Location along the muscle's length is a determinant of myofibril size

In a previous study of myofibril size in 'Pale' (fast-twitch-glycolytic) fibers of rabbit extraocular muscle (EOM), it was found that individual long Pale fibers demonstrate a substantial increase in the size of myofibril profiles from their proximal to their distal halves (Davidowitz et al., 1996b). That finding raised the question of whether such proximal-to-distal increase of myofibril size in the Pale fibers is determined by: (1) longitudinal position within the individual muscle fibers themselves or (2) location along the length of the muscle as a whole? This question was tested in the present study by comparing the original group of long Pale fibers, which extend the full length of the muscle, with two groups of short Pale fibers, which are respectively confined to the proximal and distal halves of the muscle. It was found that (a) in the proximal half of the muscle, the short fibers and the adjacent portions of the long fibers have the same smaller size of myofibrils, and (b) in the distal half of the muscle, the short fibers and the adjacent portions of the long fibers have the same larger size of myofibrils. This finding indicates that the proximal-to-distal increase of myofibril-profile size in these EOM Pale fibers is determined by location along the length of the muscle as a whole, and is not related to longitudinal position within the individual fibers themselves.