A Golgi study of the early postnatal development of the visual cortex of the hooded rat.

Although neuroanatomical plasticity has been demonstrated in the rat visual cortex, no systematic data on the dendritic development of the area are available. In the present study, the visual cortex of hooded rats at 1, 3, 5, 7, 10 and 15 postnatal days of age (P1-P15) was impregnated with the rapid Golgi method. The cortex was divided into the superficial layers, II-IV, and the middle layer V. At P1, pyramidal neurons had apical shafts and the beginning of the apical terminal arch. Analysis of both basilar and oblique dendritic number showed that pyramidal neurons of the middle layer developed more quickly than those in the superficial layers. The number of lower order basilar dendritic branches reached asymptote over the examined time period, whereas the higher order branches were still increasing in number but at a decelerating rate by P15. Dendrites at all ages exhibited varicosities which were especially prominent on the thin dendritic branches of the earlier ages. Some thin, filamentous processes, termed protospines, were found on dendrites and cell bodies at P1 to P5. They seemed to decrease by P7, when a few mature spines appeared. Spines increased in number on days P10 and P15. A comparison of the data from this study with quantified Golgi studies in adult rats indicates that by P10 and P15 the number of basilar branches is in the range seen in the adult.