Histology and ultrastructure of the neural lobe of the lizard,Klauberina riversiana

Summary The pars nervosa of Klauberina riversiana belongs to a primitive tetrapod type which is characterized by the deep penetration of the infundibular recess, a thin-walled structure, and the virtual absence of pituicytes. The differential response of this gland to aldehyde fuchsin and periodic acid Schiff suggests the presence of two types of neurosecretory nerve endings. Ultrastructurally four kinds of nerve endings are distinguishable. Type I, probably a cholinergic nerve ending, contains only small clear vesicles ca. 400 Å in diameter. The relative abundance of cholinergic nerve endings in this pars nervosa may be related to the necessity of transporting hormone through the ependymal cell. Type II, containing granulated vesicles about 1,000 Å in diameter and probably aminergic, is very rare. The two remaining types apparently secrete neurohypophysial hormones. They are Type III, containing dense granules ca. 1,500 Å in diameter and Type IV containing pale granules ca. 1,500 Å in diameter. Evidence is reviewed which suggests that Type III nerve endings may secrete arginine vasotocin while Type IV endings may secrete (an)other hormone(s).All these axons end only on the ependymal cells, the vascular processes of which form a continuous cuff over the basement membranes of the blood vessels. Hence the ependymal cells link the cerebrospinal fluid, the nerve endings and the blood vessels. Particles resolvable with the electron microscope are traced through a possible transport pathway from the granules, through the ependymal cells to the basement membrane. It is suggested that pituicytes replace ependymal cells and assume their transport functions in animals with massive neural lobes containing large numbers of nerve endings and blood vessels.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina.This investigation was supported in part by a Public Health Service fellowship 1 FZ HD 32,949-01 REP from the national Institute of Child Health and Human Development.The authors wish to thank Professor H. Heller for his constant interest and constructive criticism.