Fig. 2. Myosin expression and striation in primary cells. A fraction of primary cells crawled on top of other cells while in culture. (A) A sketch of the two-cell system indicating that only the upper cell was striated. (B,C) Two different fields with SG^–/– cells growing on top of other cells. The upper layer of cells is in focus in (B,C) and the lower layer of cells is in focus in (B',C'). There is 6 µm height difference between the two focal planes. The arrowheads point to nuclei that are in focus in the lower layer. Myosin appeared highly expressed and often exhibited some structure (B) or complete striation (C) only in cells growing on top of other cells. The arrow in B indicates a fiber of structured myosin in the myotube and the arrow in C indicates a fully striated myofiber. (D) Almost twice as many SG^–/– cells were striated as C57 cells. For both cell types, only the upper cell was striated. Bar, 20 µm.