypophysis directly
- Drained by hypophyseal veins
The adenohypophysis has no direct arterial supply - it depends entirely on the hypothalamic-hypophyseal portal system.
Quick Summary for 5 Marks
| Point | Key Fact |
|---|---|
| 1. Two parts | Adenohypophysis (Rathke's pouch) + neurohypophysis (diencephalon) |
| 2. Adenohypophysis cells | Acidophils (GH, PRL), basophils (ACTH, TSH, FSH, LH), chromophobes (~50%) |
| 3. Pars intermedia | Colloid cysts + MSH-producing corticotrophs |
| 4. Neurohypophysis | Pituicytes + Herring bodies; stores/releases ADH + oxytocin |
| 5. Blood supply | Portal system (no direct artery to adenohypophysis); inferior arteries to neurohypophysis |
Sources: Junqueira's Basic Histology 17e, p. 1002-1017 | Color Atlas of Human Anatomy Vol. 2 (Thieme), p. 566-568 | Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 1, p. 660-661
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