Development of the rabbit visual cortex: a quantitative Golgi analysis.

Lamina IV stellate cells and lamina V pyramidal cells were studied in Golgi material of visual cortex of rabbits ranging in age from 10 days to adult. Spine density counts revealed that primary branches have lower spine density than secondary or tertiary branches in both stellate and basilar pyramidal dendrites. Most areas sampled showed an increase in spine density from age 10 days to a peak at 25-30 days. In some areas this was followed by a plateau, but in most dendritic areas sampled there was a significant decrease from peak levels to adult levels. Measurements of dendritic length revealed that basilar dendrites undergo changes in length which parallel the changes in spine density counts: a peak in the length of basilar dendrites was followed by a decline to adult levels. However, the dendritic length of stellate cells showed much less change with age after 10 days. We propose that the time period during which spine density and pyramidal cell dendritic organization peaks above adult levels may coincide with, and provide a morphological correlate of, the critical period.