Anatomy of the Hypophysis

Hypophysis (Pituitary Gland)

• Develops from oral ectoderm and is subdivided into three portions: pars distalis (anterior lobe), pars tuberalis, and pars intermedia
• Blood supply:
  • Superior hypophyseal arteries supply the neural stalk from above
  • Inferior hypophyseal arteries supply the neurohypophysis from below
  • Hypophyseal portal system carries neurohormones from the neural stalk to the adenohypophysis

Adenohypophysis

• Pars distalis:
  • Composed of cords of epithelial cells interspersed with capillaries
  • Hormones produced by these cells are stored as secretory granules
  • Accounts for 75% of the mass of the hypophysis
  • Cells can be recognized as chromophobes, basophils, and acidophils based on staining
  • Basophils and acidophils are named for the hormones they produce (e.g., gonadotropic cells produce two hormones)
• Control of the pars distalis:
  • Cells are controlled by peptide hormones produced in the hypothalamic aggregates of neurosecretory cells and stored in the neural stalk
  • Hormones are transported to the pars distalis through the capillary plexus
  • Additional control mechanisms include:
    • Direct effect of hormones secreted by endocrine cells on the release of peptides from the neural stalk and the pars distalis
    • Action of nerve impulses or molecules produced neither in the hypothalamic nuclei nor in the target tissue (e.g., inhibin and activin produced in the gonads, dopamine produced in the central nervous system)

Pars Tuberalis

• Funnel-shaped region surrounding the infundibulum of the neurohypophysis
• Most cells secrete gonadotropins (FSH and LH)

Pars Intermedia

• Develops from the dorsal portions of Rathke's pouch
• Rudimentary region made up of cords and follicles
• Probably produces MSH (Melanocyte Stimulating Hormone)

Neurohypophysis

• Consists of the pars nervosa and the neural stalk
• Composed of unmyelinated axons of secretory neurons situated in the supraoptic and paraventricular nuclei
• Secretory neurons have well-developed Nissl bodies related to the production of neurosecretory material
• Neurosecretions are transported along the axons and accumulate at their nerve endings to form Hering's bodies
• Hering's bodies contain neurosecretory granules which are released and enter the fenestrated capillaries
• Neurosecretory materials consist of two hormones: Anti-diuretic hormone (vasopressin) and oxytocin

Cells of the Neurohypophysis

• Consists mainly of axons from hypothalamic neurons
• About 25% of its volume consists of a specific type of highly branched glial cells called pituicytes

Actions of the Hormones of the Neurohypophysis

• ADH (vasopressin):
  • Released in response to increased tonicity of the blood
  • Main effect is to increase the permeability of collecting tubules of the kidney to water, leading to reabsorption of water and regulation of osmotic balance
• Oxytocin:
  • Stimulates contraction of the myoepithelial cells that surround the alveoli and ducts of the mammary glands during nursing and of the smooth muscle of the uterine wall during copulation and childbirth
  • Secretion of oxytocin is stimulated by nursing or by distension of the vagina or the uterine cervix

Adrenal (Suprarenal) Gland

• Paired organs that lie near the superior poles of the kidneys
• Consists of two concentric layers: a yellow peripheral layer, the adrenal cortex, and a reddish-brown central layer, the adrenal medulla
• Adrenal cortex and medulla can be considered two organs with distinct origins, functions, and morphological characteristics

Blood Supply of the Adrenal Gland

• Supplied by several arteries that are divided into three groups: arteries that irrigate the capsule, cortical arteries, and medullary arteries
• Medullary arteries pass through the cortex and form an extensive capillary network in the medulla
• Cells in the medulla are bathed with both arterial blood from the medullary arteries and venous blood originating from the capillaries of the cortex

Adrenal Cortex

• Cells have the typical ultrastructure of steroid-secreting cells
• Synthesize and secrete steroid hormones upon demand
• Steroids are lipid-soluble molecules that diffuse through the plasma membrane and do not require the specialized process of exocytosis for their release
• Can be subdivided into three concentric layers: zona glomerulosa, zona fasciculata, and zona reticularis

Cortical Hormones and Their Action

• Adrenal steroids originate from cholesterol
• Converted to final hormones partly in the mitochondria and partly in the SER
• Secreted steroids can be divided into three groups: mineralocorticoids (aldesterone), glucocorticoids (cortisol), and androgens (DHEA)
• Mineralocorticoids act mainly on the distal tubules and stimulate the reabsorption of sodium by epithelial cells
• Glucocorticoids affect the metabolism of carbohydrates, promote protein and lipid degradation, and suppress the immune response
• DHEA is a weak androgen that exerts its action after being converted into testosterone in several tissues

Adrenal Medulla

• Composed of polyhedral cells arranged in cords or clumps and supported by reticular fiber network
• Profuse capillary supply intervenes between adjacent cords
• Cells arise from the neural crest as do the postganglionic neurons of sympathetic and parasympathetic ganglia
• Cells of the adrenal medulla can be considered modified sympathetic postganglionic neurons that have lost their axons and dendrites during embryonic development and become secretory cells
• Secretory granules contain one or the other type of catecholamine, epinephrine or norepinephrin

Learn about the different parts and blood supply of the hypophysis, including the adenohypophysis and neurohypophysis.