Heart-shaped clones

Heart-shaped clones he intricate shape of the heart is formed by oriented cell growth patterns, according to Meilhac et al. on page 97. The heart is formed during embryo-genesis 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 T Membrane fusion SNAREs an inhibitor wo membranes fuse into one through the action of SNARE proteins. As only particular combinations of t-SNAREs (on the target membrane) and v-SNAREs (on the vesicle) will cause fusion between bilayers, restricting their localization promotes fusion in the right place. On page 79, Varlamov et al. add a new layer to the regulation by showing that some SNAREs actually block fusion that would otherwise occur in the wrong place. SNARE specificity forms two mutually exclusive systems T in the Golgi—one enriched at the cis and one at the trans face of the Golgi. The concentration of cis t-SNAREs at the cis face of the Golgi, where trans t-SNAREs are scarce, helps prevent vesicles bearing trans v-SNAREs from fusing there and vice versa. But even an imperfect distribution of t-SNAREs may be offset by inhibitory SNAREs (i-SNAREs), based on the new results. The authors found that some trans-Golgi SNAREs inhibit fusion reactions by cis-Golgi SNAREs and vice versa. At least some of these iSNAREs block fusion by substituting for one of the The addition of iSNAREs (red) inhibits normally active SNARE complexes. 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 …