DURING SECONDARY MYOTUBE FORMATION, PRIMARY MYOTUBES PREFERENTIALLY ABSORB NEW NUCLEI AT THEIR ENDS

Developing muscles contain at least two types of myoblasts. Early myob lasts are the first myoblast to form and are the only myoblasts presen t during primary myotube formation. By the time secondary myotube form ation begins, early myoblasts are rare and late myoblasts are common. The late myoblasts have been postulated to give rise to secondary myot ubes. While this is generally accepted, it is unclear whether late myo blasts also contribute to the growth of primary myotubes. One study ha s produced evidence that myoblasts present during secondary myogenesis selectively fuse with each other or with secondary myotubes, but not with primary myotubes (Harris et al. [1989a] Development 107:771-784). However, the sizes of primary myotubes increase during secondary myot ube formation. We have therefore re-examined the question of whether p rimary myotubes absorb new nuclei during secondary myotube formation. Pregnant rats were given a single intraperitoneal injection of 5 mg of B-bromodeoxyuridine (BrdU) on one embryonic day (from E13 to E19) and their embryos removed on E20. The brominated-nuclei were labelled wit h an antibody to BrdU and the myotubes were marked with anti-myosin an tibodies. Double labelled sections from the soleus, tibialis anterior, and extensor digitorum longus muscles were examined with a confocal m icroscope. The numbers and locations of labelled nuclear profiles in p rimary and secondary myotubes were counted and recorded. The results s how: (1) that primary myotubes absorb nuclei at all stages of developm ent, including the period of secondary myotube formation; (2) that in the early stages of secondary myotube formation, more myoblasts fuse w ith primary than secondary myotubes whereas this situation is reversed by the end of secondary myotube formation; and (3) that the nuclei ad ded to primary and secondary myotubes during the early stages of their formation are located within the middle of E20 muscles. The nuclei ad ded to growing myotubes are preferentially located at the ends of the muscles. (C) 1995 Wiley-Liss, Inc.