Posterior pituitary
The posterior pituitary is the posterior lobe of the pituitary gland which is part of the endocrine system. The posterior pituitary is not glandular as is the anterior pituitary. Instead, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior pituitary, and serve as a site for the secretion of neurohypophysial hormones directly into the blood. The hypothalamic–neurohypophyseal system is composed of the hypothalamus, posterior pituitary, and these axonal projections.
Structure
The posterior pituitary consists mainly of neuronal projections of magnocellular neurosecretory cells extending from the supraoptic and paraventricular nuclei of the hypothalamus. These axons store and release neurohypophysial hormones oxytocin and vasopressin into the neurohypophyseal capillaries, from there they get into the systemic circulation. In addition to axons, the posterior pituitary also contains pituicytes, specialized glial cells resembling astrocytes assisting in the storage and release of the hormones.Classification of the posterior pituitary varies, but most sources include the two regions below:
;Pars nervosa: Also called the neural lobe or posterior lobe, this region constitutes the majority of the posterior pituitary and is the storage site of oxytocin and vasopressin. Sometimes considered synonymous with the posterior pituitary, the pars nervosa includes Herring bodies and pituicytes.
;Infundibular stalk: Also known as the infundibulum or pituitary stalk, the infundibular stalk bridges the hypothalamic and hypophyseal systems.
The median eminence is only occasionally included as part of the posterior pituitary. Other sources specifically exclude it from the pituitary.
A few sources include the pars intermedia as part of the posterior lobe, but this is a minority view. It is based upon the gross anatomical separation of the posterior and anterior pituitary along the cystic remnants of Rathke's pouch, causing the pars intermedia to remain attached to the neurohypophysis.
Function
Hormone secretion
Two hormones are classically considered as being related to the posterior pituitary: oxytocin and vasopressin. These hormones are created in the hypothalamus and released in the posterior pituitary. After creation, they are stored in neurosecretory vesicles regrouped into Herring bodies before being secreted in the posterior pituitary via the bloodstream.| Hormone | Other names | Symbol | Main targets | Effect | Source |
| Oxytocin | OT | Uterus, mammary glands | Uterine contractions; lactation | supraoptic and paraventricular nuclei | |
| Vasopressin | Arginine vasopressin, antidiuretic hormone | VP, AVP, ADH | Kidneys and arterioles | Stimulates water retention; raises blood pressure by contracting arterioles | supraoptic and paraventricular nuclei |