SKELETAL MORPHOGENESIS IN THE SEA-URCHIN EMBRYO - REGULATION OF PRIMARY MESENCHYME GENE-EXPRESSION AND SKELETAL ROD GROWTH BY ECTODERM-DERIVED CUES

The skeleton of the sea urchin embryo is synthesized by the primary me senchyme cells (PMCs), Previous studies have shown that local interact ions between PMCs and the neighboring ectoderm regulate several aspect s of skeletal morphogenesis, including PMC distribution in the blastoc oel, the size of the skeleton and its branching pattern, In the presen t study, we have further examined the regulation of skeletogenesis by the ectoderm, We generated a 'rate map' of skeletal growth, which reve aled stereotypical changes in the rates at which specific skeletal ele ments elongate during development, We showed that three transcripts en coding PMC-specific gene products known to be involved in the synthesi s of the skeleton exhibited dynamic, spatially regulated patterns of e xpression within the PMC syncytium, All three gene products showed hig h levels of expression at sites of skeletal rod growth, although the s pecific patterns varied among the genes, We present direct evidence, b ased upon cell transplantation experiments, that the expression of one of these genes, SM30, is responsive to local, ectoderm-derived cues, Based upon our studies, we suggest that short-range signals from diffe rent ectodermal territories may regulate the expression of PMC-specifi c gene products that are rate-limiting in skeletal biosynthesis, there by locally influencing skeletal rod growth.