Fig. 2. Fluorescence microscopy of transverse tibialis anterior (TA) muscle sections at P1 (A,B) and P28 (C,D) using laminin immunohistochemistry for muscle fiber size measurement (A,C) and phalloidin staining for measurement of myofibrillar packing (B,D). Extensive muscle hypertrophy and accumulation of myofibrillar material is evident between P1 and P28. Arrows in B indicate examples of how immature muscle fibers typically begin to accumulate myofibrillar material first in the subsarcolemmal region and then inward toward the central axis of the cell. Image B is shown at a higher magnification for clarity. Postnatal time-courses of fiber size (E) and the fraction of the cross-sectional area occupied by contractile material (F) are also presented. ^*P<0.05 relative to the previous time-point.