Fig. 2. In twister mutant embryos, slow and fast muscle fiber development is severely affected. Confocal micrographs of 26 hpf wild-type and mutant embryos stained with antibodies specific for slow muscle fibers (F59; A,C,E) or fast muscle fibers (F310; B,D,F; all lateral views). (A) In wild-type embryos, slow muscle forms a superficial monolayer of striated fibers arranged in parallel. (B) In wild-type embryos, fast muscle fibers are oriented obliquely and form the bulk of the myotome. (C,D) In twister heterozygous mutants, individual fibers are thinner, such that gaps appear between them (arrowheads), and somite boundaries are irregular (arrow). (E,F) twister homozygous mutants exhibit severely disrupted slow and fast myofiber organization. Some fibers have detached from the somite boundaries, which are irregular (white arrow). Many muscle fibers have contraction clots of myofibril material, as shown by densely stained clots (arrowheads). Scale bar: 50 µm.