Gradient isolation of glial cells: evidence that flat epithelial cells are astroglial cell precursors.

Discontinuous gradients of metrizamide were used to separate the cell components of monolayers of primary cultures of embryonic rat brains. These primary cell cultures were of two types: long-term cultures (more than a year) of embryonic rat brain, which contained several glial cell types, and monolayers of cell cultures (several weeks old), which contained a complex population of cells, including neuronal elements. The gradient separation produces fractions of pure flat epithelial cells that are able to survive and proliferate. After a few days, all flat epithelial cells become confluent and show a positive reaction to glial fibrillary acidic protein (GFAP); this indicates that these cells astroglial precursor cells. Following their maintenance in vitro for several months, all cultures give rise to a pure population of astrocytes identified not only by their characteristic morphology, but also by their content of GFAP. It is proposed that the differentiation controls are dependent on cell interactions that are influenced by the composition of the cell population and/or the molecular growth and differentiation factors released by these cells into the medium.