Early neurogenesis and synaptogenesis in cerebral cortex.

Early cortical neurogenesis and synaptogenesis are discussed with reference to observations made in fetal and postnatal rat temporal cortex. Thymidine autoradiography was used to determine the time of origin of cortical neurons, particularly of layer I. Earliest neurons--CRs--form between FD 12 and 15, whereas the period of maximal formation of layer I neurons other than CRs is situated between FD 17 and FD 19. Neurons destined for the deepest part of layer VI begin to form at FD 13. Both types of early-forming neurons develop synaptic contacts by FD 16. From this stage onwards, the temporal cortex taken as a whole continuously contains synaptic contacts. However, a more detailed analysis shows that some at least of the early formed synapses are transitory: for example, the CRs, which receive dendritic synapses on FD 16 and somatic synapses on FD 17, apparently lose most--if not all--of these contacts towards the end of fetal life. Whereas CRs develop an important protein producing equipment during the neonatal period, younger neurons located in the same (first) layer form dendritic, somatic and somatofugal synapses.