Pituitary Hormones and Posterior Pituitary Physiology

Vocabulary flashcards covering pituitary anatomy, embryological origin, anterior and posterior pituitary hormones, cell types, hypothalamic portal control, and major neuroendocrine axes (HPT, HPA, HPG, HPO).

Pituitary Gland Structure

A small endocrine organ (~1 cm, 0.5–1 g) located in the sella turcica at the base of the brain, connected to the hypothalamus via the hypophysial stalk and divided into the anterior pituitary (adenohypophysis), posterior pituitary (neurohypophysis), and pars intermedia.

Rathke's Pouch

An embryonic invagination of pharyngeal epithelium that gives rise to the epithelial cell structure of the anterior pituitary (adenohypophysis).

Somatotropes

Anterior pituitary cells that comprise 30–40% of the gland's endocrine cells and synthesize and secrete Growth Hormone (GH).

Corticotropes

Anterior pituitary cells comprising ~20% of the total cell population that secrete Adrenocorticotropic Hormone (ACTH).

Thyrotropes

Anterior pituitary cells comprising 3–5% of the total cell population that secrete Thyroid-Stimulating Hormone (TSH).

Gonadotropes

Anterior pituitary cells comprising 3–5% of total cells that secrete both Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH).

Lactotropes

Anterior pituitary cells comprising 3–5% of total cells that synthesize and secrete Prolactin (PRL).

Growth Hormone (GH)

An anterior pituitary hormone secreted by somatotropes that stimulates body growth, increases $IGF\text{-}1$ secretion and lipolysis, and decreases insulin effects on metabolism.

Adrenocorticotropic Hormone (ACTH)

A 39-amino-acid polypeptide secreted by corticotropes that stimulates glucocorticoid and androgen secretion from the adrenal cortex, maintaining the zona fasciculata and reticularis.

Thyroid-Stimulating Hormone (TSH)

A glycoprotein hormone (~$28,000\text{ MW}$) composed of $\text{alpha}$ and $\text{beta}$ subunits that stimulates thyroid hormone synthesis and maintains thyroid follicular cells.

Prolactin (PRL)

An anterior pituitary hormone secreted by lactotropes that stimulates milk production in mammary glands.

Pituicytes

Glial-like supporting cells found in the posterior pituitary that do not synthesize hormones.

Supraoptic Nucleus (SON)

A hypothalamic nucleus containing magnocellular neurons primarily responsible for synthesizing Antidiuretic Hormone (ADH).

Paraventricular Nucleus (PVN)

A hypothalamic nucleus containing magnocellular neurons primarily responsible for synthesizing Oxytocin and Corticotropin-Releasing Hormone (CRH).

Neurophysins

Carrier proteins that transport ADH and oxytocin along axons from hypothalamic nuclei to nerve terminals in the posterior pituitary.

Median Eminence

The inferior portion of the hypothalamus containing the primary capillary plexus, serving as the link where hypothalamic releasing and inhibitory hormones enter portal blood.

Hypothalamic-Hypophysial Portal System

A vascular network carrying hypothalamic releasing and inhibitory hormones from the primary capillary plexus in the median eminence directly to the secondary plexus in the anterior pituitary.

Dopamine (Prolactin-Inhibiting Hormone)

A hypothalamic factor that exerts continuous inhibitory control over prolactin secretion by lactotropes.

Thyrotropin-Releasing Hormone (TRH)

A tripeptide synthesized in the paraventricular nucleus that activates the phospholipase C pathway ($Ca^{2+}$ and DAG) in thyrotropes to stimulate TSH secretion.

HPT Axis Negative Feedback

A feedback mechanism where elevated circulating $T_3$ and $T_4$ levels directly suppress pituitary TSH secretion; when thyroid hormone levels reach ~$1.75 \times$ normal, TSH secretion drops to near zero.

Corticotropin-Releasing Hormone (CRH)

A 41-amino-acid peptide secreted by the paraventricular nucleus of the hypothalamus in response to physical or mental stress to stimulate ACTH release.

Pregnenolone Conversion

The rate-limiting step in adrenal steroidogenesis, activated when ACTH stimulates cAMP and Protein Kinase A (PKA) to convert cholesterol into pregnenolone.

Gonadotropin-Releasing Hormone (GnRH)

A 10-amino-acid decapeptide secreted in a pulsatile manner (every 1–3 hours in males, 1–2 hours in females) that stimulates LH and FSH release from gonadotropes.

Leydig Cells

Testicular cells stimulated by Luteinizing Hormone (LH) to produce testosterone.

Sertoli Cells

Testicular cells stimulated by FSH and testosterone to support spermatogenesis, which also secrete inhibin to selectively suppress FSH secretion.

LH Surge

A $6 \times$ to $8 \times$ increase in LH secretion triggered by high estrogen levels sustained for 2–3 days via positive feedback, directly inducing ovulation.

Antidiuretic Hormone (ADH / Vasopressin)

A 9-amino-acid peptide stored in the posterior pituitary that binds receptors in kidney collecting ducts to insert aquaporin channels via cAMP, increasing water reabsorption.

Central Diabetes Insipidus

A condition resulting from the absence of ADH, leading to impermeable collecting ducts, failure of renal water reabsorption, and excretion of large volumes of dilute urine.

Osmoreceptors

Sensory neurons in the AV3V region of the hypothalamus that detect changes in extracellular fluid osmolarity to regulate ADH secretion.

Oxytocin

A 9-amino-acid peptide stored in the posterior pituitary that stimulates uterine smooth muscle contraction during labor and induces milk ejection during lactation.

Milk Ejection Reflex (Letdown Reflex)

A neuroendocrine reflex triggered by suckling, causing hypothalamic release of oxytocin, which contracts myoepithelial cells surrounding mammary alveoli to force milk into ducts within ~1 minute.