Development of rabbit hippocampus: anatomy.

The postnatal development of the CA1 region of rabbit hippocampus was studied using a variety of light and electron microscopic (EM) techniques. Nissl and Golgi stains showed high cellular density, small cell soma area, and sparse dendritic branching in neurons of immature animals (less than 1 week old); dendritic spines were also relatively infrequent during this period. Cell branching and spine frequency reached near-adult levels by 3 weeks, with the major area of hippocampal expansion seen in the apical dendritic layer. EM examination of synapse patterns was made using osmicated and E-PTA-treated tissue. Both techniques showed that the vast majority of synapses in immature animals (less than 2 weeks old) occurred in the dendritic region and were of the asymmetric type. Axosomatic synapses became less rare by 2 weeks; they were usually of the symmetric synapse type. The pattern of synaptic contacts in immature hippocampus resembled the mature pattern by 3-4 weeks. These data suggest a relatively late development of inhibitory postsynaptic potentials in CA1 pyramidal cells.