Distribution of developmental myosin heavy chains in adult rabbit extraocular muscle: identification of a novel embryonic isoform absent in fetal limb.

PURPOSE: To identify embryonic and neonatal/fetal myosin heavy chains (MyHCs) in rabbit extraocular muscle (EOM) by electrophoretic and immunochemical analyses and to describe the distribution of these two MyHC isoforms in the endplate zone (EPZ) and the distal and proximal segments of EOM fibers.
METHODS: SDS-PAGE and Western blot analysis using monoclonal antibodies (mAbs) against embryonic and neonatal/fetal MyHCs were performed on MyHC isoforms from rabbit adult and neonatal EOM and fetal limb muscles. Immunohistochemical analysis was performed along the entire length of the rabbit superior rectus muscles, using these and other mAbs.
RESULTS: Western blot analysis showed that adult rabbit EOM had two embryonic MyHC bands: a weakly stained band that comigrated with the embryonic MyHC from fetal limb muscles, and a strongly stained band of lower electrophoretic mobility for which there was no limb counterpart. Three anti-embryonic MyHC mAbs stained muscle fibers, predominantly in the orbital layer, and staining was localized distal and proximal to the EPZ but not in the EPZ itself. There, most fibers expressed the EOM-specific fast MyHC, although some fibers expressed alpha-cardiac MyHC. Anti-neonatal/fetal MyHC mAb failed to stain in Western blot analysis but stained scattered fibers predominantly in the global layer, and there was no specific absence of staining at the EPZ.
CONCLUSIONS: There are two electrophoretically distinct isoforms of embryonic MyHCs in adult rabbit EOM. These isoforms are expressed in orbital fibers but are excluded from the EPZ, where EOM-specific fast MyHC is strongly expressed. Neonatal and fetal MyHC is weakly expressed in the EOM, but is not excluded from the EPZ.