Anatomy of the hypothalamic-pituitary axis

The recent discovery of several hypothalamic factors involved in the regulation of anterior pituitary function and the development of sensitive immunocytochemical techniques have greatly contributed to a better identification of the hypothalamic nuclei and individual neurons involved in the production of the hypophysiotropic factors. Each peptidergic factor is produced by a group of parvocellular neurons located in different hypothalamic areas and projecting to the external zone of the median eminence. These results obtained in the last few years fully confirm previous data obtained on the basis of lesion experiments which had established that the hypophysiotropic areas were localized in the hypothalamus. Studies performed at the electron-microscopic level indicate that hypophysiotropic peptidergic factors are contained in dense core vesicles within the cytoplasm of neuronal cell bodies. From there, the dense core vesicles migrate along the axons to reach the external zone of the median eminence where they are stored in endings. On appropriate stimulation, the hormones are released into the capillaries of the pituitary portal plexus. These parvocellular neurosecretory systems then appear to be very similar to the classic magnocellular neurosecretory systems involved in the secretion of vasopressin and oxytocin (fig. 1).