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hybrid fibers), and a number of these fibers increase with certain physiological
and disease conditions (
Bloemberg and Quadrilatero, 2012
;
Staron et al.,
2012
). The naming conventions of the fiber andmyosin protein subtypes were
developed in the decades prior to molecular biology techniques; in contrast,
the names of the genes, which encode the myosin heavy-chain proteins, were
named using a different system. Thus, there is little correlation between the
fiber types and gene names (with the exception of Type IIA fibers, which con-
tainmyosin coded for by
MYH2
). Type IIX fibers were originally designated as
“X-fibers” by their lack of reactivity with certain myosin subtype-specific
monoclonal antibodies that were then available, which selectively recognized
Type IIA and Type IIB fibers (
Schiaffino et al., 1989
); “Type IID” fibers were
also proposed, but these proved to be identical to the previously named Type
IIX (
Termin et al., 1989
). Also, while
MYH4
, the gene which encodes the
myosin isotype which is prevalent in Type IIB fibers, is present in the human
genome, Type IIB fibers have yet to be demonstrated at the protein level in
humans (
Schiaffino and Reggiani, 2011
). In addition to these major subtypes,
additional fiber subtypes which express unique myosin subtypes are found in
the jaw, heart, and extraocular muscles, and during embryonic development
(
Schiaffino and Reggiani, 2011
).
Antibodies against several myosin heavy-chain isoforms, which work well
in species as diverse as humans and mice, are commercially available and also
from the Developmental Studies Hybridoma Bank at the University of Iowa.
However, all of the currently available isoform-specific myosin antibodies are
mouse monoclonal, making it difficult to stain the same tissue section with
combination of several antibodies. This also makes it difficult to distinguish
between so-called hybrid muscle fibers expressing two or more different myo-
sin isoforms (
Schiaffino, 2010
). Using mouse monoclonal antibodies for
immunostaining also often results in high background staining when used
on mouse skeletal muscle sections. In addition, there is variability in fiber-type
recognition by the same antibody in different species (
Smerdu and Soukup,
2008
) that should be carefully considered when planning research in species
which have not been previously tested with a particular antibody.
Despite the limitations mentioned above, immunostaining is still the most
accurate method for the evaluation of fiber-type composition. Examples of
recent studies conducted with this method include an evaluation of muscle
adaptations during spaceflight, which utilized antibodies against Type I, Type
2A, and Type 2B myosin heavy-chain isoforms (
Sandona et al., 2012
); the
evaluation of muscle-subtype changes during fetal development in mice using
antibodies specific for MyHC Type I, MyHC embryonic, and MyHC
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