Index
It is the structural basis of a neurohumoral system that coordinates fluid balance and reproductive function through the action of two peptide hormones:
- Vasopressin.
- Oxytocin
Vasopressin is the primary endocrine regulator of renal water excretion, facilitating adaptive physiological responses to maintain plasma volume and plasma osmolality.
Oxytocin is essential in labor and breastfeeding. The data support a broader role for both peptides in the neuroregulation of complex behavior.
Clinically, deficits in vasopressin production or action manifest as diabetes insipidus.
Understanding the physiology and pathophysiology of vasopressin is also essential to addressing the diagnosis and treatment of hyponatremia, the most common electrolyte disturbance in clinical practice.
Parts of the Neurohypophysis
The neurohypophysis consists of three parts:
- The supraoptic and paraventricular nuclei of the hypothalamus.
- The supraoptic-pituitary tract.
- The posterior pituitary.
Histology of the Neurohypophysis
Anatomists distinguish between the structure of the neurohypophysis in three relevant areas that begin closer to the hypothalamus:
- The median eminence.
- The infundibular stem.
- The infundibular process.
The infundibular process of most neurohypophysis is generally known as the posterior pituitary.
The bulk of the neurohypophysis is composed of largely unmyelinated axons of hypothalamic neurosecretory neurons.
These axons have their cell bodies in the paraventricular and supraoptic nuclei of the hypothalamus.
These neurons secrete oxytocin or antidiuretic hormone. Approximately 100,000 axons participate in this process to form the posterior pituitary. In addition to axons, the neurohypophysis contains glial cells and other ill-defined cells called so-called pituicytes.
Compared with the adenohypophysis, the histological appearance of the neurohypophysis shows large numbers of axons dotted with glial and capillary cells.
The neurohypophysis contains abundant capillaries, particularly in its ventral portion, where most hormones release occurs.
Many of these capillaries are fenestrated (contain holes), which facilitates the supply of hormones to the blood.
An interesting histological feature of the neurohypophysis is the presence of herring bodies.
When viewed with an electron microscope, these are dilated areas or bulges in the axons’ terminal portion containing clusters of neurosecretory granules.
The granules contain oxytocin or antidiuretic hormone, together with its associated neurophysins. Herring bodies are often associated with capillaries and are somewhat challenging to identify by light microscopy unequivocally.
Anatomy of the Neurohypophysis
The posterior pituitary derives from the forebrain during development and is predominantly made up of nervous tissue.
It is located below the hypothalamus, thus forming a structural and functional unit: the neurohypophysis.
The supraoptic nucleus is located along the proximal part of the optic tract. It consists of the cell bodies of discrete magnocellular vasopressinergic and oxytocic neurons that project toward the posterior pituitary along the supraoptic-pituitary tract.
The paraventricular nucleus also contains discrete magnocellular vasopressinergic and oxytocic neurons, which also project to the posterior pituitary along the supraoptic-pituitary tract.
The supraoptic-pituitary tract contains additional and smaller parvocellular neurons that project toward the median eminence and other extrahypothalamic areas, including the forebrain, brainstem, and spinal cord.
Some of these parvocellular neurons are vasopressors. A group of those that project through the median eminence secrete a corticotropin-releasing hormone (CRH) and end in the pituitary-portal bed of the anterior pituitary.
These and other vasopressinergic parvocellular neurons that terminate in the pituitary-portal bed regulate adrenocorticotropin (ACTH) release from the anterior pituitary gland, acting synergistically with corticotropin produced by other hypothalamic neurons.
The posterior pituitary receives an arterial blood supply from the inferior pituitary artery and the trabecular artery (a branch of the superior pituitary artery), derived from the internal carotid artery and its components.
Venous drainage of the neurohypophysis is through the dural, cavernous, and inferior petrosal sinuses.
Importance of Vasopressin
Vasopressin is a critical component in regulating fluid balance and electrolytes through direct effects on renal water management.
However, its physiology has a broader context, encompassing roles in the integrated response to changes in cardiovascular status.
A blunted appreciation of thirst may accompany age-related changes in vasopressin production, decreased fluid intake, reduced ability to excrete a free water load, and decreased renal concentrating capacity.
These changes predispose the elderly to hyper and hyponatremia. As it befits its primary and physiological function, the production of vasopressin by the neurohypophysis is influenced by sensory signals that reflect the osmotic state and blood pressure / circulating volume.
An osmosensory input feeds additional data on the osmolar state peripheral to the neurohypophysis. The blood vessels of the hepatic portal contain sensory neurons that respond to changes in the osmolality of the peripheral blood.
Unlike the central mechanisms, the critical transduction element of the peripheral process is the stretch-sensitive ion channel, TRPV4.
Plasma osmolality is frequently elevated in patients after liver transplantation in which the donor organ is denervated, demonstrating the role of this peripheral pathway.
