The caudal neurosecretory system of fishes

Abstract

The comparative microanatomy, cytology, and ultrastructure of the caudal neurosecretory system of teleost fishes are surveyed, and some of the variations in morphologic features of the urohypophysis are indicated, based on the study of 125 teleost species. The neurohemal organ, the urohypophysis, was present in every teleost specimen studied. The general morphology of the teleost caudal system parallels the cranial (hypothalamo-neurohypophyseal) system in the tetrapods. The ultrastructural characteristics are also strictly comparable; the “Gomori-negative” neurosecretion of the caudal system is composed largely of elementary neurosecretory granules in the 1000 to 3000 A range. These granules appear to be organized in the perikaryon by the Golgi apparatus. No urohypophysis has been found in any nonteleost fish. The caudal neurosecretory neurons are present in elasmobranchs; here the neurochemal area is a diffuse capillary bed on the ventral surface of the spinal cord. Some primitive isospondylous teleosts, as well as some highly specialized forms, have a diffuse and extended urohypophyseal region. The possible evolution of this system is considered. Experimental studies on urohypophysectomized Tilapia mossambica exposed to hypertonic saline, with and without attempted replacement therapy, support an osmoregulatory role for the urohypophysis. Tilapia without their urohypophyses, subjected to a hypertonic environment, showed increased mortality, increased loss of body weight, increased serum chloride levels, and increased prominence of the preoptic nucleus, as compared with sham-operated or intact controls.