1.3 The postierior pituitary gland

Structural and Functional Relationship Between the Hypothalamus and Posterior Pituitary

• The relationship between the hypothalamus and posterior pituitary is straightforward and simpler compared to the anterior pituitary.

• The posterior pituitary is also known as neurohypophysis, indicating its neural tissue nature.

• The term "hypophysis" refers to the entire pituitary structure, hence the name "hypophyseal fossa" in the sphenoid bone where the pituitary is located.

Anatomy of the Posterior Pituitary

• The posterior pituitary gland consists predominantly of neural tissue, including:

  • Axons

  • Neuroglial cells

  • Extension of brain tissue, which can be observed under a microscope.

Neurosecretory Cells

• Key nuclei involved in hormone production are:

  • Paraventricular Nucleus: Contains neurosecretory cells that synthesize hormones.

  • Supraoptic Nucleus: Another group of neurosecretory cells for hormone synthesis.

• These nuclei are collections of cell bodies (gray matter) located in the hypothalamus.

Hormone Production and Secretion Process

• Hormones are synthesized in the hypothalamus by neurosecretory cells, then transported down axons to the posterior pituitary.

Process Overview:

1. Hormones (like ADH and oxytocin) are synthesized in hypothalamic nuclei.

2. They are transported down the full length of the axons to axon terminals in the posterior pituitary.

3. Upon stimulation (action potential), hormones are secreted into interstitial fluid, functioning similarly to neurotransmitters.

Blood Supply and Hormone Release

• Hormones do not form synapses with other neurons but are released into interstitial fluid where they diffuse into the bloodstream.

• The posterior pituitary has a capillary bed known as the capillary plexus of the infundibular process.

  • Blood flow involves:

    • Arterial blood entering

    • Capillary beds absorbing hormones

    • Venous blood exiting

  • This system does not involve a portal system, making it a direct component of systemic circulation.

Hormones of the Posterior Pituitary

• The posterior pituitary releases two main hormones:

  • Antidiuretic Hormone (ADH), also known as vasopressin.

  • Oxytocin (OT).

• Important to note:

  • Oxytocin is commonly abbreviated as OT in medical texts.

Key Characteristics of Posterior Pituitary Tissue

• The posterior pituitary resembles neural tissue and contains:

  • Glial cells

  • Nerve fibers and endings

  • Neurosecretory vesicles that store hormones for release.

Summary of Functionality

• Hormone Synthesis Location: Hypothalamus

• Hormone Release Location: Posterior Pituitary

  • Hormones are synthesized in the hypothalamus, packaged in vesicles, and stored until an action potential triggers their release into the bloodstream from the posterior pituitary.

Comparison Table