Cell proliferation time and orientation. (a) In total, 244 individual fiber-associated satellite cells were tracked over 24 hours, from 24 to 48 hours after activation by fiber harvest. Raw data are available as an Excel file (see Additional File 1, Table S1). For each cell, time and orientation of division (if any) were noted, and each division is represented by a marker coded to order (shape) and orientation (shade) of division. Individual cells were sorted from earliest to latest initial division time; 37 cells were not seen to divide at all (left end of x axis) although they were demonstrably activated, as evidenced by their morphology, position above the exterior lamina and motility. Subsequent divisions (if any) for each cell are also noted above the initial division at the time (hours) at which they occurred. Direction of division (horizontal = cells divide within the plane of the myofiber surface; vertical = axis of division is perpendicular to the myofiber) determines the marker fill color. (b) Average time between observed divisions (first to second, and second to third) in hours. Bar = SEM. (c) Quantification of total number of cells dividing either horizontally or vertically with respect to the plane of the myofiber for each observed division. (d) Sequential tagged image format (TIF) files (10 minutes apart) showing examples of (top) vertical and (bottom) horizontal division.