Elevated DNA excision repair capacity in the extraembryonic mesoderm of the midgestation mouse embryo

In order to determine whether there is differential cell-type-specific DNA repair we measured the nucleotide excision repair capacity of the four distinct cell lineages that comprise the extraembryonic yolk sac using the unscheduled DNA synthesis assay. Yolk sacs from mouse embryos at 11.5-12.5 days gestation were microdissected to yield purified trophoblast, parietal endoderm, mesoderm, and visceral endoderm, as well as fetal skin fibroblasts which were then grown as primary explants. At this midgestational stage of development, the yolk sac provides essential functions for the sustenance of the embryo while the complex process of organogenesis is proceeding in the liver, kidney, and gut. Trophoblast giant cells, parietal endoderm, and visceral endoderm all demonstrated low levels of unscheduled DNA synthesis consistent with levels measured in adult mouse skin fibroblasts. As has previously been documented, embryonic mouse skin fibroblasts were reproducibly 2- to 3-fold higher than adult mouse skin fibroblasts in levels of DNA excision repair. The extraembryonic mesoderm, however, displayed a statistically significant level of unscheduled DNA synthesis 10-fold higher than adult mouse skin fibroblasts or the other lineages of the midgestation yolk sac. Further, the S-indexes of these lineages were also determined to assess the possible relevance of differential repair to the proliferative status of the cells. These data demonstrate that DNA excision repair capacity is lineage-specific during embryogenesis in the mouse. These studies may begin to provide a context for understanding the perplexing developmental aspects such as the characteristic congenital abnormalities associated with the human heritable DNA repair deficiency diseases.