Endocrine system

General Organization

The endocrine system comprises discrete glands and dispersed endocrine cells that secrete hormones—chemical messengers—directly into the bloodstream. Unlike exocrine glands, these organs lack ducts; their richly vascularized stroma ensures rapid hormone dissemination. Endocrine structures are broadly divided into:

• Neuroendocrine organs (hypothalamus, pituitary, pineal)

• Classical endocrine glands (thyroid, parathyroids, adrenals, pancreas, gonads)

• Diffuse endocrine system (scattered enteroendocrine and paracrine cells in heart, gut, kidney, thymus)

Each gland is characterized by:

1. Location & relations in the body

2. Macroscopic architecture (lobation, capsule, blood supply)

3. Microscopic (histological) zonation or cell types

4. Innervation and lymphatic drainage

5. Embryological origin

6. Principal hormones and their target effects

2. Hypothalamus & Pituitary

2.1 Hypothalamus

• Location & Relations: Forms the floor and inferolateral walls of the third ventricle, superior to the sella turcica; bounded laterally by the optic tracts and mammillary bodies posteriorly.

• Gross Anatomy: Composed of discrete nuclei (e.g., paraventricular, supraoptic, arcuate) that synthesize releasing/inhibiting factors.

• Vascular Supply: Superior hypophyseal arteries (branches of internal carotid) form the primary capillary plexus.

• Function: Integrates neural and humoral stimuli (light, osmolarity, stress); secretes hypophysiotropic hormones into the primary capillary plexus of the hypophyseal portal system.

2.2 Pituitary (Hypophysis)

• Location & Relations: Resides within the sella turcica of the sphenoid bone, covered by the diaphragma sellae.

• Divisions:

• Adenohypophysis (anterior lobe): Pars distalis (majority of cells), pars tuberalis, pars intermedia (residual in humans).

• Neurohypophysis (posterior lobe): Pars nervosa (axon terminals), infundibular stalk.

• Histology:

• Anterior: Acidophils (somatotrophs, lactotrophs), basophils (corticotrophs, thyrotrophs, gonadotrophs), chromophobes.

• Posterior: Unmyelinated axons from supraoptic/paraventricular nuclei; Herring bodies store ADH & oxytocin.

• Hormones & Axes:

• GH, ACTH, TSH, FSH, LH, Prolactin secreted by adenohypophysis under hypothalamic control.

• ADH, Oxytocin released from neurohypophysis in response to neural stimuli.

• Blood Flow & Drainage: Hypophyseal portal veins drain anterior lobe; posterior lobe drains to cavernous sinus.

3. Pineal Gland

• Location & Relations: On the dorsal aspect of the diencephalon, in the quadrigeminal cistern, attached to the roof of the third ventricle.

• Structure: Small, conical; encapsulated; lobulated parenchyma.

• Histology: Pinealocytes (secrete melatonin), interstitial glial cells, occasional corpora arenacea.

• Function: Transduces photoperiodic information via the sympathetic chain; melatonin regulates circadian rhythms and seasonal reproductive cycles.

4. Thyroid Gland

• Location & Relations: Anterolateral trachea, spanning C5–T1; two lobes connected by an isthmus over the 2nd–3rd tracheal rings; pyramidal lobe present in ~50%.

• Capsule & Lobation: Fibrous capsule with septa dividing the gland into lobules.

• Vascular Supply: Superior thyroid arteries (external carotid) and inferior thyroid arteries (thyrocervical trunk); rich anastomoses.

• Histology:

• Follicular cells: form spherical follicles containing colloid (thyroglobulin); produce T₃/T₄.

• Parafollicular (C) cells: interfollicular clusters; secrete calcitonin.

• Innervation: Sympathetic fibers from cervical ganglia; parasympathetic via the vagus.

• Function:

• T₃/T₄: increase basal metabolic rate, potentiate catecholamine effects, essential for CNS development.

• Calcitonin: lowers serum Ca²⁺ by inhibiting osteoclastic bone resorption (minor role in humans).

5. Parathyroid Glands

• Location & Variations: Four small ovoid glands on the posterior thyroid capsule—two superior (near cricothyroid junction), two inferior (near lower thyroid pole).

• Capsule & Stroma: Minimal capsule; delicate connective tissue septa.

• Histology:

• Chief cells: small, centrally nucleated; secrete PTH.

• Oxyphil cells: larger, eosinophilic; function unclear.

• Vasculature & Drainage: Shared with the thyroid; drain into thyroid veins.

• Function: PTH elevates serum Ca²⁺ by increasing bone resorption, renal Ca²⁺ reabsorption, and activating renal 1α‑hydroxylase for vitamin D.

6. Adrenal (Suprarenal) Glands

6.1 Cortex

• Location & Relations: Superomedial poles of kidneys; enveloped by renal (Gerota’s) fascia.

• Zones & Histology:

• Zona glomerulosa: ovoid clusters; mineralocorticoids (aldosterone).

• Zona fasciculata: long radial cords; glucocorticoids (cortisol).

• Zona reticularis: anastomosing cords; adrenal androgens (DHEA).

• Blood Supply: Suprarenal arteries (superior, middle, inferior) → subcapsular plexus → sinusoidal capillaries → medulla.

6.2 Medulla

• Histology: Clusters of chromaffin cells (derived from neural crest), supported by sustentacular cells; secretes epinephrine (80%) and norepinephrine.

• Innervation: Pre‑ganglionic sympathetic fibers (celiac plexus) synapse directly on chromaffin cells.

7. Pancreatic Islets (Islets of Langerhans)

• Location & Relations: Scattered throughout pancreatic parenchyma, most numerous in the tail.

• Histology & Cell Types:

• β‑cells (60–70%) secrete insulin; centrally located.

• α‑cells (20–25%) secrete glucagon; peripheral.

• δ‑cells secrete somatostatin.

• PP cells secrete pancreatic polypeptide.

• Vascular Arrangement: Islet arterioles → fenestrated capillary network → draining venules → exocrine tissue → portal circulation to liver.

• Function: Insulin and glucagon maintain glucose homeostasis; somatostatin modulates islet hormone secretion.

8. Gonads

8.1 Testes

• Location & Relations: In scrotum; each suspended by spermatic cord.

• Structure: Tunica albuginea → lobules → seminiferous tubules (spermatogenesis).

• Interstitial (Leydig) Cells: secrete testosterone under LH stimulation.

8.2 Ovaries

• Location & Relations: In ovarian fossae on lateral pelvic walls; attached by suspensory ligament and ovarian ligament.

• Cortex & Medulla: Cortex houses follicles; medulla contains stroma, vessels, and nerves.

• Hormones: Estrogens and progesterone from developing follicles and corpus luteum; inhibin from granulosa cells.

9. Diffuse (Enteroendocrine) System

Scattered endocrine cells in the gastrointestinal tract (e.g., G cells, I cells), heart (atrial natriuretic peptide), kidneys (juxtaglomerular renin), thymus (thymosins), and placenta (hCG) contribute additional hormonal regulation.

10. Functional Integration & Clinical Correlates

• Hormonal feedback loops (predominantly negative) maintain homeostasis (e.g., pituitary–thyroid, pituitary–adrenal axes).

• Neurovascular relationships guide surgical approaches (e.g., preserving recurrent laryngeal nerve in thyroidectomy).

• Pathologies (adenomas, hyperplasia, neoplasia) reflect disruptions in anatomical structure, vascular supply, or feedback control.