Neurosecretion in birds

Abstract

Morphologically the hypothalamic neurosecretory system of birds includes (1) two rather diffuse neurosecretory nuclei, the paraventricular nucleus and supraoptic nucleus which, according to the species concerned, consist of several more or less distinct divisions; (2) two hypothalamo-hypophysial neurosecretory tracts, the paraventriculo-hypophysial tract and the supraoptico-hypophysial tract, which are not separately identifiable in many species; (3) the neurosecretion-rich infundibular branches of the hypothalamo-hypophysial neurosecretory tract in the median eminece; and (4) the neurohypophysis and the neurosecretory fibers leading thereto. Specific functions cannot be assigned as yet to the individual neurosecretory nuclei or to their divisions. For the distal parts of the system, functions may be divided into those of the neurohypophysial and eminential components. The former is identified primarily with the regulation of water balance and apparently also with the ecbolic function in ovulation. The hormone involved is probably arginine vasotocin which is probably produced by hypothalamic neurosecretory cells and transported in association with the stainable neurosecretory material via the hypothalamo-hypophysial neurosecretory tract to the posterior lobe. Although oxytocin occurs in the posterior pituitary its function is unknown. The eminential component, whose radial neurosecretory fibers emanate from the eminential plexus and extend outwardly into juxtaposition with the primary capillaries of the hypophysial portal system, is primarily involved in control of the function of the adenohypophysis. This component is best known from the studies on species in which the gonadotropic function of the adenohypophysis is photoperiodically controlled. To us, the best hypothesis of the general functional mechanism of the eminential component is that the “releasing factor” or “releasing factors” are formed in hypothalamic neurosecretory cells where they become associated with neurosecretory material, which is usually stainable with aldehyde-fuchsin or chromalum hematoxylin, and which passes via neurosecretory fibers into the radial fibers of the zona externa of the median eminence. It seems evident that the control of the adenohypophysis is exerted primarily through the transfer of such “releasing factors” from these fibers into the primary capillaries of the portal system. It is possible that this transfer involves a functional relationship between the neurosecretory fibers and fibers from the nonneurosecretory tubero-hypophysial tract. The very fragmentary data available suggest that the gonadotropic function of the adenohypophysis is almost completely dependent on such “releasing factors” although the gland does have basic adrenocorticotropic and thyrotropic functions that are independent of the median eminence. However, higher levels of thyrotropic and adrenocorticotropic activity by the adenohypophysis apparently require the appropriate “releasing factors”.