Fibronectin organization at the somite boundary precedes the assembly and organization of new myofibrils. Zebrafish embryos at 24 hpf were immunostained for fibronectin (red: B,C,E) and α-actinin (green: A,C,D). Fibronectin was organized at the somite boundary before the assembly of the first striated myofibrils (B,C: arrowheads) and becomes less abundant in the more mature boundaries (B: arrow). In the most recently formed somites, nascent myofibrils (D,E), with a characteristic irregularly dotted pattern of α-actinin staining, extend between the recently formed fibronectin-containing nascent somite/myotome boundaries (E: arrow). (F) Insertion of actin filaments (arrows) into membrane-based adhesion domains at the somite boundary is associated with the accumulation of electron-dense material (arrowheads) on the inner and outer surfaces of the sarcolemma. (G) The membrane compartment at the somite boundary undergoes progressive development in preparation for myofibril assembly. Note the absence of electron dense material in an undeveloped region of the membrane (*) with deposition of some electron-dense material in a region without well-developed actin filaments (black arrowhead) followed by more accumulation of material in a region (white arrowhead) with definite insertion of well-developed actin filaments (arrow). Scale bars are 50 μm (A–C) and 2 μm (F). (D,E) Are 3 × magnifications of corresponding regions in (A,C). (G) Is a 2.5 × magnification of the image in (F).