An electron microscope study of the organization of the cerebral cortex of the 60 mm rabbit embryo

Summary The embryonic cerebral cortex was examined for signs of a columnar organization. In vertical montages extending from the intermediate lamina to the pia, nerve cells arranged in columns are separated by bundles of radial processes. However, sections tangential to the pial surface revealed little evidence for repeating modular units; bundles of radial processes are not regular in outline, and surrounding groups of cells display little symmetry in their distribution.Tangential processes representing the development of a rudimentary neuropile are present in the marginal lamina and at lower levels of the cortical plate. However, well defined synapses were not seen at this stage of development when proliferation and migration of neuroblasts continues to provide cells to the upper part of the cortical plate. The association of neuroblasts with bundles of radial fibers suggests that these vertical elements serve as preferential pathways for cells migrating to the cortex.