Corpus: Posterior pituitary

The neurohypophysis or posterior pituitary is part of the pituitary gland in which axons of hypothalamic neurons end. In contrast to the adenohypophysis, which is an endocrine gland, the neurohypophysis is part of the brain in terms of developmental history.

The neurohypophysis develops from a protrusion of the base of the third ventricle (processus infundibularis). From the third month of development, the epithelial cells differentiate into specific glial cells (pituicytes). At the same time, unmyelinated axons grow in from the nuclei of the hypothalamus.

The neurohypophysis is subdivided into:

• Infundibulum: forms the pituitary stalk
• Lobus nervosus: forms the posterior lobe of the pituitary gland (HHL)

The HHL is connected to the hypothalamus via the hypothalamohypophysial tract.

In the area of the infundibulum is the eminentia mediana, an area with capillary loops that originate primarily from the superior hypophysial artery. As a circumventricular organ, the capillaries have a fenestrated endothelium, so that there is no blood-brain barrier here. Blood flows from this first capillary network via portal veins to capillaries of the adenohypophysis. This pituitary portal vascular system forms the basis for the functional connection between the hypothalamus and adenohypophysis.

The pars nervosa of the neurohypophysis is mainly supplied by the inferior hypophysial artery and individual branches of the superior hypophysial artery. They also form a dense capillary plexus with a fenestrated endothelium without a blood-brain barrier.

Over 70 % of the neurohypophysis consists of unmyelinated axons with perikarya in the hypothalamus. In addition to axons that run to the adenohypophysis, there are also axons of magnocellular neurones of the supraoptic nucleus and the paraventricular nucleus that run to the lobus nervosus (magnocellular neuroendocrine system).

The neurohypophysis also contains special glial cells (pituicytes), which make up approx. 25 % of the volume of the lobus nervosus. They often contain lipid droplets and are characterised by pigment granules and densely packed intermediate filaments. Their processes form a network and a basic framework for the axons. They are also connected via gap junctions.

Like the eminentia mediana, the lobus nervosus has many sinusoidal capillaries with fenestrated endothelium.

The magnocellular hypothalamic neurones produce oxytocin and ADH. They are synthesised as a prohormone, from which another peptide is also produced, neurophysin I and II respectively. The hormones are bound to their neurophysins, packaged in secretory vesicles and transported to the lobus nervosus via axonal transport. Larger accumulations of hormone granules can be detected as so-called Herring bodies. In response to neuronal excitation, proteolytic cleavage of the hormone from neurophysin occurs, with pituicytes playing an important role in this process. The hormones are then secreted into the bloodstream via the fenestrated endothelial cells (neurosecretion).

Various neurotransmitters and neuropeptides are also present in the axons, e.g. galanin, dynorphin, enkephalins and catecholamines.

Hormones released by the neurohypophysis include:

• ADH: promotes water reabsorption in distal tubules and collecting ducts of the kidney by incorporating aquaporin 2 into the apical membrane.
• Oxytocin: leads to uterine contractions and triggers labour; postnatally it causes milk to be ejected from the lactating mammary gland by contraction of the myoepithelial cells. The role in men has not yet (2019) been conclusively clarified.

The activity of the magnocellular neuroendocrine neurones of the hypothalamus is controlled by humoral and neuronal information:

• ADH secretion is primarily regulated by blood osmolarity. Neuronal afferents from the osmosensitive circumventricular organs and from the autonomic centres of the brain stem (e.g. nucleus solitarius) convey information about the water and electrolyte balance as well as the state of the cardiovascular system.
• The release of oxytoxin is regulated by afferents from sensory systems and limbic areas; for example, tactile stimuli from the breast and the baby's cry lead to secretion.

• Image source podcast: © Rachael Gorjestani / Unsplash