Fig. 3. Id3-depleted embryos form an excess of CNS progenitors. (A,B) In situ hybridization of Id3-depleted embryos. In the absence of Id3, the injected side of the embryo shows greatly expanded expression of Sox3 (A, arrowhead), a marker of the proliferating multipotent CNS progenitor pool. Similarly, expression of Opl, a marker of early CNS and border cell fates is also greatly expanded (B). (C) Phosphohistone H3 immunocytochemistry of Id3-depleted embryos. No difference is seen in the number of actively cycling cells on the control versus Id3-depleted (arrowhead) side of the embryo. (D) Whole-mount TUNEL staining of Id3-depleted embryos. No change in the numbers of apoptotic cells is noted on the Id3-depleted side of the embryo (arrowhead) as compared with the control side. Red staining is lineage tracer ß-gal.