Fig. 2. Fate mapping of the cloacal muscles using chick/quail chimeras and retroviral labelling. Identification of the contribution of the 31st somite to the cloacal musculature using chick/quail chimeras. Transverse parallel sections (see approximate levels in Fig. 1C) stained for quail cells (QCPN antibody) and counterstained with Eosin (A, B) and Alcian Blue (C). (A) Section through the level of the cloaca, showing all four cloacal muscles to be populated with quail cells. (B) Section through the level of the thigh and shank, showing quail derived cells from the somite 31 in some leg muscles. (C) Contribution of somite 31 to the hemivertebra and the adjacent epaxial muscle (arrow). (D) Detail of cloacal region of a chick embryo after marking somite 30 (at HH18) with lacZ-encoding retrovirus and X-gal whole-mount staining at HH36. X-Gal staining was detected in cloacal and leg muscles, see Table 1. Orientation is marked: V, ventral; D, dorsal. CFC, M. caudofemoralis (pars caudofemoralis of m. caudo-ilio-femoralis); CFP, m. caudofemoralis - pars pelvica; FCLP, M. flexor cruris lateralis - pars pelvica; FL, fibularis luingus; IF, iliofibularis; LC, levator cloacae; PCE, m. pubocaudalis externus; PCI, m. pubocaudalis internus; PIL, m. iliotibialis lateralis pre-acta bularis; SCC, m. sphincter circular; SCS, m. sphincter sling-shaped; TCL, m. transversus cloacae.