The posterior lobe of pituitary gland is known as neurohypophysis. This is so, because it is composed mainly of pituicytes, which are glial cells (neuroglia) that have a supporting role, and unmyelinated axons. The pituicytes are elongated cells which have long cytoplasmic processes and, in aging people, contain yellowish brown pigment granules. The unmyelinated nerve fibers, which originates in the supraoptic and paraventricular nuclei, are intermingled with the pituicytes and capillaries. (Bear in mind that the anterior lobe of pituitary is the glandular portion of the hypophysis and it is called 'adenohypophysis').
The neurohypophysis releases two hormones: vasopressin and oxytocin, which are secreted as peptide pro-hormones by cells of the hypothalamic nuclei and stored in the posterior lobe of pituitary gland. They are released into the bloodstream according to the physiological needs of the body. Both hormones are transported from the hypothalamus to the neurohypohysis by neurophysin carrier proteins along the nerve cell axons and can be visualized by immunocytochemistry in the dilated axon terminals called 'Herring bodies'.
Thus, the posterior lobe of the pituitary takes part in the regulation of blood pressure and urine output, through the hormone vasopressin, and activity of the uterine muscles, through the hormone oxytocin. Oxytocin is also important in stimulating milk release through its contractile action on muscle elements in the mammary gland. It also stimulates uterine smooth muscle contraction at parturition. As mentioned before, the vasopressin and oxytocin are formed in the paraventricular and supraoptic nuclei of the hypothalamus, from which they enter the posterior lobe of the pituitary.
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| Above, the pituitary gland at the base of the brain, being divided into adenohypophysis and neurohypophysis. |