Pituitary & Hypothalamus — Anterior Pituitary

Anterior Pituitary Gland

• Anterior lobe of pituitary gland (aka. adenohyophysis) → forms 80% of pituitary gland → consists of 3 parts:
  • Pars tuberalis
  • Parts intermedius (intermediate lobe)
  • Pars distalis (anterior lobe) → produces most anterior pituitary hormones
• Derived from buccal ectoderm → it extends upwards from the epithelial lining of the primitive mouth cavity (as Rathke’s pouch), then fuses with the downward growing infundibulum of the hypothalamus (which forms the posterior pituitary)
• Consists of non-neural secretory epithelial cells connected INDIRECTLY to the hypothalamus via the “hypothalamic-hypophysial portal system”:
  • Non-neural secretory epithelium:
    • Granular secretory cells (chromophils)
      • Acidophils (80%) → red-staining cells → synthesise and store large peptide hormones in secretory granules
        • Lactotropes → PRL
        • Somatoropes → GH (somatotropins)
      • Basophils (20%) → blue-staining cells → synthesise and store glycoprotein hormones in secretory granules
        • Gonadotropes → FSH and LH
        • Thyrotropes → TSH
        • Corticotropes → ACTH (and MSH)
    • Agranular secretory cells (chromophobes)
      • Weakly staining cells → inactive or degranulated secretory cells → does not synthesise any hormones
  • Hypothalamic-hypophysial portal system
    • Branches of superior hypophysial artery → forms “1° capillary plexus” near median eminence of hypothalamus → then forms lesser portal veins that gives rise to “2° capillary plexus” in anterior lobe of pituitary gland
    • Functions of portal venous system
      • Blood supply of 90% of anterior pituitary (and infundibulum)
      • Pathway for hypophysiotropic hormones (from median eminence of hypothalamus) to reach the anterior pituitary gland, and
      • Allows hormones secreted by anterior pituitary gland to enter systemic circulation
• Blood supply → 90% supplied by hypothalamic-hypophysial portal system (from superior hypophysial artery)

Hypophysiotrophic hormones: Control of anterior pituitary gland

• Hypophysiotrophic hormones are small peptides (EXCEPT for DA) produced by cell bodies (containing “parvocellular neurons”) in the hypothalamus located near the median eminence → small amounts are secreted from the tuberoinfundibular tract into the hypothalamic-hypophysial portal system → reaches anterior pituitary where it exerts either an “excitatory” or “inhibitory” effect on non-neural secretory epithelial cells

Nb. Only trace levels of these hormones are found in systemic circulations b/c → (i) small amounts are secreted, and (ii) they are secreted into the hypothalamic-hypophysial portal system

• Nature of release from hypothalamus:
  • Pulsatile manner → can vary with circadian rhythm, but induced in the presence of specific stimuli (Eg. cold, stress, Etc.)
  • Regulated by –ve feedback (esp from hormones secreted by peripheral organs) EXCEPT PRL (see below)
• Types of hormones:
  • Thyrotrophin-releasing hormone (TRH) → tripeptide (3 a.a.) → stimulates TSH secretion from thyrotropes
  • Corticotrophin-releasing hormone (CRH) → 41 a.a. peptide → stimulates ACTH secretion from corticotropes
  • Gonadotrophin-releasing hormone (GnRH) → 10 a.a. peptide → stimulates LH and FSH secretion from gonadotropes
  • Growth-hormone releasing hormone (GHRH) → 43 a.a. peptide → stimulates GH secretion from somatoropes
  • Somatostatin (GHIH) → 14 a.a. peptide → suppresses GH secretion from somatotropes (also ACTH, TSH and PRL release; also inhibits insulin/glucagon)
  • Prolactin releasing factor (PRF) → stimulates PRL secretion from lactotropes
  • Dopamine (DA) → suppresses PRL secretion from lactotropes

Bianca 2016

Answer by Bianca