Allocation of cells to proliferation vs. differentiation and its consequences for growth and development.

A model relating the recruitment of skeletal muscle fibers from precursor cells to growth and development of the whole muscle is presented. The pattern of growth throughout ontogeny is analyzed for differences in: (1) initial number of precursor cells, (2) timing of onset of differentiation, (3) timing of offset of differentiation, and (4) differentiation rate (number of precursor cells that differentiate each cell cycle). The initial number results in a larger muscle but has no effect on relative growth rate. Later onset time and slower differentiation rate result in relatively slow growth early in ontogeny but rapid growth for most of ontogeny. A later offset time results in faster growth late in ontogeny as new fibers continue to be recruited late into ontogeny. The pattern derived from onset time and differentiation rate matches that of the precocial-altricial continuum in birds in which selection for functional ability early in ontogeny results in slow growth late in ontogeny. Methods for recognizing the different developmental parameters in the size distribution of muscle fibers are described and three empirical examples interpreted in terms of the model.