1.4 Adrenal Glands: Zona Fasciculata

Zona Fasciculata Overview

  • Composed of cells arranged in bundles resembling fascicles.

  • Cells here are target cells for ACTH and produce glucocorticoids.

Glucocorticoids

  • Definition: Hormones produced from the adrenal cortex that have significant effects on glucose levels.

  • Cortisol: The primary human glucocorticoid.

Functions of Cortisol

  1. Regulation of Metabolism during Long-term Stress

    • Ensures cells receive necessary nutrients to manage prolonged stress conditions (from hours to even decades).

  2. Increases Protein Metabolism & Fat Breakdown

    • Enhances lipolysis (fat breakdown) predominantly through adipocytes.

    • Encourages protein catabolism, especially in muscle tissue, leading to muscle wasting.

  3. Promotes Gluconeogenesis

    • Stimulates the synthesis of glucose from non-carbohydrate sources (e.g., amino acids).

  4. Resistance to Stress

    • Provides the body with nutrients during stress by mobilizing resources into the bloodstream.

Additional Functions of Glucocorticoids

  • Increased Blood Pressure

    • Enhances blood vessel sensitivity to hormones like angiotensin II, promoting vasoconstriction.

  • Anti-Inflammatory & Immunosuppressive Effects

    • Reduces histamine release from mast cells, decreases capillary permeability, and inhibits phagocytosis, reducing inflammation.

    • Example usage: Cortisone shots for inflammation reduction.

Regulation of Glucocorticoid Release

  • Governed by a negative feedback loop involving hypothalamus and anterior pituitary.

    • Increased glucocorticoid levels suppress CRH and ACTH production, reducing further glucocorticoid secretion.

Clinical Implications of Glucocorticoid Administration

  • Example: A heart transplant patient receiving prednisone (a modified glucocorticoid) will have low ACTH and CRH levels due to negative feedback suppression.

Endocrine Pathologies

  1. Cushing's Syndrome

    • Caused by hypersecretion of glucocorticoids (especially cortisol).

    • Symptoms:

      • Redistribution of fat (e.g., moon face, buffalo hump).

      • Muscle atrophy due to protein catabolism.

      • Poor wound healing and increased bruising due to immune suppression.

  2. Addison's Disease

    • Caused by hyposecretion of glucocorticoids and possibly aldosterone.

    • Symptoms:

      • Hypoglycemia from low cortisol levels.

      • Muscle weakness and low blood pressure from low aldosterone (due to low sodium retention).

      • Dehydration and potential cardiac arrest from high potassium levels due to low aldosterone.

      • Skin darkening due to high ACTH levels as a compensatory response (due to low glucocorticoid production).

Notable Example

  • JFK and Addison's Disease: History of JFK exhibiting symptoms consistent with Addison's, including skin pigmentation changes and autoimmune destruction of adrenal cortex.