The development of pyramidal neurons after eye opening in the visual cortex of hooded rats: a quantitative study.

Dendritic maturation of pyramidal neurons in layers III and V of the rat visual cortex was quantified in rapid Golgi-stained tissue from socially housed hooded rats at ages 15, 30, and 60 days. A complex pattern of changes emerged. Dendritic branching analyses revealed that in layer III both bifurcating and terminal basilar branches increased in length over the sampled days. Likewise, total dendritic length increased in the layer III basilar dendritic tree between 15 and 30 days of age and peaked at 30 days of age in the layer III apical oblique branches. No changes in dendritic length were observed in layer V pyramidal neurons. Neither layer III nor layer V neurons exhibited any changes in the number of basilar or apical oblique branches over the time period studied. Dendritic spine density increased on terminal basilar branches in both cell populations between 15 and 30 days. There also was an increase in spine density on the layer V pyramidal apical shaft between 15 and 30 days, while on layer III bifurcating basilar branches, spine density fell between 30 and 60 days. These data indicate that while dendrites may continue to grow longer, pyramidal neurons do not on average generate additional branches after day 15. However, the pattern of spine density is still changing between 15 and 60 days of age. Thus no single developmental pattern holds for all cell populations or even across areas of the dendritic field within a population.