Indispensability of Trophectoderm-Synthesized Hyaluronan for Blastocyst Attachment in Mice

Embryo implantation, which is an essential prelude for successful pregnancy, is initiated by the apposition and attachment of the blastocyst followed by its invasion into the uterine wall. Hyaluronan, a large negatively charged polysaccharide, is a major ECM component known to regulate numerous adhesion associated biological processes in a number of physiological settings. We hypothesized that hyaluronan may contribute to blastocyst attachment, an essential prerequisite for successful implantation. To challenge our hypothesis we used lentiviral incorporation for deletion of genes, encoding for hyaluronan synthesizing enzymes, selectively in the embryonic trophectoderm. The mutant blastocysts were transferred to uteri of foster mothers, further monitored for the success of pregnancy. Implantation in mice takes place on embryonic (E)day 4.5. Uterine flushing of pregnant mice, performed on this day resulted in recovery of a relatively high number of blastocysts, reflecting their failure to attach. This observation was further supported by histological analysis of E4.5 uteri, revealing impaired implantation. Decreased attachment of mutant blastocysts was also observed upon their incubation on top of human uterine epithelium cells, in vitro. Furthermore, decreased expression of osteopontin, an obligatory protein for blastocysts attachment, was detected in trophectoderm of mutant embryos by immuno-fluorescence. Functional MRI inspections revealed a decrease in uterine blood vessels permeability, which constitute the immediate response to blastocyst apposition in mice. Taking these observations into account, we suggest that hyaluronan, synthesized by the trophectoderm, is absolutely necessary for the attachment of blastocysts into the uterine epithelium in mice.