Fig. 6. Cell proliferation and digit regeneration. BrdU incorporation was studied in 2-day regenerating digits. (A-D) Frontal sections with distal toward the top of the image. (A) BrdU incorporation identifies a population of proliferating cells at the apex of a regenerating wild-type blastema. (B) The absence of a regeneration response in Msx1 mutant digits is associated with very little BrdU incorporation. (C) BrdU-labeled cells are shown in the regenerating blastema of Msx1 mutant digits treated with BMP4. (D) Noggin treatment inhibits cell proliferation in amputated wild-type digits.