The heart is formed during embryogenesis from a simple tube-like structure. As it grows, the tube morphs into the complicated but familiar shape of our four-chambered heart. How one set of cells in the tube expands to acquire the shape of a ventricle while others become the atrium, for example, is unclear. Recently, oriented cell growth was shown to be the main factor in shaping the fly wing and the petals of the Antirrhinum flower. Meilhac et al. now find that directed growth patterns similarly affect the shaping of the heart.
Using clonal analyses of embryonic cardiac cells, the authors were able to trace the patterns of cell growth in the developing heart. They find that new cells are not randomly scattered around their progenitors. Rather, growth is oriented in a pattern specific for each area of the heart, and that pattern reflects the final shape of its region. For instance, in the outflow tract of the heart, which is a tubular region, the clones grew longitudinally along the long axis. In contrast, in the rounded ventricle, the clones were oriented like the spokes of an umbrella.
A computer simulation revealed that similar clone shapes could be formed by simply restricting the plane of cell division. Thus, as long as daughter cells continue to divide along the original mitotic orientation, outside signaling among cardiac cells may not be necessary to attain the clonal shapes seen in vivo. It remains to be determined how each region of cells chooses its mitotic plane.
Other mammalian organs, such as the brain or kidney, may also be shaped by oriented cell divison. Cruder shapes might be formed by variances in proliferation rates, but the group suggests that fine shaping is more easily accomplished by oriented cell division. ▪