DISRUPTION OF PRIMARY MESENCHYME CELL PATTERNING BY MISREGULATED ECTODERMAL EXPRESSION OF SPMSX IN SEA-URCHIN EMBRYOS

The patterning of the mesoderm of the sea urchin embryo is a classical paradigm of epithelial mesenchymal interactions in organogenesis, yet Little is known of its molecular basis. Here we address the role of t he homeobox gene, SpMsx, a member of the highly conserved Msx gene fam ily, in this process. Msx genes have been shown to function in the dor soventral patterning of the central nervous system in Drosophila and i n a variety epithelial-mesenchymal interactions in vertebrates. We sho wed previously that the SpMsx gene is expressed during embryogenesis i n a complex and dynamic pattern consistent with roles in the developme nt of subpopulations of endoderm, mesoderm, and oral ectoderm. To pert urb this pattern of expression and thus probe the function of SpMsx, w e injected SpMsx mRNA into single-cell zygotes and monitored developme nt morphologically and with a series of territory-specific molecular m arkers. RT-PCR analysis revealed that injected SpMsx transcripts persi sted at least until the gastrula stage in amounts comparable to endoge nous levels, injected embryos exhibited deficiencies in the organizati on of primary and secondary mesenchyme cells within the blastocoelic c avity, as well as abnormalities in spicule number and shape. Defects i n the endoderm were also common, including reduced or absent archenter ons. Micromere transplantation experiments revealed that the defects i n skeletogenic mesenchyme patterning were non-cell autonomous, consist ent with findings that cell- cell interactions between ectoderm and th e progenitors of the skeletogenic mesenchyme, the primary mesenchyme c ells (PMCs), are important both for PMC guidance and spicule morphogen esis. Our data, taken together with observations in other organisms on the role of Msx genes in embryonic signaling processes, particularly involving the BMP pathway, suggest that SpMsx may be a part of the mec hanism by which the ectoderm influences both the arrangement of primar y mesenchyme cells within the blastocoel and the shapes of the skeleta l rods, (C) 1998 Academic Press.