Changes in the proportion and number of Pax(7+ve) and MF20(+ve) myoblasts during chick myogenesis in the head and limb.

Previous studies suggested that all myoblasts are present in the head and limb prior to the commencement of primary myotube formation. As a consequence, these myoblasts must be in various developmental states during myogenesis, i.e. proliferating, differentiating or terminally differentiated. There are few in vivo studies investigating dynamic quantitative changes of subgroups of these myoblasts during myogenesis. In this report, using anti-Pax7 and anti-myosin heavy chain antibodies, we examined the quantitative change of proliferating (Pax7(+ve)) and terminally differentiated (MF20(+ve)) myoblasts during primary and secondary myogenesis in the chick head and limb. Our results show that during primary myogenesis, less than 30% of myoblasts are in the proliferating phase, but as soon as secondary myogenesis begins, over 95% of myoblasts start to proliferate. Moreover, we have found that the proportion of terminally differentiated myoblasts is maintained at a low level (less than 3%) during primary and secondary myogenesis.