Age‐dependent changes in the proliferative response of S‐100 protein‐positive glial cells to injury in the rat brain

A mechanical injury was inflicted to the left cerebral hemisphere in rats of four age groups: newborns, 6, 14 and 30 days old. The injury was followed by [ 3 H]thymidine injections at different time intervals. Brain sections were immunostained for S‐100 protein and subjected to autoradiography. During microscopic observations of the injury region, locations and numbers of the autoradiographically labeled astrocytes expressing S‐100 protein were recorded. On the basis of the observations, injury‐induced changes in the total number of proliferating astrocytes, as well as in their distribution, were analysed quantitatively. In rats injured neonatally, as well as those injured on postnatal days 6 and 14, the reactive increase in the number of proliferating astrocytes began on the first post‐traumatic day. In 30‐day‐old rats the increase was slower and appeared on day 2. The maximal increase in the astrocyte proliferative activity occurred in 6‐day‐old rats as early as day 1 after injury and was about eight times higher than that recorded in newborns, and nearly twice as high as that recorded in brains of 30‐day‐old rats. The results suggest that the intensity of astrocyte proliferative response to injury cannot be regarded as simply being proportional to the developmental progress of the brain tissue. Rather, these results indicate that changes in glial proliferative responses to injury follow a developmental time course, with a peak around the end of the first postnatal week.