Describe the histology of the pituitary, thyroid, parathyroid and suprarenal glands.

Pituitary Gland

• Mnemonic for acidophils — "GPA" (Growth hormone, Prolactin are Acidophil); for basophils — "B-flat" (FSH, LH, ACTH, TSH)
• Herring bodies are accumulations of neurosecretory material in the axon terminals of the posterior lobe — the diagnostic feature
• The posterior pituitary makes nothing; it stores and releases ADH and oxytocin made in the hypothalamus

Thyroid Gland

• The only endocrine gland that stores its hormone extracellularly
• Follicles lined by simple cuboidal epithelium and filled with colloid (thyroglobulin), which is eosinophilic and PAS positive
• Active gland — tall columnar cells, little colloid, peripheral resorption vacuoles
• Inactive gland — flat squamous cells, abundant dense colloid
• Parafollicular (C) cells — larger, pale, between the follicles or within the follicular epithelium but not touching the colloid; secrete calcitonin; derived from the neural crest

Parathyroid Gland

• Chief (principal) cells — small, pale, most numerous; secrete parathyroid hormone
• Oxyphil cells — larger, eosinophilic, packed with mitochondria; appear after puberty; function uncertain
• Fat cells appear in the stroma with age, and are a useful clue to the identity of the gland

Suprarenal Gland

• Mnemonic — "GFR — Salt, Sugar, Sex; the deeper you go the sweeter it gets"
• The medulla is a modified sympathetic ganglion, its cells being postganglionic neurones without axons — which is why they are supplied by preganglionic fibres
• The cortex is mesodermal, the medulla neural crest — two entirely different organs in one capsule

Islets of Langerhans

• The islets are pale-staining islands in the darker exocrine acinar tissue — the feature that identifies the pancreas at a glance
• They are richly vascular, and the blood flows from the centre outward, so insulin reaches the alpha cells first and restrains them

General Features of Endocrine Glands

• Ductless — secretion passes directly into the blood
• Richly vascular, with fenestrated capillaries lying close to the cells
• Cells arranged in cords or follicles rather than acini
• The thyroid is the only one storing hormone extracellularly, as colloid
• Steroid-secreting cells (suprarenal cortex, Leydig, corpus luteum) have abundant smooth endoplasmic reticulum, mitochondria with tubular cristae, and lipid droplets
• Peptide-secreting cells have abundant rough endoplasmic reticulum, a large Golgi, and membrane-bound secretory granules
• That ultrastructural difference identifies what a cell secretes even without knowing the organ

Applied Aspects

• Pituitary adenoma — classified by the hormone secreted; prolactinoma is the commonest. Upward extension compresses the optic chiasma, giving bitemporal hemianopia
• Acromegaly from a somatotroph (acidophil) adenoma; Cushing disease from a corticotroph (basophil) adenoma
• Graves disease — tall columnar follicular cells with scalloped colloid and lymphoid aggregates; the histology reflects intense stimulation by TSH-receptor antibody
• Medullary carcinoma of the thyroid arises from the parafollicular C cells, secretes calcitonin, and contains amyloid; part of men-2
• Phaeochromocytoma — a tumour of the chromaffin cells of the medulla; episodic hypertension, headache, palpitations and sweating
• Type 1 diabetes — autoimmune destruction of the beta cells with insulitis; in type 2 the islets show amyloid deposition