Craniofacial Muscles

40 L.K. McLoon et al.
Fig. 3.8 Co-expression of multiple myosin heavy chain isoforms in single “hybrid” fi bers in serially
sectioned cross-sections of rabbit inferior rectus muscle immunostained for the following six
MyHC isoforms: EOM speci fi c, fast type IIA (IIA), embryonic (developmental, EMB), slow tonic,
slow type I (I), neonatal (perinatal, NEO). Green arrow indicates a myo fi ber that is positive for
IIA, EMB, NEO and slightly positive for slow tonic MyHC isoforms. The red arrow indicates a
myo fi ber positive for EMB, slow tonic, I and NEO MyHC isoforms
Zhou et al. 2010 ) . To make this picture even more complex, it is clear that not all
myo fi bers within the EOM course from tendon to tendon (Davidowitz et al. 1977 ;
McLoon et al. 1999 ; Shall et al. 2003 ; Harrison et al. 2007 ) . This has physiological
implications, and may explain the loss of predicted force that occurs in EOM under
experimental conditions (Goldberg et al. 1997 ; Milller et al. 2002 ) .
One of the most distinctive aspects of MyHC expression patterns in EOM is the
presence of multiple isoforms within single myo fi bers, referred to in the limb skeletal
muscle literature as “hybrid” fi bers (Fig. 3.8 ). Limb skeletal muscles also contain
hybrid fi bers, which tend to increase when the muscle is subjected to stress or injury,