Since the early 1980s, glial research has centered around an oligodendrocyte type 2 astrocyte progenitor (0-2A) first discovered in culture of the optic nerve. 0-2A progenitors in culture generate oligodendrocytes constitutively but can give rise to process-bearing astrocytes (so-called type 2 astrocytes) when treated with 10% fetal calf serum. Attempts to find cells in vivo with the type 2 astrocyte phenotype during normal development have failed. Most astrocytes are generated prenatally and during the first postnatal week, before oligodendrocyte formation, suggesting that there is not a second wave of type 2 astrocytes in vivo as suggested by previous in vitro studies. Franklin and Blakemore noted that the apparent discrepancies between the data obtained in vitro and in vivo highlights an important aspect of the approach in vitro. When a progenitor cell is grown in tissue culture, the environments to which it can be exposed are restricted only by the imagination of the experimenter. By exposing the cell to a variety of signals, the differentiation potential of the cell can be examined. In contrast, during development, a progenitor cell is exposed to a restricted sequence of signals that are spatially and temporally programmed.
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