Meeting Abstract

P1.65  Sunday, Jan. 4  Size and distribution of muscle fiber types within chondrichthyan muscles HERNANDEZ, L. P.*; MORGAN, R. J.; George Washington Universty, Washington, DC phernand@gwu.edu

Vertebrate morphologists have long appreciated the importance of muscle fiber type composition. Although they vary widely in their size and distribution, different isoforms of slow and fast myosins comprise the bulk of all skeletal muscle tissue. Combined, these different myosins coordinate to perform a variety of important functions associated with locomotion, feeding and breathing. Evolutionarily, amniotes and anamniotes have shown a remarkable disparity in muscle fiber type distribution. In amniotes and adult amphibians muscle fibers show a mosaic distribution with interspersed slow and fast fibers. In fishes, as well as in larval amphibians, muscle fibers show a zoned distribution whereby specific fiber types group together within muscles. Here we describe the distribution and relative proportion of fast and slow fibers in adult shark and skate muscles. Muscles were stained for a variety of specific myosins using standard immunohistochemical methods. Antibodies considered to stain all vertebrate myosins (based on data from a large number of amniotes and anamniotes) did not recognize myosins within many irregularly shaped muscle fibers in skate. While our results do not indicate the mosaic pattern seen in amniotes, they also do not fully support the strictly zoned fast and slow regions seen in most anamniotes. Antibodies against slow myosin stained the peripheries of certain larger muscle fibers, but stained entire smaller diameter fibers in other muscles as seen in anamniotes. Thus skate muscles may be comprised of a number of intermediate fiber types and consist of a unique pattern of distribution. It is likely that a range of intermediate muscle fiber types may be an ancestral character. We discuss the functional, evolutionary, and developmental implications of our data.