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First published online January 14, 2005
doi: 10.1242/10.1242/dev.01545

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A dual fate of the hindlimb muscle mass: cloacal/perineal musculature develops from leg muscle cells

¹ Veterinary Basic Sciences, Royal Veterinary College, London NW1 0TU, UK
² Anatomy Institute, First Medical Faculty, Charles University, Prague, Czech Republic
³ Brighton and Sussex Medical School, University of Sussex, Falmer, Brighton BN1 9PX, UK
⁴ INSERM UMR 623, Developmental Biology Institute of Marseille, Campus de Luminy, Case 907, 13288 Marseille Cedex 09, France

^* Authors for correspondence (e-mail: valasekpetr{at}hotmail.com; kpatel{at}rvc.ac.uk)

Accepted 26 October 2004

The cloaca serves as a common opening to the urinary and digestive systems. In most mammals, the cloaca is present only during embryogenesis, after which it undergoes a series of septation events leading to the formation of the anal canal and parts of the urogenital tract. During embryogenesis it is surrounded by skeletal muscle. The origin and the mechanisms regulating the development of these muscles have never been determined. Here, we show that the cloacal muscles of the chick originate from somites 30-34, which overlap the domain that gives rise to leg muscles (somites 26-33). Using molecular and cell labelling protocols, we have determined the aetiology of cloacal muscles. Surprisingly, we found that chick cloacal myoblasts first migrate into the developing leg bud and then extend out of the ventral muscle mass towards the cloacal tubercle. The development of homologous cloacal/perineal muscles was also examined in the mouse. Concordant with the results in birds, we found that perineal muscles in mammals also develop from the ventral muscle mass of the hindlimb. We provide genetic evidence that the perineal muscles are migratory, like limb muscles, by showing that they are absent in met^d/d mutants. Using experimental embryological procedures (in chick) and genetic models (in chick and mouse), we show that the development of the cloacal musculature is dependent on proximal leg field formation. Thus, we have discovered a novel developmental mechanism in vertebrates whereby muscle cells first migrate from axially located somites to the pelvic limb, then extend towards the midline and only then differentiate into the single cloacal/perineal muscles.

Key words: Striated sphincter muscles, Cloaca, Perineum, Chick/quail chimera, Somite, Segmental origin, limbless, MyoD, Pax3, Pax7, Met, Lbx1, Meox2

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