Development of the diencephalon in the rat. III. Ontogeny of the specialized ventricular linings of the hypothalamic third ventricle.

The development of the specialized linings of the hypothalamic third ventricle was examined autoradiographically in mature rats that were labelled with 3H-thymidine during the developmental period, and in a closely spaced series of embryonic and infant rats. We distinguished in mature rats, apart from the typical ependymal wall, three specialized linings: the convoluted ependyma, the laminated epithelium, and the tanycytic epithelium. The ventricular wall of most of the anterior hypothalamus, and of the dorsal portion of the posterior hypothalamus, is composed of ciliated ependymal cells and most of them are generated several days before birth, soon after the cessation of neurogenesis in the adjacent hypothalamic nuclei. The cells of the rostral convoluted ependyma adjacent to the paraventricular nucleus are produced at about the same time as the neighboring cells of the smooth ependyma. Its cells come from the same germinal region that we have assumed to generate the neurons of the magnocellular neurohypophysial secretory system. The structural differentiation of the convoluted ependyma starts after birth and is completed by the beginning of the second week. Many of the ependymal cells of the laminated epithelium are produced postnatally, and the production of the specialized cells that form a parallel subependymal row extends into the third week. These cells appear to arise from the same matrix that generates earlier the neurons of the dorsomedial and ventromedial hypothalamic nuclei; their structural differentiation begins during the second week. Also the cells of the tanycytic epithelium are produced mostly postnatally, predominantly during the first week. They appear to arise from the same matrix that generated earlier the neurons of the hypophysiotropic tuberomammillary and arcuate nuclei. It is postulated that these three specialized ventricular linings are specifically related to the three cpmponents of the endocrine hypothalamus with which they have shared neuroepithelial sites of origin.