Endocrinology 2/25/26

Overview of the Pituitary Gland

  • The pituitary gland is divided into two main parts: anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).

Anterior Pituitary Cells

  • The anterior lobe of the pituitary consists of five different types of cells.
    • The classification is based on histological staining methods, particularly H and E (Hematoxylin and Eosin) staining.

Histological Staining

  • Hematoxylin: Stains basophilic structures blue, primarily the cytoplasm of basophils.
  • Eosin: Stains acidophilic structures red, primarily the cytoplasm of acidophils.

Cell Types in Anterior Pituitary

  1. Acidophils:

    • Stain red (eosinophilic); contain hormones that give off this color.
    • Subtypes:
      • Lactotrophs: Produce Prolactin
      • Somatotrophs: Produce Growth Hormone

  2. Basophils:

    • Stain blue (basophilic); primarily contain hormones that give off this color.
    • Subtypes:
      • Corticotrophs: Produce Adrenocorticotropic hormone (ACTH)
      • Thyrotrophs: Produce Thyroid-stimulating hormone (TSH)
      • Gonadotrophs: Produce Luteinizing hormone (LH) and Follicle-stimulating hormone (FSH)

  3. Chromophobes:

    • Do not take up stain; possibly depleted acidophils or basophils with no visible hormones.

Cell Functions

  • Hormones are stored in the cytoplasm and are released in response to specific stimuli.

Comparison of Anterior and Posterior Pituitary

  • The anterior lobe is comprised mostly of endocrine cells, while the posterior lobe consists mainly of axonal endings from neurons.

Structure of the Posterior Pituitary

  • Comprised of:
    1. Axon terminals: These are long extensions from neurons in the hypothalamus (supraoptic nuclei and paraventricular nuclei).
    2. Glial cells: Less defined role, but present in the posterior lobe.

Hormones of the Posterior Pituitary

  1. Vasopressin (ADH):

    • 9 amino acid peptide responsible for regulating water retention and blood pressure.

  2. Oxytocin:

    • Plays roles in uterine contraction during childbirth and milk ejection during breastfeeding.

Hormonal Release Mechanisms

  • Hormones are transported down the axon and stored in secretory vesicles at the axonal terminal.
  • Neuronal stimulation leads to depolarization, opening calcium channels and triggering exocytosis of hormones into capillaries.

Exocytosis Process

  1. Depolarization of neuron leads to an action potential.
  2. Sodium ions flow into the cell, causing further depolarization and opening of calcium channels.
  3. Calcium influx triggers exocytosis of hormone-containing vesicles.
  4. Hormones enter circulation via capillary beds ensuring broad systemic effects.

Recovery of Vesicles

  • After hormone release, vesicles are retrieved via endocytosis and reused, often involving a clathrin coat for stability during transport.

ADH and Oxytocin Production

  • Both hormones originate from different nuclei in the hypothalamus:
    • Vasopressin: Produced by supraoptic nuclei.
    • Oxytocin: Produced by paraventricular nuclei.

Structure of ADH and Oxytocin

  • Both are nonapeptides (nine amino acids) with similar structures, with minor differences impacting their function:
    • Vasopressin: Contains Isoleucine at position 3 and Arginine at position 8.
    • Oxytocin: Contains Leucine at position 3 and Proline at position 8.
  • Both contain a glycoprotein component that aids transport down the axon.

Functions of Vasopressin (ADH)

  • Regulates osmotic pressure and blood pressure by:
    • Enhancing water reabsorption in the kidneys, thus reducing urine output and increasing body hydration.
    • Acts through osmoreceptors that detect changes in blood osmolality (normal osmolality ≈ 300 mOsm/L).

Physiological Implications

  • Dehydration leads to increased ADH release, reducing urine output and retaining water to normalize blood osmotic pressure.
  • Alcohol inhibits ADH secretion, leading to increased urination and dehydration.
  • Hormonal regulation and the balance of these hormones are critical for maintaining hydration and blood pressure homeostasis.

Importance of the Pituitary Gland

  • Often referred to as the "master gland" because its hormones regulate various physiological functions in the body, affecting multiple organ systems, despite being controlled by the hypothalamus.