Genome-wide mapping of unselected transcripts from extraembryonic tissue of 7.5-day mouse embryos reveals enrichment in the t-complex and under-representation on the X chromosome

Mammalian embryos can only survive if they attach to the uterus(implantation) and establish proper maternal-fetal interactions. Tounderstand this complex implantation pathway, we have initiated genomicanalysis with a systematic study of the cohort of genes expressed inextraembryonic cells that are derived from the conceptus and play a majorrole in this process. A total of 2103 cDNAs from the extraembryonic portionof 7.5-day post-conception mouse embryos yielded 3186 expressed sequencetags, approximately 40% of which were novel to the sequence databases.Furthermore, when 155 of the cDNA clones with no homology to previouslydetected genes were genetically mapped, apparent clustering of theseexpressed genes was detected in subregions of chromosomes 2, 7, 9 and 17,with 6.5% of the observed genes localized in the t-complex region ofchromosome 17, which represents only approximately 1.5% of the mousegenome. In contrast, X-linked genes were under-represented.Semi-quantitative RT-PCR analyses of the mapped genes demonstrated that onethird of the genes were expressed solely in extraembryonic tissue and anadditional one third of the genes were expressed predominantly in theextraembryonic tissues. The over-representation of extraembryonic-expressedgenes in dosage- sensitive autosomal imprinted regions andunder-representation on the dosage-compensated X chromosome may reflect aneed for tight quantitative control of expression during development.