Fig. 5. Disrupted sclerotomal differentiation in Meox1;Meox2 mutants. Whole-mount preparations of in situ hybridization analysis of markers expressed at E9.5 in the sclerotome: Pax1 (A-F), Pax9 (G-J), Twist (K-N) and Foxc2 (O-R). Representative transverse cryosections of whole-mount preparations of control (C,H,L,P) and mutant (F,J,N,R) embryos are shown. (A-C) Pax1 is expressed at high levels in the sclerotome of control embryos (arrowheads), but is not detected in the somites of Meox double mutants (D-F), although branchial arch and limb bud expression persists (arrows). The expression of Pax1 is first seen shortly after epithelial somites form in control embryos (B,C; arrowhead), but is not induced in Meox double mutants (E,F; arrowhead). (G-J) Pax9 is also expressed at high levels in the sclerotome of control embryos (G,H), especially in the caudal half-somites (arrowheads). By contrast, double Meox mutant embryos show greatly reduced Pax9 expression, most evident in the caudal half-somites (I,J). The residual Pax9 expression is restricted to the sclerotomal cells closest to the neural tube (J). (K-N) Twist RNA is detected throughout epithelial somites, and in the sclerotome and dermomyotome of differentiated somites (K,L). In Meox double mutants (M,N), Twist expression is greatly reduced in somites, while expression persists in branchial arches and limb buds. (O-R) Foxc2 is expressed in control embryos in sclerotomal cells (O) preceding Pax1 and Pax9, while its expression in the posterior half of somites of mutant embryos is reduced dramatically in the mutant (Q). The distribution of the Foxc2 signal on sectional analysis is, however, similar in control (P) and mutant (R) embryos.