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Fig. 1. Anatomy of cloacal and perineal muscles. (A) Experimental procedures using
newly formed quail somites for heterotopic single somite grafting into the
chick recipient. Viral injection into interlimb somite was carried out for
future exact determination of the operated segmental level. (B) Result of the
single somite marking by retroviral alkaline phosphatase transfection. The
second intercostal muscle is a derivative of somite 21
(Evans, 2003). Operated somite
was 10 somites further caudally, therefore somite 31 was transplanted. (C)
Lateral view of a chick HH36 with cloacal muscles (red), two leg muscles
(green) and the tail region. Lines `a' and `b' show the approximate level of
sections in Fig. 2A and
2B respectively. Orientation is
marked: V, ventral; D, dorsal; Ca, caudal. The m. sphincter cloacae
has 2 parts: m. sphincter circular (SCC) and m. sphincter
sling-shaped (SCS). The sling region originates in the supracloacal septum and
on the apex of pubis and forms a slightly dorsally positioned sling around the
cloacal tubercle. These layers merge in the dorsocaudal aspect. The m.
transversus cloacae (TCL) in the adult is a very prominent superficial
muscle and with its almost transverse course it forms the boundary between the
abdomen and the tail. The m. levator cloacae (m. retractor phalli
caudalis) (LC) is a long narrow muscle arising from the underside of
feather bulge of the lateral rectrices, inserting near the midline of the
cloacal tubercle - phallus. Sexual dimorphism was not evident at the stages
examined. Because of close topographical relationship to the cloacal muscles,
we also focus on two hindlimb muscles. CFC begins from the proximal half of
the femur and inserts on the ventral side of pygostyl on the fascia of m.
depressor caudae in the vicinity of the rectrices. FCLP is the
caudal-most muscle of the leg (C,D). (D) Lateral view of a skinned HH36 chick
after in situ hybridisation with MyoD. (E) Caudolateral view of a
mouse E15.5 with perineal muscles (red), leg muscles (green) and the tail
region. (F) Caudolateral view of a skinned mouse E15.5 after in situ
hybridisation with MyoD. g.t., genital tubercle; BS, m.
bulbospongiosus; CFC, M. caudofemoralis (pars
caudofemoralis of m. caudo-ilio-femoralis); FCLP, M. flexor
cruris lateralis - pars pelvica; ISC, m. ischiocavernosus; LA,
m. levator ani; LC, levator cloacae; PCE, m.
pubocaudalis externus; PCI, m. pubocaudalis internus; SA, m.
sphincter ani externus; SCC, m. sphincter circular; SCS, m.
sphincter sling-shaped; TCL, m. transversus cloacae.
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