Neurohypophysis

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Main organs of the endocrine system.

The neurohypophysis forms the posterior portion of the pituitary gland (hypophysis). It contains the posterior lobe of the pituitary gland and differs in both structure and function to its anterior counterpart, the adenohypophysis.

The neurohypophysis is formed by pituicytes, a type of glial cell and is embedded with a mass of unmyelinated neurons. It is divided into two main elements: the pars nervosa and the infundibular stalk. Some texts may also include the pars intermedia and the median eminence as components of the neurohypophysis. 

The infundibular stalk connects the posterior pituitary to the hypothalamus and contains unmyelinated axonal bodies of magnocellular neurosecretory cells which originate in the hypothalamus. These neurons carry neuroendocrine hormones namely oxytocin and arginine vasopressin (AVP), from the hypothalamus to the pars nervosa where they are released into the systemic circulation via fenestrated sinusoidal capillaries. AVP is responsible for renal excretion while oxytocin controls parturition and lactation. It is important to note that the neurohypophysis is not composed of secretory cells and therefore does not produce or secrete hormones, but rather stores and releases these hormones into the blood stream. Therefore, as a whole, the neurohypophysis functions to coordinate fluid balance and reproductive actions through the storage and release of neuroendocrine hormones. 

Extending through the entirety of the neurohypophysis is a capillary bed which is supplied by the descending branch of the superior hypophyseal artery and the inferior hypophyseal artery. Venous drainage of the capillary bed is conducted by the hypophyseal vein which drains to the dural venous sinuses.