Executioner caspases are expressed in the developing heart and their conditional deletion in the myocardium reduces myocyte number at birth.

<p><b>(A)</b> Expression of caspases-3,-6 and -7 in total protein extracts of hearts of mice at different ages from embryonic day 16 (E16) to postnatal day 30 (P30). Alpha-actin is a marker of myocyte differentiation. Glyceraldehyde-phosphate dehydrogenase (GAPDH) is a loading control. <b>(B)</b> Subcellular distribution of caspase-3 (Casp-3) and caspase-7 (Casp-7). Tot: total extract, Nuc: nucleus, Mit: mitochondria, Cyt: cytosol. DLD: dihydrolipoamide dehydrogenase (mitochondrial marker); GAPDH (cytosolic marker); lamin (nuclear marker). <b>(C)</b> Heart weight of 3-day-old and 7-month-old wild type (WT) and caspase-3 and caspase-7 knockout (KO) mice. N = 6 per age and genotype. <b>(D)</b> Cardiomyocyte number in hearts of male and female wild type (WT) and double mutant (KO) mice. N = 5 per gender, age and genotype. <b>(E)</b> PCNA (Proliferating Cell Nuclear Antigen) immunostaining of heart tissue slices from 3-day-old wild type and double mutant mice and graph showing percent PCNA⁺ nuclei in cardiomyocytes (fibroblasts have small and compressed cytosol). Bar: 20 µm. **, p<0.01 Student’s-t test KO vs. WT. Bars are means ± s.e.m.