Neuronal differentiation in somatosensory cortex of the rat. I. Relationship to synaptogenesis in the first postnatal week.

Newborn (P-0 and P-1) through 6-day-old (P-6) rats were studied using light microscopic (Golgi) and ultrastructural methods. Previous studies demonstrated that early-formed synapses are concentrated at specific cortical depths, i.e. in strata. The present study shows that the synaptic stratum in the marginal zone corresponds to a dense fiber plexus and few somata (Cajal-Retzius cells). Axons in this zone almost exclusively form synapses on distal branches of dendrites originating in deeper lamina. In newborn neocortex there is a second synaptic stratum located deep to the cortical plate. It contains numerous axosomatic and axoproximal dendritic synapses as well as the most highly differentiated somata and proximal dendrites. By age P-6 there are 3 synaptic strata; one each in the marginal zone, cortical plate and 'subplate' layers. For all 3 strata a neuron's most differentiated dendrites are directed towards, traverse or run within, the nearest synaptic stratum. We conclude that, throughout the first postnatal week, the most mature dendrites of a given neuron generally occur at depths where synapse density is highest. At P-0 the most mature somata are similarly related to synaptic density.