QUANTITATIVE-ANALYSIS OF PROTEIN-SYNTHESIS IN MOUSE EMBRYOS .2. DIFFERENTIATION OF ENDODERM, MESODERM, AND ECTODERM

The changes in protein synthesis that occur during differentiation of the primitive germ layers were examined by high-resolution, two-dimens ional gel electrophoresis of proteins synthesized in 6.5 and 7.5 days postcoitum (d.p.c.) mouse embryos. For 6.5 d.p.c. embryos, protein syn thesis patterns were compared between whole extraembryonic and embryon ic regions and between embryonic visceral endoderm and embryonic ectod erm. For 7.5 d.p.c. embryos, comparisons were made between extraembryo nic and embryonic regions and between isolated embryonic endoderm, mes oderm, and ectoderm. Each of the isolated 7.5 d.p.c. germ layers was d ivided into anterior and posterior fragments in order to evaluate poss ible regional differences in gene expression along the anterior-poster ior axis. Comparisons of protein synthesis patterns revealed the great est difference between isolated endoderm and ectoderm, indicating that by as early as 6.5 d.p.c. patterns of gene expression differ signific antly between these tissues. The greatest similarities were found betw een ectoderm and whole embryonic regions and between endoderm and whol e extraembryonic regions, which most likely reflects the overall cellu lar compositions of the embryonic and extraembryonic regions. Based on their patterns of synthesis, four groups of proteins were identified that were preferentially synthesized in either endoderm or ectoderm. T hese provide useful markers for studying differentiation in these tiss ues. One other protein, migrating at the position expected for vimenti n, was synthesized at an elevated rate in isolated mesoderm. We also o bserved differences in rates of synthesis of alpha-tubulin and tropomy osin-5 indicative of potential differences in cytoskeletal composition among the germ layers beyond those previously described. The differen ce in overall protein synthesis patterns between anterior and posterio r regions was greatest in the embryonic endoderm, indicating that diff erentiation along the anterior-posterior axis may be initiated sooner or may proceed more rapidly in the endoderm than in the other germ lay ers. These data provide the first quantitative evaluation of the degre e to which differentiation of the three primitive germ layers affects protein synthesis patterns and reveal potentially useful markers of en doderm and ectoderm differentiation.