A MORPHOMETRIC INVESTIGATION OF MYOTUBE FORMATION IN RABBIT EMBRYO MEDIAL PTERYGOID MUSCLE

To determine the times of the appearance of myoblasts, early myotubes, late myotubes, and myofibers, we studied a region between two aponeur oses of the medial pterygoid masticatory muscle in embryos of two stra ins of rabbits, without disturbing the normal innervation. The objecti ves of this study were to define the quantitative relations among thes e cells and to determine their kinetics statistically. We used Fauve d e Bourgogne and New Zealand rabbit embryos on day 17, day 17 plus 12 h ours, day 18, day 18 plus 12 hours, and days 20, 22, and 28 of gestati on. Cell proliferation was studied with a light microscope, by means o f counting methods. Similar development was observed in the two strain s of rabbits. The numbers of myoblasts decreased as follows: (i) a mar ked decrease; (ii) a sudden cessation of the decrease, marked by a reb ound at 18 days, and lasting less Man 24 hours; and (iii) a plateau be tween embryonic days 22 and 28. The onset of reduction in the number o f early myotubes coincided with the rebound of myoblasts. The number o f late myotubes increased at the time of maximal early myotube density and during rebound of the myoblasts. Myofiber densities were similar to late myotube densities on day 22. We suggest that early myotubes ar e formed Very gradually by fusion of myoblasts, and that the significa nt increase in the numbers of myoblasts corresponds to the second gene ration of myoblasts necessary for differentiation of late myotubes.