The freeze-fractured median eminence. I. Development of intercellular junctions in the ependyma of the 3rd ventricle of the rat.

Ependymal junctions in both the ventro-lateral wall and infundibular floor of the 3rd ventricle were examined in adult, young, neonatal, and fetal rats in freeze-fracture preparations. During late fetal and early postnatal life, tight junctions developed in both ventro-lateral and floor ependyma with, first, the appearance of an area of waffle-like or honeycomb texture in the membrane where tight junctions will appear; second, the decoration of P-face crests of this honeycomb with individual particles; and lastly, fusion of these particles into smooth strands. In adults, tight junctions were rare in the lateral ependyma, but persisted on the floor as a loose network. Many small ependymal gap junctions were already present at the earliest fetal stage (17 days) examined. They occupied significantly larger proportions of the membranes on the ventro-lateral wall than on the floor throughout the time course due to an increase in their size but not in their numbers. The smallest gap junctions were regarded as the newest in a process of renewal throughout life. The development of both junctional types parallels what is known on the onset of neuroendocrine functions in the median eminence. In the rat, this is apparently just before, at, or just after birth.