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 conserved Msx gene family, in this process. Msx genes have been shown to function in the dorsoventral patterning of the central nervous system in Drosophila and in a variety epithelial-mesenchymal interactions in vertebrates. We showed previously that the SpMsx gene 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 of SpMsx, we injected SpMsx mRNA into single-cell zygotes and monitored development morphologically and with a series of territory-specific molecular markers. RT-PCR analysis revealed that injected SpMsx transcripts 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 of Msx genes in embryonic signaling processes, particularly involving the BMP pathway, suggest that SpMsx may 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.

RNA-Seq identifies SPGs as a ventral skeletal patterning cue in sea urchins

“…1B). This approach was previously used to show ectoderm-specific skeletal patterning defects with NiCl 2 (Armstrong et al, 1993) and overexpression of the homeobox protein Msx (Tan et al, 1998). Patterning defects were observed only in chimeras in which the hull was exposed to SB (Fig.…”

Novel gene expression patterns in hybrid embryos between species with different modes of development

“…Oral ectoderm is involved in induction events in specification of aboral ectoderm (Wikramanayake et al 1995;Wikramanayake and Klein 1997) and in patterning of the skeleton-secreting mesenchyme cells (Armstrong et al 1993;Ramachandran et al 1997;Tan et al 1998;Zito et al 1998;Di Bernardo et al 1999). Changes in a few higher level regulatory genes that specify modules such as gene expression territories may be sufficient to change developmental architecture.…”

Cloning and characterization of cDNA for syndecan core protein in sea urchin embryos

“…Recently, the Msx gene of sea urchin embryos was cloned and characterized ( SpMsx; Dobias et al 1997 ). SpMsx transcripts were found to be distributed ubiquitously in blastula‐stage embryos and then restricted to the prospective oral ectoderm and vegetal plate by the late blastula stage, when it is assumed to be involved in PMC patterning and spicule morphogenesis ( Tan et al 1998 ). Transplantation experiments show that the ectoderm is implicated in PMC patterning and morphogenesis ( Ettensohn 1990; Hardin et al 1992 ; Armstrong et al 1993 ).…”