During avian and mammalian embryogenesis, the cloaca forms a common opening to the digestive, urinary and reproductive systems, which, in mammals,subsequently develops into separate openings to each system. Valasek and co-workers now reveal that the skeletal muscles surrounding the cloaca develop from leg muscle cells in both chicks and mice (see p. 447). The researchers show that chick cloacal muscle originates from somites 30-34, a domain that overlaps the one giving rise to leg muscles. By marker gene analysis, they show that chick and mouse cloacal muscles derive from the ventral muscle mass of the hind limb. Further labelling, genetic and surgical experiments lead the researchers to conclude that in both species (and presumably humans), myogenic precursors migrate from the somites into the pelvic limb, where they extend towards the midline, only then differentiating into cloacal/perineal muscles.

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