Figure 3

Forced p57^{Kip 2}expression in the embryonic heart does not impair cardiomyocyte proliferation. Localization of p57^Kip2 and Ki-67 in ventricular cardiomyocytes of E9.5 wild type (WT) and R26loxpTA-p57^k/+;Mlc2v-Cre^k/+animals by indirect immunofluorescence. In WT E9.5 cardiomyocytes p57^Kip2 is not yet expressed (A-C). However, p57^Kip2 is expressed in most cardiomyocytes of the R26loxpTA-p57^k/+;Mlc2v-Cre^+/- mouse embryos as early as E9.5 (D-F). Expression of p57^Kip2 in embryonic cardiomyocytes does not preclude Ki-67 expression (E, F). DNA synthesis in the heart of R26loxpTA-p57^k/+;Mlc2v-Cre^k/+transgenic mice is similar to the hearts of wild type animals despite the abundance of p57^Kip2 protein in the cardiomyocytes of the transgenic hearts. In the merged pictures (third column), the yellow nuclei represent cardiomyocytes co-labeled by both antibodies. High power magnification of the same cardiomyocyte labeled with both antibodies, illustrates that although there are many cells stained with both p57^Kip2 and Ki-67, in most cells p57^Kip2 and Ki-67 immunoreactivity occur in exclusive nuclear domains (G, H, I).