Maturation of rat visual cortex. II. A combined Golgi-electron microscope study of pyramidal neurons.

Tissue removed from 3, 6, 9, 12, 15, and 21-day-old rats has been prepared for correlative light and electron microscopy to examine the maturation of cell bodies, dendrites, and axons of pyramidal neurons in layer V of rat visual cortex. As the size of the cell body increases steadily during the first 3 postnatal weeks there is an equivalent growth in nuclear volume. By day 15, there are infoldings in the nuclear envelope which may be induced by eye opening on day 14. Nucleoli increase in size until day 9, after which they appear to condense. Within the perikarya, the most conspicuous change is the amount and organization of the rough endoplasmic reticulum. Symmetric axosomatic synapses are evident by day 6. The ultrastructure of dendrites does not change substantially with age. Dendrites form synapses with symmetric densities as early as day 3 and asymmetric ones by day 9. It seems that dendritic spines begin as low, broad protrusions having symmetric junctions with smaller diameter axonal processes. With time they become taller stumps, before acquiring their mature lollipop shape and participating in asymmetric synapses with axonal varicosities. Other dendritic appendages, filopodia, and growth cones are transient structures, being conspicuous only between days 3 and 12. "Terminal" growth cones are essential for extension of dendritic processes, whereas "en passant" growth cones and filopodia seem important for dendritic branching. Boutons of mature pyramidal cell axons form asymmetric synapses with dendritic shafts and spines, but the developing synapses formed by these axons have more symmetric junctions. The maturation of pyramidal cell features progresses in concert with such extrinsic determinants as afferent input and is probably influenced by the competency of synaptic connections.