🧠 Pituitary Tumours
Anatomy, classification, functional syndromes, mass effects, pituitary apoplexy, trans-sphenoidal surgery, and the MEN1 connection.
Anatomy & Embryology of the Pituitary Gland
Understanding pituitary tumour presentations requires knowing the gland’s geography intimately. The clinical signs of a growing pituitary mass are entirely dictated by which surrounding structure gets compressed first. The local anatomy is the examination curriculum.
Location
The pituitary gland sits in the hypophyseal fossa — a saddle-shaped bony recess in the body of the sphenoid bone called the sella turcica (Latin: “Turkish saddle”). The roof is formed by a dural fold, the diaphragma sellae, through which the pituitary stalk (infundibulum) passes to connect the gland to the hypothalamus above.
The Two Lobes — Different Embryological Origins
The pituitary is not one organ — it is two embryologically distinct structures fused together, which explains why tumours arising from each behave completely differently:
| Lobe | Proportion | Embryological Origin | Key Tumours |
|---|---|---|---|
| Anterior lobe (Adenohypophysis) |
75% of gland | Ectodermal outpouching of the primitive oral cavity — Rathke’s pouch. Migrates superiorly to fuse with the downgrowth from the brain. | All pituitary adenomas (prolactinoma, GH-oma, ACTH-oma, TSH-oma). Craniopharyngioma from Rathke’s pouch remnants. |
| Posterior lobe (Neurohypophysis) |
25% of gland | Downward diverticulum of the floor of the diencephalon — neural tissue, not glandular. It stores and releases hormones made in the hypothalamus. | Does not form adenomas. Damage causes Diabetes Insipidus (loss of ADH storage/release). |
Rathke’s Pouch — Why It Matters
Rathke’s pouch is the ectodermal outpouching that migrates upward from the roof of the primitive mouth to form the anterior pituitary. Remnants of this tract can persist along its migratory path — particularly at the pituitary stalk and floor of the third ventricle.
These remnants can proliferate into a Craniopharyngioma — a histologically benign but behaviourally aggressive tumour arising from squamous epithelial remnants. It typically affects children and young adults, grows in the suprasellar region, and characteristically contains calcification, cysts, and cholesterol (“motor oil”) fluid on imaging. Despite its benign histology, it causes devastating compression of the optic chiasm, hypothalamus, and pituitary stalk.
The Cavernous Sinus — Know Your Contents
The cavernous sinuses sit on either side of the sella. Their contents are the most important anatomical memory in this topic — lateral invasion by a pituitary tumour will damage these structures in order of vulnerability:
CN VI — Why It Is Hit First by Lateral Pituitary Invasion
The abducens nerve (CN VI) runs through the cavernous sinus in the most medial position — closest to the pituitary. When a pituitary tumour invades the cavernous sinus laterally, CN VI is the first nerve encountered and the first to be damaged. The result is a lateral rectus palsy — the eye cannot abduct, causing a convergent squint (esotropia) and diplopia on lateral gaze.
The other nerves (CN III, IV, V1, V2) run in the lateral wall of the cavernous sinus and require more extensive invasion before being affected.