Morphogenetic cell movements in the middle region of the dermomyotome dorsomedial lip associated with patterning and growth of the primary epaxial myotome

The morphogenetic cell movements responsible for growth and morphogenesis i n vertebrate embryos are poorly understood, Myotome precursor cells undergo myotomal translocation; a key morphogenetic cell movement whereby myotomal precursor cells leave the dermomyotome epithelium and enter the subjacent myotome layer where myogenic differentiation ensues. The precursors to the embryonic epaxial myotome are concentrated in the dorsomedial lip (DML) of the somite dermomyotome (W. F. Denetclaw, B. Christ and C, P, Ordahl (1997) Development 124, 1601-1610), a finding recently substantiated through surg ical transplantation studies (C. P. Ordahl, E. Berdougo, S, J. Venters and W. F.Denetclaw, Jr (2001) Development 128, 1731-1744). Confocal microscopy was used here to analyze the location and pattern of myotome cells whose pr ecursors had earlier been labeled by fluorescent dye injection into the mid dle region of the DML, a site that maximizes the potential to discriminate among experimental outcomes. Double-dye injection experiments conducted at this site demonstrate that cells fated to form myotome do not involute arou nd the recurved epithelium of the DML but rather are displaced laterally wh ere they transiently intermingle with cells fated to enter the central epit helial sheet region of the dermomyotome. Time- and position-dependent label ing experiments demonstrated that myotome precursor cells translocate direc tly from the middle region of the DML without prior intra-epithelial 'trans lational' movements of precursor cells to either the cranial or caudal lips of the dermomyotome epithelium, nor were any such translational movements evident in these experiments. The morphogenetic cell movements demonstrated here to be involved in the directional growth and segmental patterning of the myotome and dermomyotome bear interesting similarities with those of ot her morphogenetic systems.