Glial cells: modulators of neuronal environment.

Studies of glial cells in neural tissue culture systems suggest that glial cells subserve different functions during development and aging of the central nervous system and that they may help modulate the neuronal environment by virtue of their responsiveness to hormones and other intrinsic factors. There is a marked proliferation of glial cells during early stages of brain development, probably reflecting the involvement of glial cells in myelination and other growth processes. Studies in culture suggest that proliferation of glial cells can be induced by steroid hormones. The migration rate of glial cells from cerebellar explants of embryonic chick brain grown in organotypic culture was measured in control and hormone-treated explants. Treatment with cortisol, corticosterone, estradiol, and progesterone significantly elevated glial cell migration from the tissue explants. The influence of steroid hormones on glial cells may be mediated via a steroid intracellular mechanism. In C-6 glioma cells and in chick embryo dissociated brain cell cultures consisting predominantly of glial cells, 3H-corticosterone was shown to accumulate by a saturable but non-specific retention mechanism. In contrast, the accumulation of 3H-corticosterone by predominantly neuronal cultures was both saturable and specific. Glial cells in culture exhibit certain age-related changes, including changes in resting membrane potentials and in cellular responses to hormone treatment, as measured by changes in incorporation of 3H-leucine into protein and incorporation of 3H-uridine into RNA. The possibility that glial cells in vivo may likewise exhibit differential responses to hormones throughout the lifespan suggests that hormones may markedly influence cellular aging.