Structure
The pituitary gland has two parts with different origins and different control systems. It is roughly 1 cm across and joined to the floor of the third ventricle by the infundibulum, which passes through an opening in the diaphragma sellae.
| Feature | Adenohypophysis (anterior) | Neurohypophysis (posterior) |
|---|---|---|
| Origin | Rathke pouch, an upgrowth of oral ectoderm from the roof of the primitive mouth | Downgrowth of the diencephalon (neuroectoderm) |
| Parts | Pars distalis, pars intermedia, pars tuberalis | Pars nervosa, infundibular stem, median eminence |
| Tissue | Glandular epithelial cells | Axons and pituicytes (glial cells) |
| Control | Hypothalamic releasing and inhibiting hormones carried in portal blood | Direct nerve impulses in hypothalamic axons |
| Products | GH, prolactin, ACTH, TSH, FSH, LH | Stores and releases ADH (vasopressin) and oxytocin |
The pars tuberalis wraps around the front of the stalk. The pars intermedia is rudimentary in adult humans and may contain small colloid cysts, remnants of the cleft in Rathke pouch.
Relations
The pituitary is surrounded on almost every side by structures that its tumours can compress.
- Above: the diaphragma sellae, a fold of dura roofing the fossa, then the optic chiasm, which lies above and slightly in front of the stalk. Above the chiasm are the hypothalamus and the third ventricle.
- Below: the body of the sphenoid and the sphenoidal sinus, separated from the gland by a thin plate of bone.
- On each side: the cavernous sinus, containing the internal carotid artery and the abducens nerve, with the oculomotor, trochlear, ophthalmic and maxillary nerves in its lateral wall.
- In front: the tuberculum sellae, anterior clinoid processes and anterior intercavernous sinus.
- Behind: the dorsum sellae, posterior intercavernous sinus, and beyond them the basilar artery and pons.
The intercavernous sinuses link the two cavernous sinuses around the gland, forming a venous ring that can bleed during surgery.
Blood supply and innervation
The anterior lobe receives most of its blood indirectly, through a portal system, which is how the hypothalamus controls it. The superior hypophysial arteries arise from the internal carotid arteries just above the cavernous sinus and form a capillary plexus in the median eminence and stalk. Hypothalamic neurons release their hormones into these capillaries. Long and short portal veins carry the blood down the stalk to a second capillary bed in the pars distalis, delivering the hormones straight to the anterior lobe cells.
The inferior hypophysial arteries arise from the internal carotid within the cavernous sinus and supply the posterior lobe directly. Venous blood from both lobes drains into the cavernous and intercavernous sinuses.
Innervation
The posterior lobe is made of the axon terminals of the hypothalamo-hypophysial tract. Neurons in the supraoptic and paraventricular nuclei synthesise ADH and oxytocin, transport them down their axons and release them into capillaries in the pars nervosa. The anterior lobe has no secretomotor nerves; only vasomotor sympathetic fibres from the internal carotid plexus reach it.
Clinical relevance
Pituitary adenomas are the commonest pituitary tumours, and their effects depend on size and on the hormone they secrete. A macroadenoma growing upwards through the diaphragma sellae presses on the optic chiasm from below, damaging the crossing nasal retinal fibres and producing a bitemporal hemianopia. Upper temporal quadrants are usually lost first because the lower fibres of the chiasm are compressed first. A craniopharyngioma, derived from remnants of Rathke pouch, typically presses from above and affects the lower quadrants first.
Other effects of pituitary disease:
- Hormone excess: prolactinoma (the commonest functioning adenoma), acromegaly from GH, and Cushing disease from ACTH.
- Lateral spread: invasion of the cavernous sinus can affect the oculomotor, trochlear and abducens nerves and encase the internal carotid artery.
- Stalk compression: blocks dopamine reaching the anterior lobe, causing a modest rise in prolactin.
- Posterior lobe or stalk damage: causes diabetes insipidus.
- Pituitary apoplexy: sudden haemorrhage into a tumour, with headache, visual loss and ophthalmoplegia.
- Sheehan syndrome: infarction of the enlarged gland after severe postpartum haemorrhage.
Most adenomas are removed by the transsphenoidal route, through the nasal cavity and sphenoidal sinus into the floor of the fossa, avoiding brain retraction. The surgeon must stay between the carotid arteries on each side.