Fig. 5. Attachment structures and attachment structure components in vab-19 mutants. (A-F) Localization of intermediate filaments (MH4 immunostaining) in the wild type and in vab-19 mutants. (A) During early elongation in the wild type, IFs accumulate in each epidermal cell adjacent to one muscle quadrant. By the 1.5-fold stage, IFs were organized in circumferential bands in regions of epidermis adjacent to muscle, although these bands are not as regular as in later stages. (C) After the twofold stage, IFs localize to regularly spaced bands. (D-F) In vab-19 mutants, IF staining appears normal until after the twofold stage, when it expands into regions of epidermal cells that do not overlap muscle, compared with the wild type (brackets). (G-M) Myotactin expression (visualized using the MH46 antibody) in wild-type and vab-19 embryos. During early (comma to 1.5-fold; G,K) and intermediate (twofold; H,L) elongation stages, Myotactin appears normal in vab-19 mutants. In wild-type threefold-stage embryos, Myotactin localizes to circumferential bands in muscle-adjacent regions of the epidermis (I, inset). In vab-19 mutants, Myotactin is still localized to muscle-adjacent regions but remains in longitudinal rows rather than circumferential bands (M, inset). Scale bars, 10 µm. Electron microscopy (longitudinal sections) of attachment structures in wild-type L1 larva and vab-19(e1036) arrested larvae. In the wild type (N), attachment structures are confined to epidermis in muscle-adjacent regions, which is 200 nm thick. In vab-19 mutants (O), attachment structures are present and in phase with cuticle annuli but can be longer than normal. Scale bar, 500 nm (N), 150 nm (insets).