Early development of the myotome in the mouse

The structure and development of the myotome has been extensively studied i n birds and amphibians but few studies have systematically addressed its de velopment in mammals. We have used a transgenic mouse carrying an nLacZ mar ker coupled to a myosin light chain 3F promoter to describe the structure o f the developing mammalian myotome, Through studies of transgene expression pattern, coupled with immunohistochemistry for the muscle structural prote ins desmin and slow myosin heavy chain we describe a gradient of maturity f or the cells within the developing myotome, Our results show that, the earl iest myocytes of the mammalian myotome span the rostrocaudal extent of the somite and have single large nuclei which localise:centrally within the myo tome, Throughout the period of study the myotome is more mature ventrally t han dorsally and cells comprising the medial aspect of the myotome are youn ger than those lying laterally. Immunohistochemistry for the earliest expre ssed muscle regulatory factor (myf-5) is used to de fine areas of the myoto me contributing new myogenic cells. In the early myotome small, round, myf- 5-expressing cells are found extensively within the dorsomedial aspect of t he dermamyotome and also within the entire rostral and caudal dermamyotomal lips. They subsequently appear within the central zone of the myotome, adj acent to the medially curled rostral and caudal dermamyotomal lips, and the re bean to elongate symmetrically. As the myotome enlarges, myf-5 expressio n is always restricted to the most medial aspect of the myotome, adjacent t o the least mature myocytes, marking the site of addition of new myogenic c ells. Together, these results allow development of a model of mammalian myo tome formation where growth occurs medially by addition of new cells from b oth rostral and caudal dermamyotome lips, while more mature myocytes are di splaced laterally. Furthermore, early myotomal myocytes differentiate in th e absence of MyoD expression, unlike later myotomal myocytes. This, along w ith their distinct morphology, suggests these cells may form a separate lin eage of pioneer myogenic cells. Dev Dyn 1999; 216:219-232, (C) 1999 Wiley-L iss, Inc.