Postnatal maturation of nonpyramidal neurons in the visual cortex of the cat.

A Golgi study of nonpyramidal neurons in the visual cortex of kittens aged from 1 to 80 days revealed that different neuronal types undergo a differential sequence of maturation. The earliest nonpyramidal cells to differentiate are large multipolar cells of layers 3-5, which appear around birth and whose axons gradually establish long lateral, intracortical connections. They are followed by spiny stellate cells of layer 4, which appear in the first postnatal, week, and by neuroglioform cells in layers 4 and 5, a cell type which at 10 days displays a highly differentiated axonal plexus. In general, most classes of local axon cells can be identified by the end of the second week, though still possessing a very immature morphology, the axonal-tuft cells of layer 2 maturing later, in the third week. With some exceptions, most neurons exhibit an adultlike axonal arborization by the end of the first month; however, immature chandelier terminals are observed until the 40th day, and in kittens aged from 30 to 80 days, the vertical terminal segments of chandelier cells are larger than in the adult. Some neuronal types seem to present an exuberant growth of axonal fibers in the late postnatal period and a subsequent reduction up to the adult stage.