Fig. 6.

Model of the possible relationships between Fgf2 availability and cell cycle events. A, Salt-and-pepper appearance of Fgf2 mRNA in the early PVE (Vaccarino et al., 1999a), suggesting that cell-to-cell variability in internal sources of Fgf2 may represent a primary factor governing Fgf2 exposure within these cells and their neighbors. B, At later stages of development, Fgf2 is downregulated within the PVE except for the cells close to the apical surface. The interkinetic nuclear movement coupled to the cell cycle is shown in relation to the hypothesized action of Fgf2. In a subset of progenitor cells, Fgf2 is necessary in early G1 to promote commitment to a subsequent cell cycle (DeHamer et al., 1994). During early G1, cells are also close to the ventricular surface, which could be a source of Fgf2 protein. Fgf2 may be released into the CSF by PVE cells or by cells of the choroid plexus, which contain high amounts of Fgf2. The increased nuclear shading represents the association of Fgf ligands with the nucleus, also occurring during the G1 phase (Zhan et al., 1993; Prudovsky et al., 1994).