An in vitro model of wound healing in the CNS: analysis of cell reaction and interaction at different ages.

We have developed an in vitro model in which cells responding to trauma in the immature and mature CNS can be isolated, placed into serum-free culture, and characterized. By implanting nitrocellulose filters into the brains of neonatal and adult rats under different conditions, we are able to harvest populations of cells responding to trauma in the neonate (critical period implant), in the adult (scar implant), and in implants that have remained in vivo past the critical period (postcritical period implant). Upon placement in culture, we have found that astrocytes represent the majority of cells occupying both the critical period and postcritical period implants, whereas fibroblasts and macrophages represent the majority of cells in the glial-fibroblastic scar. The morphologies of the astrocytes on the surface of the different implants, after 3 days in culture, differs markedly--the critical period astrocytes exhibiting a more ordered distribution compared to the haphazard arrangement of astrocyte processes on the surface of the postcritical and scar implants. After migration from the implant, critical period astrocytes assume an epithelioid morphology and cluster together setting up definite boundaries between themselves and the endothelial cells. In contrast, postcritical period astrocytes exhibit a more elongated morphology under the same culture conditions and appear to be randomly dispersed among the endothelial cells. The scar astrocytes exhibit a wide range of morphologies and, although they tend to cluster, do not exhibit the ordered association seen with the critical period astrocytes. We propose that the plasticity of the neonatal astrocytes and the rapid and ordered cellular response seen in vitro reflect the ability of the immature CNS in vivo to respond to injury without the formation of a glial-fibroblastic scar.