Fig. 6. Myoblast cell proliferation is controlled by lbx1. An anti-phospho histone H3 antibody (brown) in combination with 12/101 (green) shows cells proliferating in the hypaxial muscle region (A-D, arrows). A transverse section through a stage 24 tadpole, before lbx1 expression is on in myoblasts, shows very little cell proliferation in the ventrolateral region of the somite (A, arrow). At stages 28 (B), 33 (C) and 37 (D), an increasing number of proliferating cells can be seen in the ventrolateral region of the somite (arrows). The remaining panels show tadpoles injected in one cell at the two-cell stage with either lbx1-splice MO (E,F), lbx1 mRNA (G,H,K-N) or a control MO (I,J). All of these panels are transverse sections with the injected side on the right. Stage 33 tadpoles that have been injected with lbx1-splice MO show a decreased number of proliferating cells in the ventrolateral region of the somite (E). Consistent with the loss of proliferating cells in E, a slightly smaller differentiated muscle mass can be seen in the same section in F (12/101, green). (G) Stage 28 tadpoles injected with lbx1 mRNA exhibit a dramatic increase in the number of proliferating cells in the region of the lineage tracer (red). (H) Active cell proliferation seen in G inhibits muscle differentiation (12/101, green). (I,J) Control injected tadpoles at stage 33 show no difference in cell proliferation or size of differentiated muscle. (K,L) By stage 37, tadpoles injected with lbx1 mRNA have the same amount of cell proliferation, but the differentiated muscle mass on the injected side is much larger. (M,N) A stage 41 tadpole injected with lbx1 mRNA has a larger differentiated muscle mass on the injected side in both the trunk (M) and tail (N).