In situ screening for genes expressed preferentially in secondary mesenchyme cells of sea urchin embryos.

In sea urchin embryos, four types of non-skeletogenic mesodermal cells are derived from secondary mesenchyme cells (SMCs). Although determining the complete lineage of SMCs is currently a high-priority goal, specific markers for each type of SMC-derived cell in Hemicentrotus pulcherrimus are unavailable. To identify genes preferentially expressed in the various types of SMC-derived cells, we constructed a cDNA library of the archenteron isolated from late gastrulae. Both the 5' and 3' ends of 1,050 cDNAs randomly selected from 7,500 picked clones were sequenced. Based on the sequence at the 3' end, the cDNAs were grouped into 671 independent clusters. Among these, 605 clusters were analysed by whole-mount in situ hybridisation; 28% (170 clusters) exhibited differential expression patterns, while 24% were ubiquitously expressed and 48% did not show any staining. Of 170 clusters showing differential expression patterns, 33 clusters were differentially expressed in SMC-derived cells. From these clusters, several genes were obtained that were specifically or predominantly expressed in each type of SMCs, including coelomic pouch cells in which specific expression patterns have not been reported previously, and hence will be useful for lineage studies. Furthermore, in situ hybridisation revealed the existence of a new type or subpopulation of SMCs distributed sparsely in the blastocoel.