DLM myotube–myoblast association and contact involves several distinct configurations. (A–D) WT DLMs. Views of myotube (MT)–myoblast (MB) interfaces before (A and B) and after (C and D) formation of contact sites (yellow asterisks). n, nucleus. Contact site formation appears to be associated with tighter apposition of the cells. B and D include a color-coded heat map of the distances between neighboring membranes (see Materials and methods section Cell surface distance analysis). Membrane-associated electron-dense plaques, reminiscent of a structure observed in Drosophila embryonic preparations (Doberstein et al., 1997), are occasionally observed (arrowheads in A, but their association with contact sites is not clear. (E–G) DLMs after simultaneous knockdown of the adhesion elements Sns and Hbs (mef2-GAL4>UAS-sns-i,UAS-hbs-i). (E) A low magnification view demonstrates the resulting fusion arrest phenotype with myoblasts congregating around thin myotubes (MT). (F and G) High magnification views reveal the relatively wide gap between myotubes and neighboring myoblasts, which do not flatten their apposed surface. (H) Bar graph showing the distribution of myotube–myoblast intermembrane distances (see Materials and methods section Cell surface distance analysis). n, number of cells analyzed in single TEM sections. A distance distribution profile in which most (50–70%) membrane separations are of intermediate value (22–50 nm; mean distance = 27.2 ± 15.5 nm) is observed in WT DLM preparations where the cells do not make contact (left bar). The profile is clearly biased toward smaller distances (0–22 nm; mean distance = 15.0 ± 4.7 nm) for WT cell pairs that are in contact with each other (middle bar) and toward larger distances (>50 nm; mean distance = 65.4 ± 33.0 nm) in adhesion-defective (sns + hbs RNAi) DLM preparations (right bar). Bars: (A, B, D, and G) 500 nm; (C and F) 200 nm; and (E) 10 µm.