ABSTRACT
This chapter analyzes spatial and temporal patterns of expression of a large number of individual mRNAs during embryogenesis of the sea urchin. Hybridization of radioactive probes for individual mRNAs shows that 90% of the moderately abundant messages have restricted spatial distributions in the embryo. About half of these increase significantly in abundance during cell differentiation and encode tissue-specific products. The other half of this class of mRNAs initially are expressed uniformly in the embryo and after blastula stage gradually are enriched in or confined to regions of oral ectoderm and endoderm. Analysis of the capacities of blastomeres separated at 16-cell stage to accumulate mRNAs with different spatial distributions in normal embryos shows that mRNAs that initially appear uniformly in the embryo and those restricted to differentiating mesenchyme accumulate to normal levels, whereas patterns of synthesis and decay of mRNAs characteristic of ectoderm differentiation are interrupted.
