Maturation of rat visual cortex. III. Postnatal morphogenesis and synaptogenesis of local circuit neurons.

The postnatal development of 3 types of local circuit neurons in rat visual cortex was examined in Golgi and electron microscopic preparations. During the first postnatal week, smooth and sparsely spinous stellate, bitufted and bipolar neurons were identified in Golgi material by their characteristic dendritic arborizations. Morphological differentiation begins during this week, as each neuron sprouts dendrites which extend, branch and produce spines, and ends by day 21. This differentiation was traced by quantifying the somatic area and number of primary dendrites on stellate, bitufted and bipolar neurons in layer II/III or layer V. Neurons in deep cortex differentiate earlier than those in superficial laminae. On day 3, axons are evident as short, straight processes, however, by day 6, many axons have branches and varicosities. The increase in the complexity of the axonal trees continues during the second and third postnatal weeks. Since the axons of stellate and bitufted neurons form synapses with the somata of pyramidal neurons, an index of the synaptogenesis of these neurons was traced by counting the numbers of synapses on the somata of pyramidal neurons. The mean number of axosomatic synapses increases steadily from day 3 to day 30. Layer V pyramidal neurons form axosomatic synapses before pyramidal neurons in layer II/III. In conclusion, the morphology of local circuit neurons develops during the period after they migrate into cortex. The principle that cortical local circuit neurons develop after projection neurons only applies for the synaptogenesis of the axon, but not for the maturation of the cell body and dendrites.