M

Anterior Pituitary & Hypophyseal Portal System Study Notes

Cell Types of the Anterior Pituitary (Adenohypophysis)

  • Five histologically-distinct secretory cell populations:
    • Somatotrophs (somatotrophic cells) → secrete Growth Hormone (GH) a.k.a. somatotropin.
    • Thyrotrophs (thyrotrophic cells) → secrete Thyroid-Stimulating Hormone (TSH).
    • Gonadotrophs (gonadotrophic cells) → secrete Follicle-Stimulating Hormone (FSH) & Luteinizing Hormone (LH).
    • Lactotrophs (prolactin / lactotrophic cells) → secrete Prolactin (PRL).
    • Corticotrophs (corticotrophic cells) → secrete Adrenocorticotropic Hormone (ACTH) & Melanocyte-Stimulating Hormone (MSH).
  • Mnemonic for order (optional): "Some Teachers Get Paid Consistently" (Somatotroph, Thyrotroph, Gonadotroph, Prolactin cell, Corticotroph).

Hormones Secreted by the Anterior Pituitary

  • 7 total hormonal products (all peptides/proteins):
    1. Growth Hormone (GH)
    2. Thyroid-Stimulating Hormone (TSH)
    3. Follicle-Stimulating Hormone (FSH)
    4. Luteinizing Hormone (LH)
    5. Prolactin (PRL)
    6. Adrenocorticotropic Hormone (ACTH)
    7. Melanocyte-Stimulating Hormone (MSH)
  • Collectively termed tropic hormones (note: the cells are “trophic,” the hormones are “tropic”).
  • FSH + LH together = Gonadotropins.

Hypothalamic Regulation: Releasing vs. Inhibiting Hormones

  • 7 hypophysiotropic hormones originate from neurosecretory cells in the hypothalamus and act on the anterior pituitary.
  • 5 are stimulatory (contain the word “Releasing”):
    • Growth Hormone-Releasing Hormone (GHRH)
    • Thyrotropin-Releasing Hormone (TRH)
    • Gonadotropin-Releasing Hormone (GnRH)
    • Prolactin-Releasing Hormone (PRH)
    • Corticotropin-Releasing Hormone (CRH)
  • 2 are inhibitory:
    • Growth Hormone-Inhibiting Hormone (GHIH) = Somatostatin (SST)
    • Prolactin-Inhibiting Hormone (PIH) = Dopamine
  • Functional logic:
    • Releasing hormones → + stimulation of corresponding adenohypophyseal cell → secretion of its tropic hormone.
    • Inhibiting hormones → - suppression of secretion (notably GH & PRL pathways).

Melanocyte-Stimulating Hormone (MSH): Special Note

  • Produced by corticotrophs but physiological function in humans remains obscure.
  • Known to cause rapid color change in amphibians (e.g., frogs) – classic comparative example.
  • Mentioned for completeness; will not be discussed further in this course block.

Hypophyseal (Pituitary) Portal System

  • A portal system = two capillary beds connected by a portal vein (cf. hepatic portal system).

  • Anatomical components:

    1. Primary capillary plexus (median eminence of hypothalamus)
    • Supplied by the Superior Hypophyseal Artery (branch of Internal Carotid).
    • Neurosecretory axon terminals release hypothalamic hormones here → hormones enter blood.
    1. Hypophyseal Portal Veins (run down the infundibulum)
    • Convey blood containing releasing/inhibiting factors to anterior pituitary.
    1. Secondary capillary plexus (anterior pituitary)
    • Site where adenohypophyseal cells discharge the 7 tropic hormones into bloodstream.
    1. Hypophyseal Veins
    • Drain the secondary plexus → deliver hormones into systemic circulation.

  • Blood-flow sequence (step-wise recap):

    1. Internal Carotid → Superior Hypophyseal Artery (oxygenated).
    2. Arteriole branches → Primary Plexus → collect hypothalamic hormones.
    3. Portal Veins → descend through infundibulum.
    4. Secondary Plexus in anterior pituitary → receives hypothalamic hormones, prompts tropic hormone release.
    5. Tropic hormones enter same plexus → Hypophyseal Veins → systemic circulation.

Mechanism of Hormone Delivery & Feedback Control

  • Stimulus (neural, hormonal, or metabolic) → activates hypothalamic neurosecretory cells.
  • Axonal transport & storage: hormones synthesized in cell body, transported to axon terminal at primary plexus.
  • Upon stimulation, vesicular exocytosis of releasing/inhibiting factors into blood.
  • Tropic hormones released from adenohypophyseal cells → systemic targets (thyroid, adrenal cortex, gonads, liver, etc.).
  • Classic long-loop feedback: peripheral gland hormones often feedback on hypothalamus and/or pituitary to modulate further release (not elaborated in transcript but fundamental concept).

Cell–Hormone Matching Table (must-memorize)

Cell TypeHormone(s)Hypothalamic StimulatorHypothalamic Inhibitor
SomatotrophGHGHRHGHIH (Somatostatin)
ThyrotrophTSHTRH—
GonadotrophFSH, LHGnRH—
LactotrophPRLPRHPIH (Dopamine)
CorticotrophACTH, MSHCRH—

Comparative & Real-World Context

  • Portal vascular architecture parallels hepatic portal system – emphasizes efficient hormone trafficking without systemic dilution.
  • Dopamine’s role as PIH highlights neurotransmitter–hormone duality; therapeutic relevance in prolactinomas (dopamine agonists).
  • Somatostatin used clinically to treat GH-secreting pituitary adenomas and diverse endocrine tumors.

Ethical / Philosophical / Practical Implications

  • Precision control of multiple endocrine axes via tiny brain region underscores evolutionary economy in physiology.
  • Dysregulation (adenomas, stalk section, ischemia) can have multisystem consequences – reinforces ethical imperative for early detection & endocrine replacement.

Key Abbreviations & Symbols

  • GH, TSH, FSH, LH, PRL, ACTH, MSH
  • GHRH, TRH, GnRH, PRH, CRH, GHIH (SST), PIH (DA)
  • + = stimulatory; - = inhibitory.

High-Yield Reminders for Exam

  • Know all 5 cell types, their 7 hormones, and the corresponding hypothalamic regulators.
  • Be able to draw/label the hypophyseal portal circuit (primary plexus, portal veins, secondary plexus, hypophyseal veins).
  • Understand difference in spelling: tropHic cells vs tropic hormones.
  • FSH & LH = gonadotropins; ACTH & MSH share precursor (POMC, though not covered in transcript).
  • MSH: important for amphibian pigmentation; human role unclear – likely distractor question.