Fig. 4.

The neuropil from postnatal day 12 (R45a) is much denser than in the first postnatal week, with many more profiles of small processes. Asymmetric synapses are seen on thin dendritic spines (t), as well as on stubby spines (s) and dendritic shafts (sh). Many dendritic spines at this age do not have the typical characteristics of adult CA1 spines. An atypical thin spine (a) receives synaptic contacts from two different axons. Multiple synapses are rarely identifiable on single sections. The atypical stubby (as) in the lower left has only one of its two synapses visible on this section, whereas two synapses (arrows) are apparent on the atypical stubby (as) in the inset. One postsynaptic process (?) could not be traced to its origin and, therefore, could not be unequivocally identified. Scale bar, 1 μm.