Growth and Differentiation During Myogenesis in the Chick Embryo

A general method for describing the complex dynamics of cell populations over an extended period of growth and differentiation in a developing tissue is presented. The measurements required to produce a unique description are defined. Skeletal muscle development in the thigh and breast of the chick embryo is analyzed, using this method, during the period of embryonic development between Day 7 and Day 17. A unique quantitation of growth and differentiation for the period from Day 11 to Day 17 is developed. The pectoralis major is compared with the averaged behavior of the thigh musculature. In each case, a single partitioning rule holds for more than 10 generations during the main myogenic period. In the pectoralis, approximately 51% of the cells entering G1 in each generation continue in the cell cycle; in the thigh, which experiences substantially greater overall growth, approximately 58% of the cells entering G1 in each generation continue in the cell cycle. No significant cell death is detected. Thus, in each case, the absolute number of myoblasts is increasing while the fractional value of myoblasts in the population is decreasing over a fivefold range. These results are discussed in terms of several quantitative models for the possible basis of the observed population dynamics.