Prenatal differentiation of colliculus superior in the rat

Up till emybryonic day (ED) the 13 the prenatal histogenesis of rat mesencephalic roof is characterized by an exclusive proliferation of ventricular cells. Then the first differentiating postmitotic cells are found above the ventricular layer, so that proliferation and differentiation occur simultaneously. On ED 15 a lamination typical for embyronic CNS with ventricular, subventricular, intermediate layers and stratum zonale can be observed in the area of presumptive colliculus superior. A definite demarcation between ventricular and subventricular layers, however, is only possible by means of autoradiograpahic methods. At the end of gestation the lamination of adult colliculus superior is already noticeable. During early prenatal development, cells which originate on one single day can autoradiographically be demonstrated in different cell layers and particularly over the whole width of the intermediate layer, while towards the end of gestation time of cell origin and location inside the mesencephalic roofs are closely correlated: cells that originate on ED 13 can be found predominantly in deeper cell layers (stratum griseum profundum, lower stratum griseum intermedium), while cells originating on EDs 15 and 17 are on ED 21 situated in the upper cell layers (upper stratum griseum intermedium, stratum griseum superficiale). This results in an inside-outside gradient of cell origin, which corresponds to an inside-outside gradient of cell differentiation. Besides, a rostro-caudal and a lateromedial gradient can be observed. On ED 13 a median ventriculaar formation (MVF) can be distinguished displaying a significantly lower mitotic activity than the cells of the neighbouring ventricular layer as could be demonstrated by autoradiographic investigation. This difference is even more pronounced in latter embryonic stages. From ED 17 on the cells of this particular structure are arranged in a typical onion-like pattern. Towards the end of gestation the MVF expands considerably as compared with the surrounding ventricular layer. As long-time experiments with 3H-thymidine reveala this increase in area is brought about by the immigration of numerous ventricular cells which originated on earlier embryonic days. The apical processes of the cells of the MVF form a thick fibre bundle which can be followed upwards to the stratum zonale. Ulstructurally these strictly parallel arranged fibers mainly resembly astroglial processes, but possess, in contrast to the latter, numerous microtubules. Because of their morphological characteristics a classification of these cells as tanycytes appears justified. The MVF is compared with similar structures in other regions of the CNS and its possible function as a guide for migrating preneurons is discussed.