The growth of the dermomyotome and formation of early myotome lineages in thoracolumbar somites of chicken embryos

Myotome formation in the epaxial and hypaxial domains of thoraco-lumbar som ites was analyzed using fluorescent vital dye labeling of dermomyotome cell s and cell-fate assessment by confocal microscopy. Muscle precursor cells f or the epaxial and hypaxial myotomes are predominantly located in the dorso medial and ventrolateral dermomyotome lips, respectively, and expansion of the dermomyotome is greatest along its mediolateral axis coincident with th e dorsalward and ventralward growth directions of the epaxial and hypaxial myotomes. Measurements of the dermomyotome at different stages of developme nt shows that myotome growth begins earlier in the epaxial than in the hypa xial domain, but that after an initial lag phase, both progress at the same rate, A combination of dye injection and/or antibody labeling of early and late-expressed muscle contractile proteins confirms the myotome mediolater al growth directions, and shows that the myotome thickness increases in a s uperficial (near dermis) to deep (near sclerotome) growth direction. These findings also provide a basis for predicting the following gene expression sequence program for the earliest muscle precursor lineages in mouse embryo s: Pax-3 (stem cells), myf-5 (myoblast cells) and myoD (myocytes), The move ments and mitotic activity of early muscle precursor cells lead to the conc lusion that patterning and growth in the myotome specifically, and in the e paxial and hypaxial domains of the body generally, are governed by morphoge netic cell movements.