Zhang M, McLennan I S
Department of Anatomy and Structural Biology, University of Otago, Dunedin, New Zealand.
Dev Dyn. 1995 Oct;204(2):168-77. doi: 10.1002/aja.1002040207.
Developing muscles contain at least two types of myoblasts. Early myoblasts are the first myoblast to form and are the only myoblasts present during primary myotube formation. By the time secondary myotube formation begins, early myoblasts are rare and late myoblasts are common. The late myoblasts have been postulated to give rise to secondary myotubes. While this is generally accepted, it is unclear whether late myoblasts also contribute to the growth of primary myotubes. One study has 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 myotube formation. We have therefore re-examined the question of whether primary myotubes absorb new nuclei during secondary myotube formation. Pregnant rats were given a single intraperitoneal injection of 5 mg of 5-bromodeoxyuridine (BrdU) on one embryonic day (from E13 to E19) and their embryos removed on E20. The brominated-nuclei were labelled with an antibody to BrdU and the myotubes were marked with anti-myosin antibodies. Double labelled sections from the soleus, tibialis anterior, and extensor digitorum longus muscles were examined with a confocal microscope. The numbers and locations of labelled nuclear profiles in primary and secondary myotubes were counted and recorded. The results show: (1) that primary myotubes absorb nuclei at all stages of development, including the period of secondary myotube formation; (2) that in the early stages of secondary myotube formation, more myoblasts fuse with primary than secondary myotubes whereas this situation is reversed by the end of secondary myotube formation; and (3) that the nuclei added to primary and secondary myotubes during the early stages of their formation are located within the middle of E20 muscles. The nuclei added to growing myotubes are preferentially located at the ends of the muscles.
正在发育的肌肉至少包含两种类型的成肌细胞。早期成肌细胞是最早形成的成肌细胞，也是初级肌管形成过程中唯一存在的成肌细胞。到次级肌管形成开始时，早期成肌细胞很少见，而晚期成肌细胞很常见。有人推测晚期成肌细胞会产生次级肌管。虽然这一点普遍被接受，但尚不清楚晚期成肌细胞是否也对初级肌管的生长有贡献。一项研究提供了证据，表明次级肌生成过程中存在的成肌细胞选择性地彼此融合或与次级肌管融合，但不与初级肌管融合（哈里斯等人[1989a]《发育》107:771 - 784）。然而，在次级肌管形成过程中初级肌管的大小会增加。因此，我们重新审视了初级肌管在次级肌管形成过程中是否吸收新细胞核的问题。在一个胚胎日（从胚胎第13天到第19天）给怀孕大鼠腹腔内单次注射5毫克5 - 溴脱氧尿苷（BrdU），并在胚胎第20天取出它们的胚胎。用抗BrdU抗体标记溴化细胞核，并用抗肌球蛋白抗体标记肌管。用共聚焦显微镜检查来自比目鱼肌、胫骨前肌和趾长伸肌的双标记切片。对初级和次级肌管中标记核轮廓的数量和位置进行计数和记录。结果表明：（1）初级肌管在发育的所有阶段，包括次级肌管形成期，都会吸收细胞核；（2）在次级肌管形成的早期阶段，与初级肌管融合的成肌细胞比与次级肌管融合的多，而在次级肌管形成结束时这种情况则相反；（3）在初级和次级肌管形成早期添加到它们中的细胞核位于胚胎第20天肌肉的中部。添加到生长中的肌管的细胞核优先位于肌肉的末端。
