Differentiation of a bipotential glial progenitor cell: what controls the timing and the choice of developmental pathway?

In the rat central nervous system (CNS) oligodendrocytes and type-2 astrocytes are thought to develop from a common precursor - the O-2A progenitor cell. Oligodendrocytes develop first and make myelin; type-2 astrocytes develop later and extend processes to nodes of Ranvier. The timing of differentiation of O-2A progenitor cells seems to depend on chemical signals secreted by another type of glial cell - the type-1 astrocyte. Type-1 astrocytes secrete platelet-derived growth factor (PDGF), which stimulates O-2A progenitor cell proliferation and drives the clock that controls the onset of oligodendrocyte differentiation, which is the constitutive pathway of progenitor cell development. Later, type-1 astrocytes are thought to secrete a CNTF-like protein that initiates type-2 astrocyte differentiation.