The choroid plexus (CP) secretes the cerebrospinal fluid (CSF) that fills the ventricles of the brain and bathes the central nervous system (CNS), but little is known about the development of this important tissue. CP forms in the ventricles at or near the embryonic dorsal midline (DM). We have previously shown DM-CP lineage relationships using Gdf7 (Bmp12) transgenic mouse embryos to fate map DM cells (Monuki et al., SFN abstract, 2004). We further analyzed the contribution of DM cells to the telencephalic CP epithelium (tCPe) and discovered that Gdf7 cell lineages are found in the small anterior domain of the tCPe, but not in its large posterior domain. In addition, we studied patterns of apoptosis in the anterior and posterior tCPe domains by TUNEL. At both E10.5 and E11.5, significant apoptosis was detectable in the anterior tCPe, but not in the posterior tCPe domain. Like the Gdf7 fate map, apoptosis studies identified an inapparent boundary between anterior and posterior tCPe domains at roughly the s...