Fig. 3. Fiber type, nuclear location and size of K19^–/– myofibers. (A,B) TA muscles from wild-type (A) and K19^–/– (B) mice were snap frozen, cryosectioned (20-µm cross sections) and fluorescently immunolabeled with antibodies against βI-spectrin to visualize the sarcolemma of the myofibers (shown in green), and counterstained with propidium iodide (PI) to visualize myonuclei (shown in red). Many fibers in K19^–/– muscle were smaller than controls but did not have central nuclei. (C,D) Cross sections of mouse TA muscle were labeled with antibodies against βI-spectrin (red) and antibodies against the myosin heavy chain of slow-twitch muscle (MHCS; green). K19^–/– muscle (D) showed 10% of fibers labeled for the slow isoform of myosin, whereas wild-type muscle showed none (C). Bars, 5 µm.