Synaptogenesis in the neocortical anlage and early developing neocortex of rat embryos

The recent finding of synapses in Monodelphis domestica (South American grey short-tailed opossum) before the establishment of the cortical plate raises the question of whether this finding is species-specific. Therefore, the establishment of the first synapses in the developing neocortex has been studied in the sensorimotor cortex of rat fetuses with a gestational age ranging from embryonic day 12 (E12) to birth. At E14, we found well-defined synapses with postsynaptic thickening and containing several vesicles in the presynaptic element in the primordial plexiform layer, before the appearance of the cortical plate. The postsynaptic elements were probably dendrites of Cajal-Retzius cells in the primordial plexiform layer and/or differentiating dendrites of presumptive cortical plate neurons. At E16, in addition to the presence of axodendritic and very few axosomatic synapses in the marginal and the subplate zones, synapses of both types were present within the cortical plate. Thus, this paper provides evidence for the presence of synapses in the developing rat brain both at an earlier stage and with a wider distribution than previously reported.