Figure 2. Cellular and subcellular behaviors that occur during myoblast fusion.

Several cellular steps occur during myoblast fusion; each step coincides with a defined series of subcellular events. (A) Cells first migrate toward their fusing partner, concurrently with actin polymerization (red lines) and expression of transmembrane attractants (small rectangles) to guide the migrating cell. (B) The cells then touch and adhere, leading to localization of the cell type-specific transmembrane proteins. (C) This leads to an accumulation of actin (red oval), and the formation of the FuRMAS (purple ring) at the site of fusion. Subsequently, a number of fusion proteins known collectively as the “fusion machinery” are localized to the site of fusion, presumably through vesicular trafficking. The area outlined by the grey box is examined in more detail in Figure 3. (D) This is followed by membrane breakdown and the removal of vesiculating membrane and the fusion machinery components. (E) Finally, the cell must reset for the next round of fusion by expressing appropriate levels of transmembrane attractant. This process will repeat iteratively until the final muscle or fiber size is achieved.