Sexual differences in the ependyma lining the third ventricle in the area of the anterior hypothalamus of adult rhesus monkeys

Summary Previous studies have shown that a circumscribed region of the anterior hypothalamus of the rhesus monkey is lined by tanycyte ependyma and it has been suggested that this ependyma which links the third ventricle with the pars tuberalis may have a functional role in the hypothalamic regulation of anterior pituitary function (Anand Kumar and Knowles, 1967a). In view of the known sexual differences in the hypothalamic regulation of pituitary gonadotropin secretion the present investigation was made to determine whether any structural differences were evident in the tanycyte ependyma in male and female rhesus monkeys.The results of this investigation are based on light and electron microscopic studies of the hypothalamus in 24 rhesus monkeys comprising 12 adult females, 11 sexually mature males and a two month old sexually immature male.The tanycyte ependyma in the rhesus monkey is double layered. There are bulbous projections on the ventricular surface of the cells in the ependymal layer nearest to the ventricle (the first layer of ependyma). These bulbous projections vary in size in relation to the menstrual cycle. They are well developed during mid-cycle and regressed during menstruation. In the males, where the secretion of pituitary gonadotropins does not occur cyclically as in the females, there was no marked variation in the bulbous projections between different individuals as in the female monkeys.In the sexually mature males, but not in the females, the two layers of ependyma are separated by a distinct space. The absence of such a space in the sexually immature male suggests that this difference may be related to sexual maturity.In the adult males the cells in the ependymal layer below the first layer of ependyma have microvilli which extend into the space between the ependymal layers. In the females where such a space is not present, microvilli were not evident.The precise functional significance of the tanycyte ependyma is not known. It is hoped that the results of the present investigation would draw attention to the need for a more detailed examination of the physiological role of the tanycyte ependyma in relation to reproduction.The expenses for this investigation were met from a grant made by the Ford Foundation to Professor Sir Solly Zuckerman and the electron microscope was provided by the Medical Research Council. I am indebted to Sir Solly for his interest in this work.