Electrophysiology and morphology of the developing hippocampus of fetal rabbits

The pyramidal neurons of fetal rabbit hippocampus were studied using intracellular electrophysiological techniques in in vitro slice preparations. Correlative light and electron microscopic analyses were carried out on hippocampus during the 21st through the 29th day of fetal gestation. In intracellular experiments, neurons with all-or-none action potentials and near-adult level resting potentials were found even in the youngest preparations. Synaptic activity, however, was rare until about 24 days of gestation. CA1 neurons showed primarily excitatory synaptic potentials during fetal development, whereas CA3 neurons displayed both inhibitory and excitatory postsynaptic potentials at early stages. Anatomical studies suggested that pyramidal cell precursors were still dividing and migrating at 21 days; by 29 days, cellular migration was completed, and cellular intercommunication in the form of synapses was increasing. These experiments demonstrate that fetal central nervous system (CNS) material can be studied electrophysiologically without growing tissue in culture. Our results suggest that the newly differentiated hippocampal neurons have a limited repertoire of activities. Such data may provide a link between in vivo studies of postnatal CNS development and cell and tissue culture investigations of the properties of immature neurons.