Fig. 3 . Late effects on proliferation and apoptosis in the Mi-2ß-depleted ventral epidermis. (A,B) The percent of Pcna-positive cells within the basal layer (K5-positive) of WT and mutant skin was estimated during development. Sagittal sections were co-labeled with antibodies against the proliferation marker Pcna and K5. Nuclei were counterstained with DAPI (A). The data (B) represent the mean percentage of Pcna-positive cells within the basal layer (K5- and DAPI-positive) from five independent animals. From E14.5 through E16.5, a similar number of Pcna-positive basal cells were seen in both the mutant and WT skin. However, starting at E18.5 and through P1, a significant reduction of Pcna-positive cells was detected in the ventral, but not the dorsal, mutant regions. (C) Apoptotic cell nuclei (brown) were detected by TUNEL analysis on ventral skin at P1. In both the WT and mutant, TUNEL-positive cells were detected in the uppermost layer of the epidermis, but the dramatic increase in their number seen in the mutant suggests persistence of nucleated dead cells and a defect in terminal differentiation. Scale bars: 50 µm.