Lindsay Carol Morton

Major: Biology Minor: Chemistry

Honors in Biology

Supervisor: Dr. Stephen T. Kinsey

CHANGES IN NUCLEAR DISTRIBUTION DURING HYPERTROPHIC FIBER GROWTH IN FISH WHITE MUSCLE

Black sea bass (Centropristis striata) muscle fibers undergo extreme hypertrophic growth, where fibers from juvenile animals are <50 μm in diameter while those of adults are >450 μm. I examined the relationship between increases in fiber size and the density/distribution of nuclei in muscle from fish that ranged in body mass over 4 orders of magnitude. Fibers increased in diameter due to hypertrophic growth throughout most of development, but in the largest size class, there was a proliferation of new, small fibers, which is indicative of mosaic hyperplasic growth. Fibers from juvenile fish contained low numbers of almost exclusively subsarcolemmal (SS) nuclei while the largest fibers of adult fish contained significant populations of intermyofibrillar (IM) nuclei with relatively low numbers of SS nuclei. Significant populations of IM nuclei appeared in fibers with a diameter that exceeded approximately 120 μm. This may be why IM nuclei have not been observed in most mammals, which have an average skeletal muscle fiber diameter that is considerably less than 100 μm. Myonuclear domain size increased with body size and reached a maximum in fish with a body mass of 310 g. Thereafter, the proliferation of new fibers and appearance of IM nuclei reduced domain size. The present study demonstrates that mosaic hyperplasia in muscle from adult black sea bass is associated with the proliferation of IM nuclei, lending support to the notion that diffusion constraints trigger the formation of new fibers. Further, the presence of IM nuclei in the largest fibers may indicate that maximal diffusion distance, and not myonuclear domain, is the major property that dictates nuclear distribution.