Apoptosis in the early human placental bed and its discrimination from necrosis using the in-situ DNA ligation technique

Extensive areas of necrosis are present in the early human placental bed. Our aim was to determine whether apoptosis is also a feature. A method was therefore required to differentiate unequivocally necrosis and apoptosis. Initially, terminal deoxynucleotide transferase-mediated dUTP nick-end labelling was used to visualize apoptotic cells. However, non-specific labelling, probably due to free DNA released by necrotic cells, was excessive; thus, in-situ DNA ligation was employed. In this technique, two DNA fragments with single-base 3' overhangs and blunt-ends were labelled with a fluorochrome and then ligated to the DNA breaks on the sections. Immunolabelling for cytokeratin or leukocyte common antigen was performed to determine the phenotype of apoptotic cells identified by the in-situ DNA ligation technique. A proportion of the dying cells was confirmed to be trophoblasts. No co-localization with leukocyte common antigen was found in this region, suggesting that maternal macrophages and natural killer cells (CD56+) were not dying by apoptosis in significant numbers. In conclusion, in-situ DNA ligation in association with immunocytochemistry can readily distinguish apoptosis from necrosis in the placental bed. The results suggest that a proportion of invading trophoblast cells are eliminated by apoptosis in early pregnancy.