Light and electron microscopical studies on specific cells of the marginal zone in the developing rat cerebral cortex

Summary

The development of specific cells in the marginal layer of the occipital cortex of the rat was studied both light and electron microscopically. These cells are the first cells of the cerebral cortex to begin differentiation on embryonal day 13 when they enter the stage of beginning ramification. The maturation of these cells proceeds rapidly; on day 17 of gestation they are in the stage of accumulation of ergastoplasm, on day 19 most of them have reached the stage of orientation of ergastoplasm. 1 to 2 days before birth the majority of these horizontally oriented cells exhibit large Nissl bodies in their cytoplasm which often surround the whole nucleus and extend into both processes. The ultrastructure of the horizontal processes of these cells cannot readily be compared with that of conventional protoplasmic processes, buth they rather appear more like extensions of the perikaryal cytoplasm, the structure of which is retained along the course of the processes.

These specific cells which are limited to and specific for the marginal zone of the cerebral cortex thus are the first cells of the cerebral cortex to reach cytological maturity—even before birth—and therefore do not fit into the concept of an inside-out gradient of neurogenesis and differentiation in the development of the cerebral cortex as proposed by a number of authors, because the deeper situated preneurons of the other cortical layers have at this time only just begun differentiation from the stage of ventricular cells. Their functional role as a ‘fetal cell’ in the morphogenesis of the cerebral cortex and their identity with the horizontal cells of Cajal-Retzius is discussed.