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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 (oxytocin and vasopressin) directly into the blood. [2]
The pituitary gland is divided into three lobes: the anterior pituitary, the intermediate pituitary lobe, and the posterior pituitary. The hypothalamus controls the anterior pituitary's hormone secretion by sending releasing factors, called tropic hormones, down the hypothalamo-hypophysial portal system. [3]
The pituitary gland or hypophysis is an endocrine gland in vertebrates. In humans , the pituitary gland is located at the base of the brain , protruding off the bottom of the hypothalamus . The human pituitary gland is oval shaped , about 1 cm in diameter, 0.5–1 gram (0.018–0.035 oz) in weight on average, and about the size of a kidney bean .
Herring bodies or neurosecretory bodies are structures found in the posterior pituitary. They represent the terminal end of the axons from the hypothalamus, and hormones are temporarily stored in these locations. They are neurosecretory terminals. [1]
Pituicytes are similar to astrocytes, another type of glial cell. Their main role is to assist in the storage and release of hormones of the posterior pituitary. Pituicytes surround axonal endings and regulate hormone secretion by releasing their processes from these endings. [1]
The AVP that is measured in peripheral blood is almost all derived from secretion from the posterior pituitary gland (except in cases of AVP-secreting tumours). Vasopressin is produced by magnocellular neurosecretory neurons in the paraventricular nucleus of hypothalamus (PVN) and supraoptic nucleus (SON).
Many of its neurons project to the posterior pituitary where they secrete oxytocin, and a smaller amount of vasopressin. Other secretions are corticotropin-releasing hormone (CRH) and thyrotropin-releasing hormone (TRH). [1] CRH and TRH are secreted into the hypophyseal portal system, and target different neurons in the anterior pituitary.
They showed this to be the case [8] by studying vasopressin secretion from the isolated posterior pituitary gland in vitro. They found that vasopressin secretion could be evoked by electrical stimulus pulses applied to the gland, and that much more hormone was released by a phasic pattern of stimulation than by a continuous pattern of stimulation.