Fig. 1. Transient PS exposure by differentiating myoblasts in mouse embryos. Primary myotubes expose PS transiently at E13 (black arrows) in the cervical area (boxed area A1,A2), in-between the developing ribs (boxed area B1,B2) and the lumbar region (boxed area C1,C2). Frequently, annexin V-positive rounded cells were found attached to myotubes (D1,D2, arrowheads). Also in these sections, indications were found that annexin V-positive extensions arise from these developing myotubes (D1,D2, open arrowheads). Mitotic cells were negative for annexin V (B2, white arrowhead). At E14, annexin V staining of myotubes is virtually absent: compare the labeling for muscle using an anti--smooth muscle actin antibody (E1) with the labeling of surface-exposed PS (E2) in the same muscle (white arrows) in an adjacent section. For comparison, F shows annexin V-labeled apoptotic cells in the fusing E11 branchial arches, both in the mesodermal compartment (arrow) and in the ectoderm (arrowhead). Bars, 25 µm (C2,D1,D2,F); 40 µm (B2); 200 µm (A2,B1,C1); 500 µm (A1,E1,E2). Abbreviations: b, brain; l, limb; r, rib.