Figure 4.
Figure 4. Protrusive extensions are a shared feature of WASp mutant and sing knockdown myoblasts. (A) Reconstruction of a FIB/SEM dataset of DLM myotube-associated myoblasts from a WASp mutant pupa (see Video 2). A sizable portion of the mutant myoblasts display extensions, mostly oriented toward the myotube, which occupies the center of the panel and was left out of the image for clarity. (B and B') TEM section of WASp mutant DLMs, demonstrating typical myoblast (MB) fusion arrest phenotypes that include an abnormally elongated morphology and finger-shaped extensions (rectangle in B, magnified and false-colored purple in B’), which protrude into the neighboring myotube (MT). The myotube and myoblast surfaces maintain separation along the extensions, and no signs of contact between the cells are apparent. (C and D) TEM sections of kette (C) and sing knockdown DLMs, demonstrating the WASp mutant-like finger-shaped protrusions that myoblasts (MB) often extend toward the myotube (MT) in these mutant backgrounds. No signs of contact between the cells or of tight apposition are apparent along the extensions. (E) Quantification of the frequency and dimensions of surface protrusions in different genetic backgrounds. Although protrusions are a common feature of the myotube-associated myoblasts in WASp mutant, kette knockdown, and sing knockdown DLMs, they are rarely observed in WT, sns-hbs, Arp2, or Ced-12/ELMO knockdown DLMs. Protrusion dimensions are similar in the different backgrounds. Lengths are shorter than those computed from the FIB reconstructions, as sections do not capture the entire structure. (F) Bar graph showing the distribution of myotube–myoblast intermembrane distances along myoblast protrusions in different genetic backgrounds, generated as in Fig. 2 H. n, number of individual protrusions analyzed in single TEM sections. Surface-membrane separation profiles along the protrusions on WASp mutant, kette knockdown, and sing knockdown DLMs match those observed for the flat portions of the myoblast surface in these backgrounds (Fig. 3 L). Bars: (A) 2 µm; (B’, C, and D) 200 nm; (B) 1 µm.

Protrusive extensions are a shared feature of WASp mutant and sing knockdown myoblasts. (A) Reconstruction of a FIB/SEM dataset of DLM myotube-associated myoblasts from a WASp mutant pupa (see Video 2). A sizable portion of the mutant myoblasts display extensions, mostly oriented toward the myotube, which occupies the center of the panel and was left out of the image for clarity. (B and B') TEM section of WASp mutant DLMs, demonstrating typical myoblast (MB) fusion arrest phenotypes that include an abnormally elongated morphology and finger-shaped extensions (rectangle in B, magnified and false-colored purple in B’), which protrude into the neighboring myotube (MT). The myotube and myoblast surfaces maintain separation along the extensions, and no signs of contact between the cells are apparent. (C and D) TEM sections of kette (C) and sing knockdown DLMs, demonstrating the WASp mutant-like finger-shaped protrusions that myoblasts (MB) often extend toward the myotube (MT) in these mutant backgrounds. No signs of contact between the cells or of tight apposition are apparent along the extensions. (E) Quantification of the frequency and dimensions of surface protrusions in different genetic backgrounds. Although protrusions are a common feature of the myotube-associated myoblasts in WASp mutant, kette knockdown, and sing knockdown DLMs, they are rarely observed in WT, sns-hbs, Arp2, or Ced-12/ELMO knockdown DLMs. Protrusion dimensions are similar in the different backgrounds. Lengths are shorter than those computed from the FIB reconstructions, as sections do not capture the entire structure. (F) Bar graph showing the distribution of myotube–myoblast intermembrane distances along myoblast protrusions in different genetic backgrounds, generated as in Fig. 2 H. n, number of individual protrusions analyzed in single TEM sections. Surface-membrane separation profiles along the protrusions on WASp mutant, kette knockdown, and sing knockdown DLMs match those observed for the flat portions of the myoblast surface in these backgrounds (Fig. 3 L). Bars: (A) 2 µm; (B’, C, and D) 200 nm; (B) 1 µm.

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