Using a new technique that allows up to 90 distinct colors to be introduced into embryonic cells, Duke cell biologists watched the multiplication of heart muscle cells as zebrafish grew. Each patch of color represents the descendant cells of a single embryonic cell (the oval dots are cell nuclei). This image reveals the unexpected finding that one population of cells, here dyed green, advanced in a sheet over the top of the other cells. "A very small number of cardiomyocytes (heart muscle cells) in the growing animal can give rise to the thousands of cardiomyocytes that form the wall of the cardiac ventricle," said Vikas Gupta, a graduate student seeking M.D. and Ph.D. degrees in the lab of Kenneth Poss of Duke Cell Biology and the Howard Hughes Medical Institute. Citation: "Clonally dominant cardiomyocytes direct heart morphogenesis," Vikas Gupta & Kenneth Poss. Nature 26 April 2012. doi:10.1038/nature11045