Myoid cell density in the thymus is reduced during mdx dystrophy and aftermuscle crush

Thymic myoid cells share structural and behavioural features with cells of the skeletal muscle lineage: they express regulatory genes and contractile proteins, and they can form myofibers in culture. Historically, those featu res suggested that myoid cells could be precursors for muscle repair in add ition to the satellite cells in muscle that are typically designated as the only muscle precursors. Muscles of the mutant mdx dystrophic mouse strain have a large demand for precursors, which is greatest at a young age. In th e present study, immunostaining for troponin T was used to localize myoid c ells. We tested the hypothesis that the myoid cell population changes when there is a demand for muscle precursors and that these changes would be ant icipated if myoid cells have a role as myogenic precursors or stem cells in muscle. Chronic demands for muscle precursors in mdx dystrophic mice were accompanied by lower myoid cell density in comparison with density in two n ormal strains (C57BL10/ScSn and Swiss Webster). Acute demand for precursors was accompanied by a sharp decline in thymic myoid cell density within 2 d ays after a crush injury to one tibialis anterior muscle in normal but not dystrophic animals. To standardize the developmental age of the thymus, den sity was determined in all animals at 28 days of age. Given the current int erest in nonmuscle sources of myogenic stem cells, these data suggest that changes in the density of thymic myoid cells may accompany acute and chroni c demands for muscle precursors. Further experiments are required to determ ine whether thymic myoid cells are participants in distant muscle cell prol iferation, new fiber formation, or the establishment of new stem cells in r egenerated muscle.