Abbreviated Title: Myonuclear Domain during Diaphragm Muscle Development

D ow nloaded from During early postnatal development in rat diaphragm muscle (DIAm), significant fiber growth and transitions in myosin heavy chain (MHC) isoform expression occur. Similar to other skeletal muscles, DIAm fibers are multinucleated, and each myonucleus regulates the gene products within a finite volume- myonuclear domain (MND). We hypothesized that postnatal changes in fiber cross-sectional area (CSA) are associated with increased number of myonuclei so that the MND size is maintained. The DIAm was removed at postnatal days 14 (P-14) and 28 (P-28). MHC isoform expression was determined by SDS-PAGE. Fiber CSA, myonuclear number and MND size were measured using confocal microscopy. By P-14, significant coexpression of MHC isoforms was present with no fiber displaying singular expression of MHCNeo. By P-28, singular expression was predominant. MND size was not different across fiber types at P-14. Significant fiber growth was evident by P-28 at all fiber types (fiber CSA increased by 61%, 93 % and 147 % at fibers expressing MHCSlow, MHC2A and MHC2X, respectively). The number of myonuclei per unit of fiber length was similar across fiber