ABSTRACT
This chapter identifies a large number of genes that are expressed specifically in ectodermal cell lineages. Zygotic expression of the eariest of these genes begins at 16–32 cell stage, close to the time of emergence of the first ectoderm founder cells, while other genes are not activated untill 250-cell mid blastula stage, 6 to 7 hrs after the last founder cell arises. While temporal expression of several ectodermally expressed genes is clearly controlled transcriptionally, spatial regulation of mRNA accumulation occurs post-transcriptionally at stages prior to early gastrula. The developmental modulations of pattern imply the presence of complex, region-specific regulatory differences, not only between oral and aboral ectodermal, but also within each of these tissues. The switch from post-transcriptional to transcriptional regulation of “ectoderm specific” genes in non-ectodermal tissues parallels the embryo’s decreasing capacity for developmental regulation and the onset of irreversible committment to specific cell fates.
