To investigate the possible role of neurons and astrocytes for oligodendrocyte development we prepared a pure population of precursor cells positive for the precursor marker GD3 with the help of fluorescence-activated cell sorting (FACS). Large numbers of highly purified cells were obtained from postnatal day 1 rat brainstems and cultured in different media and sera, and in conditioned media.
As described in the literature for optic nerve O-2A progenitors, GD3-sorted brainstem cells cultured in medium containing 10% fetal calf serum (FCS) acquired a star-shaped morphology and differentiated into GD3- and GFAP-positive type-2 astrocytes. On the other hand, in serum-free medium, most of the cells differentiated into oligodendrocytes (O1-/galactocerebroside-positive).
Sensory neuron conditioned media promoted survival and proliferation of the precursor cells. The spontaneous differentiation of progenitor cells into oligodendrocytes was retarded by the mitogen. Antibodies against platelet-derived growth factor (PDGF) completely blocked the mitotic effect and allowed spontaneous oligodendrocyte differentiation to occur.
Cultured astrocytes also secreted PDGF as a mitogen. However, postnatal astrocytes also released a potent signal promoting oligodendrocyte differentiation. The type of factor(s) released depended on the age of the astrocytes, since only conditioned medium of postnatal but not of embryonic astrocytes promoted oligodendrocyte differentiation, suggesting that astrocyte maturation directly influences oligodendrocyte differentiation. Different concentrations of PDGF could not reproduce this differentiation-inducing effect.
This study suggests that interactions between O-2A progenitor cells, neurons, and astrocytes could be required to regulate and complete the oligodendrocyte developmental pathway. Astrocytes, themselves possibly under neuronal influences, might regulate first the proliferation of the precursor cells, and, later in development, the differentiation into mature oligodendrocytes or type-2 astrocytes.