ULTRASTRUCTURE AND FUNCTION IN NEUROHYPOPHYSIS OF THE TOAD1

Abstract

An electron microscope study was carried out on hypothalamic portions of the preoptic-neurohypophyseal tract and in the infundibular process of toads in normal water balance or treated experimentally to produce release of the posterior pituitary hormones. Axons containing neurosecretory granules have been observed in the hypothalamus (region I) in the hilum of the infundibular process (region n), near the capillaries (region III) and at the ending proper (region IV). The neurosecretory granules, that are formed within special dense vesicles, have a high electron density and are surrounded by an enveloping membrane. Their size increases from 620 A in region I to 1350 A-1500 A in regions II and HI. At the ending (region IV) a second component appears which is similar to the synaptic vesicles described by De Robertis and Bennett (1954). The mean size of the vesicles is of 420 A and the histograms show no overlapping between these components. Within the axons of region I to III neuroprotofibrils and finer neuro-filaments are observed. In all parts of the axons there are bizarrely shaped mitochondria that bear no relationship to the neurosecretory material. In chronically dehydrated animals the neurosecretory granules disappear from the endings but persist in other regions of the axon. At the endings the synaptic vesicles remain and appear to increase in number after chronic dehydration. In acutely dehydrated animals (injection of hypertonic NaCl) most neurosecretory granules disappear and only some empty membranes are observed at the endings. In this case also the synaptic vesicles seem to decrease in number. Ether anesthesia produces a massive release of neurosecretion and vacuo-lization of the nerve endings. The concept of axoplasmic flow of neurosecretion is discussed on the basis of our findings. It is concluded that in addition to the streaming of secretion there should be a progressive synthesis of material along the axon. The probable mechanisms of release are discussed and compared with those in other neurohumoral secretory processes. A hypothesis is advanced about the possible role played by the synaptic vesicles in the function of neurosecretory nerve fibers.