Normal and aberrant craniofacial myogenesis by grafted trunk somitic and segmental plate mesoderm

Our research assesses the ability of three trunk mesodermal populations -- medial and lateral halves of newly formed somites, and presomitic (segmental plate) mesenchyme -- to participate in the differentiation and morphogenesis of craniofacial muscles. Grafts from quail donor embryos were placed in mesodermal pockets adjacent to the midbrain-hindbrain boundary, prior to the onset of neural crest migration, in chick host embryos. This encompasses the site where the lateral rectus and the proximal first branchial arch muscle primordia arise. The distribution and differentiation of graft-derived cells were assayed using QCPN and QH1 antibodies to identify all quail cells and quail endothelial cells, respectively. Chimeric embryos were assayed for expression of myf5, myod, paraxis and lbx1, and the synthesis of myosin heavy chain (MyHC), between 1 and 6 days later (stages 14-30). Heterotopic and control (orthotopic) transplants consistently produced invasive angioblasts, and contributed to the lateral rectus and proximal first branchial arch muscles; many also contributed to the dorsal oblique muscle. The spatiotemporal patterns of transcription factor and MyHC expression by these trunk cells mimicked those of normal head muscles. Heterotopic grafts also gave rise to many ectopic muscles. These were observed in somite-like condensations at the implant site, in dense mesenchymal aggregates adjacent to the midbrain-hindbrain boundary, and in numerous small condensations scattered deep to the dorsal margin of the eye. Cells in ectopic condensations expressed trunk transcription factors and differentiated rapidly, mimicking the trunk myogenic timetable. A novel discovery was the formation by grafted trunk mesoderm of many mononucleated myocytes and irregularly oriented myotubes deep to the eye. These results establish that the head environment is able to support the progressive differentiation of several distinct trunk myogenic progenitor populations, over-riding whatever biases were present at the time of grafting. The spatial and temporal control of head muscle differentiation and morphogenesis are very site specific, and head mesoderm outside of these sites is normally refractory to, or inhibited by, the signals that initiate ectopic myogenesis by grafted trunk mesoderm cells.