Disruption of Primary Mesenchyme Cell Patterning by Misregulated Ectodermal Expression ofSpMsxin Sea Urchin Embryos

Abstract

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 the homeobox gene,SpMsx,a member of the highly conservedMsxgene family, in this process.Msxgenes have been shown to function in the dorsoventral patterning of the central nervous system inDrosophilaand in a variety epithelial–mesenchymal interactions in vertebrates. We showed previously that theSpMsxgene is expressed during embryogenesis in a complex and dynamic pattern consistent with roles in the development of subpopulations of endoderm, mesoderm, and oral ectoderm. To perturb this pattern of expression and thus probe the function ofSpMsx,we injectedSpMsxmRNA into single-cell zygotes and monitored development morphologically and with a series of territory-specific molecular markers. RT-PCR analysis revealed that injectedSpMsxtranscripts persisted at least until the gastrula stage in amounts comparable to endogenous levels. Injected embryos exhibited deficiencies in the organization of primary and secondary mesenchyme cells within the blastocoelic cavity, as well as abnormalities in spicule number and shape. Defects in the endoderm were also common, including reduced or absent archenterons. Micromere transplantation experiments revealed that the defects in skeletogenic mesenchyme patterning were non-cell autonomous, consistent with findings that cell–cell interactions between ectoderm and the progenitors of the skeletogenic mesenchyme, the primary mesenchyme cells (PMCs), are important both for PMC guidance and spicule morphogenesis. Our data, taken together with observations in other organisms on the role ofMsxgenes in embryonic signaling processes, particularly involving the BMP pathway, suggest thatSpMsxmay be a part of the mechanism by which the ectoderm influences both the arrangement of primary mesenchyme cells within the blastocoel and the shapes of the skeletal rods.