Abstract
Morphological definitions of muscle fiber types have generally been based on the tacit assumption that the structure of a given fiber is essentially constant along it length. Recently, however, the serial sampling of rabbit extraocular muscle (EOM) has shown that some multiply innervated fibers (MIFs) display systematic variation of both the sarcoplasmic reticulum (SR) and mitochondrial content. The purpose of the present study was to determine whether systematically varying MIFs occur in rat EOM as well: such fibers had not been detected in previous studies of rat EOM by means of standard sampling methods. A young (175 gm) white Wistar rat was perfused with 2% paraformaldehyde-2% glutaraldehyde in phosphate buffer. The inferior rectus was removed intact, fixed in 4% glutaraldehyde, postfixed in 1% osmium tetroxide, dehydrated in graded ethanols, and embedded whole in Epon 812. The muscle was serially sectioned at 15u, and 114 1u sections and 42 ultrathin sections were recut from the original thick sections for sequential samplings.
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