The onset of synaptogenesis was studied in the temporal cortex of rat fetuses whose age ranged between 15 and 19 days of gestation. First synapses were found at a surprisingly early stage of cortical development: on day 16. These contacts showed relatively few vesicles and very inconspicuous membrane-thickenings. They were located in the marginal layer, above as well as below the narrow band formed by the newly arrived first neuroblasts of the prospective cortical plate. The postsynaptic structures were probably dendrites of the horizontally or obliquely orientated neurons scattered throughout the marginal layer (such neurons were seen even within the cell-dense band).
On day 17, the cortical plate separated the differentiated cells definitely into a superficial and a deep population. As on the following days, synapses were found above and below the cortical plate but not within it. In addition to contacts showing the same features as those described on day 16, there were already synapses with numerous vesicles and clearly asymmetric membrane thickenings. On days 18 and 19 the borders of the cortical plate became more clear-cut. The well-differentiated neurons situated above and below this plate could now be identified as Retzius-Cajal cells of the prospective molecular layer and as polymorphous cells of the layer VI b respectively. The presence of axo-somatic contacts on these neurons provided direct evidence that both cell types are targets for synapses.
Desmosome-like junctions were found even in the youngest fetuses studied. Their roughly symmetric membrane thickenings were clearly more conspicuous than those of earliest synapses. Desmosome-like junctions occurred very frequently between structures which subsequently were never seen to become synaptically linked. During the entire period studied, numerous coated vesicles fused with cell membranes were noted. Such “open” vesicles were seen on neurons (sometimes in the immediate vicinity of synapses) but also on non-nervous, extracortical as well as intracortical structures. Thus there does not seem to be a specific relationship between desmosome-like junctions and coated vesicles on the one hand and synapse formation on the other.