4.3 Endocrine System Anatomy

Endocrine System Anatomy

Endocrine system anatomy is 10% of General Anatomy on the NBCE Part I test plan. Official bullets are endocrine glands and tissues, development, and histology. Hormone receptors, second messengers, and feedback are Endocrine Physiology; metabolic effects of insulin, cortisol, and thyroid hormone also appear in Chemistry. Here the scoring facts are which pouch makes which gland, which zone makes which steroid, and which vein drains which adrenal.

Endocrine Glands and Tissues

Pituitary gland (hypophysis)

The pituitary sits in the sella turcica of the sphenoid, roofed by diaphragma sellae (dura) with the infundibulum passing through. Cavernous sinuses lie laterally: internal carotid artery and CN VI internally; CN III, IV, V1, and V2 in the lateral wall. An expanding pituitary mass can compress the optic chiasm above (bitemporal hemianopia is the pathway fact) and cavernous nerves laterally.

Adenohypophysis (anterior pituitary): pars distalis, pars tuberalis, pars intermedia. It is glandular, supplied by a hypophyseal portal system: superior hypophyseal arteries (from internal carotid) form a primary capillary plexus in the median eminence; long portal veins carry hypothalamic releasing and inhibiting hormones to a secondary plexus in the pars distalis. Acidophils: somatotrophs (GH) and lactotrophs (prolactin). Basophils: corticotrophs (ACTH), thyrotrophs (TSH), gonadotrophs (FSH/LH)—the B-FLAT mnemonic. Chromophobes are degranulated or supporting cells. Pars intermedia may retain Rathke cleft remnants.

Neurohypophysis (posterior pituitary): infundibulum and pars nervosa. It is neural tissue. Magnocellular neurons in the supraoptic and paraventricular nuclei synthesize ADH (vasopressin) and oxytocin; axons run the hypothalamo-hypophyseal tract and store peptide in Herring bodies. Pituicytes are the local glia. Inferior hypophyseal arteries supply the pars nervosa; posterior-lobe hormones are released into systemic capillaries, not into the portal system.

Pineal gland

The pineal is an epithalamic midline structure over the posterior third ventricle / superior colliculi. Pinealocytes secrete melatonin in darkness (sympathetic fibers from the superior cervical ganglion via the nervus conarii). Corpora arenacea (brain sand) are calcified concretions and a midline radiographic landmark. There is no blood–brain barrier here; capillaries are fenestrated.

Thyroid gland

Two lobes joined by an isthmus over tracheal rings 2–4, at about C5–T1. A pyramidal lobe (remnant of the thyroglossal duct) may ascend from the isthmus toward the hyoid. A true capsule plus a false capsule from pretracheal fascia bind the gland to the trachea, so the thyroid moves with swallowing. Posteromedial ligament of Berry tethers each lobe to the cricoid and upper trachea—and is where the recurrent laryngeal nerve is at risk.

Follicular cells make T3/T4; parafollicular C cells (between follicles, not lining the colloid) make calcitonin. Lymph drains to prelaryngeal (Delphian), pretracheal, paratracheal, and deep cervical nodes.

Parathyroid glands

Usually four, on the posterior thyroid inside the pretracheal fascia. Superior glands are more constant near the inferior thyroid artery / cricothyroid junction. Inferior glands are more variable because they migrate with the thymus and may sit in the superior mediastinum. Chief (principal) cells secrete PTH. Oxyphil cells appear after childhood, are mitochondria-rich, and have unclear function. Arterial supply is chiefly the inferior thyroid artery for all four glands.

Adrenal (suprarenal) glands

Retroperitoneal on the superomedial poles of the kidneys, inside renal fascia but separated by a septum. The right gland is pyramidal and abuts the IVC; the left is crescentic and related to the stomach and pancreas. Each has a cortex (mesoderm, yellow, steroidogenic) and a medulla (neural crest, gray-red, catecholamines).

Arteries: superior suprarenal (from inferior phrenic), middle suprarenal (from aorta), inferior suprarenal (from renal). A single vein on each side: right adrenal vein → IVC, left adrenal vein → left renal vein. Cortical sinusoids drain through the medulla, so medullary chromaffin cells are bathed in cortisol—the anatomic reason phenylethanolamine N-methyltransferase (norepinephrine → epinephrine) is induced here. Chromaffin cells are modified postganglionic sympathetic neurons that lost their axons; the preganglionic splanchnic fiber is the secretomotor nerve.

Pancreatic islets and gonads

Islets of Langerhans are scattered endocrine clusters, more numerous in the tail, draining into the portal vein so insulin and glucagon meet the liver first. Cell types: beta (insulin, majority, often central in rodents; more mixed in humans), alpha (glucagon, often peripheral), delta (somatostatin), PP/F (pancreatic polypeptide), epsilon (ghrelin). Exocrine acini and ducts are not endocrine but share the same endodermal origin and blood supply (celiac and SMA via pancreaticoduodenal and splenic arteries).

Testis: Leydig cells (LH, testosterone) in the interstitium; Sertoli cells (FSH, inhibin, AMH, androgen-binding protein) in the tubule. Ovary: theca interna (LH, androgens), granulosa (FSH, aromatase → estradiol), corpus luteum (progesterone and estradiol). These are the anatomic cell assignments; the axes themselves are Physiology.

Diffuse neuroendocrine system

DNES (APUD) cells are scattered in gut, airway, and other epithelia and share secretory-granule histology with classic glands. High-yield named cells: gastric G cells (gastrin, antrum), ECL cells (histamine), intestinal I cells (CCK), S cells (secretin), K cells (GIP), L cells (GLP-1), D cells (somatostatin), enterochromaffin cells (serotonin). Pulmonary Kulchitsky cells are the airway members (the cell of origin of bronchial carcinoid). C cells, parathyroid chief cells, and adrenal chromaffin cells are often grouped with this family on exam lists even though they live in discrete glands.

Named glands at a glance (origin, location, and the vessel Part I actually asks):

Strap muscles (sternohyoid, sternothyroid, omohyoid) cover the thyroid anteriorly; the recurrent laryngeal nerves run in the tracheoesophageal grooves immediately posterior. A midline pyramidal lobe and a lingual thyroid are the same thyroglossal path at opposite ends. The cavernous ICA and CN VI are the first neighbors of a laterally expanding pituitary, while the optic chiasm is the first neighbor of a superiorly expanding one.

Development

Adenohypophysis is an upgrowth of stomodeal (oral) ectoderm—Rathke pouch—that loses its connection to the pharynx. Residual connection can persist as a pharyngeal hypophysis or as a Rathke cleft cyst. Neurohypophysis is a downgrowth of the floor of the diencephalon (neuroectoderm) that remains attached by the infundibulum. Pineal is a dorsal diencephalic evagination (epithalamus).

Thyroid follicular epithelium arises as a midline endodermal diverticulum from the foramen cecum of the tongue. It descends in the neck anterior to the hyoid as the thyroglossal duct, which should obliterate. A thyroglossal duct cyst is therefore midline and moves with tongue protrusion; an ectopic thyroid may sit anywhere along that path, including a lingual thyroid. C cells are not from the thyroglossal diverticulum: they come from the ultimobranchial body (fourth/fifth pouch complex), populated by neural crest, and join the lateral thyroid lobes.

Pharyngeal pouches (endoderm) that Part I mixes with endocrine anatomy:

Because pouch-3 structures migrate with the thymus, an inferior parathyroid can be found in the anterior mediastinum. Superior (pouch-4) glands stay near the thyroid. DiGeorge sequence (third and fourth pouch field defect) therefore combines hypoparathyroidism, thymic aplasia, and conotruncal cardiac anomalies—anatomy that pathology will reuse.

Adrenal cortex arises from intermediate mesoderm (celomic epithelium of the urogenital ridge) as fetal and then adult cortical zones. Adrenal medulla is neural crest that invades the cortical mass and differentiates into chromaffin cells. Organ of Zuckerkandl and para-aortic paraganglia are extra-adrenal chromaffin rests.

Pancreatic islets are foregut endoderm, from the same ventral and dorsal pancreatic buds that make acini. Islet cells delaminate from duct-like epithelium; they are not neural crest (an older hypothesis). Gonads are intermediate mesoderm plus immigrating germ cells, as in the urogenital chapter.

Histology

Endocrine organs are built for secretion into fenestrated capillaries (pituitary, pineal, parathyroids, pancreatic islets, adrenal cortex). Thyroid follicles are the structural exception: hormone is stored extracellularly as colloid.

Anterior pituitary cells are identified on standard stains as acidophils, basophils, and chromophobes; immunohistochemistry is what actually names GH versus prolactin. Posterior pituitary is unmyelinated axons, Herring bodies (eosinophilic axonal dilatations packed with ADH or oxytocin granules), and pituicytes. Do not look for a glandular acinus in the pars nervosa.

Thyroid: follicles lined by simple cuboidal (active) or squamous (inactive) epithelium, filled with thyroglobulin colloid. Resorption vacuoles appear at the colloid margin when TSH is high. C cells are larger, paler, and sit in the parafollicular position—they never border the colloid lumen. Parathyroid: cords of chief cells (slightly amphophilic, glycogen) and clusters of oxyphils (deeply eosinophilic); adipocytes increase with age. No follicles.

Adrenal cortex zonation is visible on H&E: glomerulosa cells are small and clustered under the capsule; fasciculata cells are large, vacuolated spongiocytes (stored cholesterol esters); reticularis cells are smaller and often pigmented with lipofuscin. Medulla chromaffin cells form nests; they stain with chromium salts (the name) and are richly innervated. Ganglion cells may be scattered among them.

Islets: pale polygonal clusters among darker acini, with a dense capillary network. Granule ultrastructure differs (beta granules have a dense crystalline core), but light microscopy plus position is enough for Part I. Pinealocytes are arranged in cords with interstitial astrocytes; brain sand is basophilic and concentric.

DNES cells are clear or argentaffin/argyrophil cells sitting on the basement membrane of gut or respiratory epithelium, with granules polarized toward capillaries, not toward the lumen (except some gut cells that are both exo- and endocrine).

Clinical anatomy that still belongs in this chapter rather than in pathology: a pituitary stalk interruption disconnects hypothalamic dopamine from lactotrophs (prolactin rises) and disconnects releasing hormones from the other anterior-lobe cells (the rest fall). A left adrenal vein ligature is a left-renal-vein problem; a right adrenal vein ligature is an IVC problem. Recurrent laryngeal proximity to the inferior thyroid artery is why thyroid surgery is an anatomy item, not a technique item, on Part I.

Once pouch 3 versus 4, Rathke versus diencephalon, adrenal zonation, and adrenal-vein laterality are automatic, mix items on /practice/nbce-part1.