Development of layer I neurons in the primate cerebral cortex

Layer I, which plays an important role in the development of the cerebral c ortex, expands in size and diversity in primates. We found that, unlike in rodents, in the macaque monkey, neurons of this layer are generated during the entire 2 month period of corticogenesis, within the middle of the 165-d -long gestation. The large, classical Cajal-Retzius cells, immunoreactive t o reelin and calretinin but not to GABA, are generated first [embryonic day 38 (E38)-E50], with the peak of [H-3] thymidine ([H-3] TdR) labeling at E4 3. Ultrastructural analysis revealed that processes of these cells form a s tereotyped, rectangular network oriented parallel to the pial surface. Gene sis of smaller, GABAergic neurons begins slightly later (E43), reaches a pe ak of [H-3] TdR labeling between E54 and E70, and continues until the compl etion of corticogenesis (E94). These late-generated layer I cells are impor ted from outside sources such as the olfactory primordium and ganglionic em inence and via a massive subpial granular layer that may also supply some G ABAergic interneurons to the subjacent cortical plate. The ratio of large-t o-small layer I neurons changes differentially, indicating that each class is produced and/or eliminated at a different rate and suggesting that their roles in primates are diverse.