Fn and laminin are reciprocally expressed at the MTJ adjacent to fast-twitch fibers during muscle morphogenesis. A–C Anterior left, dorsal top, side-mounted, wild-type embryos. Phalloidin staining for actin to visualize the cell morphological changes during muscle morphogenesis. A1 At 16 hpf, muscle precursors are round (arrow in inset) and actin accumulates between somite boundaries (arrowheads). A2 At 24 hpf, muscle cells have elongated (arrow in inset) and actin continues to accumulate at the boundary (arrowheads). A3 At 48 hpf, muscle fibers have undergone further growth and become striated (arrow in inset). MTJs are devoid of actin staining (arrowheads). B1–B3 Focal planes, 3D projections (anterior left, dorsal top, side-mounted), and transverse views ((T) lateral right, dorsal top) of phalloidin (blue) and Fn antibody staining (red). b1 At 18 hpf, Fn accumulates throughout the medial-lateral extent of the MTJ. B2 At 24 hpf, Fn is adjacent to lateral, superficial slow-twitch fibers (arrowheads in T) as well as medial MPs (arrow in focal plane and asterisk in T). B3 At 48 hpf, Fn is downregulated throughout the medial portion of the MTJ except adjacent to MPs (asterisk in T). C1–C3 Focal planes, 3D projections (anterior left, dorsal top, side-mounted), and transverse views ((T) lateral right, dorsal top) of phalloidin (blue) and laminin-111 antibody staining (green). C1 At 18 hpf, laminin-111 begins to polymerize at the MTJ. C2 At 24 hpf, polymerized laminin-111 is adjacent to slow- and fast-twitch muscle fibers (arrowhead in focal plane points to laminin-111 adjacent to fast-twitch muscle fibers). C3 At 48 hpf, laminin-111 remains throughout the medial-lateral extent of the MTJ (arrowhead denotes laminin-111 adjacent to fast-twitch muscle fibers). D, E Models of Fn-laminin dynamics at the MTJ in a medial focal plane (D) and a 3D rendering (E) over developmental time. The presence of slow-twitch fibers (blue) along the midline confirms that focal planes are medial sections. D1 At 18 hpf, a mainly Fn myomatrix segregates somites. D2–D3 In medial focal planes at 24 and 48 hpf, because slow-twitch fibers have migrated laterally, a mainly laminin-111 myomatrix separates myotomes. E1 As slow-twitch fibers migrate laterally, myoblasts elongate and the mainly Fn matrix is replaced by a mainly laminin-111 matrix medial to the location of migrating slow-twitch fibers. E2 After slow-twitch fiber migration is complete, the MTJ is primarily laminin-111 adjacent to fast-twitch fibers while Fn remains adjacent to slow-twitch fibers. Scale bars are 50 μm
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