Grp94 and CHOP protein expression in regenerating murine myofibers. (a), (b) Indirect immunoperoxidase staining of murine cryosections with the anti-Grp94 antibody. (a) Immature skeletal muscle from a 13-day mouse embryo; embryonic myofibers appear darkly stained and the arrow indicates the presence of a sarcomeric pattern of labeling. (b) Transverse section of adult tibialis anterior muscle; myofibers appear unlabeled; the reticular immunostaining around myofibers is due to the reactivity of the secondary antibody with mouse immunoglobulins in the interstitial space. (c) Western blotting analysis with Grp94 of whole homogenates prepared from adult mouse myocardium (h), tibialis anterior muscle (sk), liver (l) and the superficial region of a tibialis anterior muscle, 4 days after injury to induce muscle regeneration (reg). Labeling of the myosin heavy chain (My) is shown as a reference for loading. (d) to (f) Indirect immunoperoxidase staining of cryosections from adult tibialis muscle, 4 days after induction of muscle regeneration, with (d) monoclonal mouse anti-embryonic skeletal myosin, (e) anti-Grp94 antibodies and (f) polyclonal rabbit anti-CHOP antibodies. Arrows in (d) to (f) indicate regenerating myofibers with centrally located nuclei. Bar: 50 μm.