Do myoblasts in vivo withdraw from the cell cycle? A reexamination.

The proliferative fraction of mononucleated cells in differentiating chick embryonic wing muscle (day 11) was measured following continuous infusion of tritiated thymidine into the embryonic circulation. During progressively longer intervals of infusion of the isotopically labeled precursor, the percentage of cells that enter S becomes larger, reaching 92% at the longest time period measured (21 hr). These observations suggest that until they are withdrawn into nonreplicative muscle syncytia, virtually all of the single cells in differentiating embryonic muscle remain in the proliferative pool. Earlier calculations of the size of this pool in developing muscle, based on the percentage of cells in S during a brief pulse, indicated, however, that less than half of the mononucleated cells are still replicating. We therefore compared the size of the proliferative fraction determined by continuous labeling with the calculation of this same parameter using our own pulse-labeling data. We find that the calculation underestimates the size of the proliferative pool and is, in fact, an estimate of only that portion of the cells whose generation times cluster around the average. This underestimate is particularly pronounced in differentiating muscle in which, concomitant with myogenic fusion, the distribution of G1 times (and consequently generation times as well) becomes longer and more highly variable. Our results suggest that the mode of administering the labeled DNA precursor profoundly affects the measurement of cell cycle parameters in vivo when these parameters exhibit considerable variability. The data presented here do not support the notion that any sizeable fraction of the myoblast population is withdrawn from the cell cycle for any significant period of time prior to fusion.