Summary. Intrauterine instillation of heat-deaggregated glycogen in virgin mice induced a marked but transient luminal infiltration of polymorphonuclear leucocytes (PMNL). Similar injections of glycogen 2 days post coitum resulted in termination of pregnancy in the majority of treated females. Embryos recovered on Day 4 of pregnancy from the glycogen-treated females, before the expected time of implantation, had developed to the cavitated blastocyst stage, and were capable of undergoing characteristic outgrowth when cultured in vitro. A small proportion, however, appeared abnormal and did not develop in vitro. Cavitated blastocysts obtained from control mated females did not undergo outgrowth in vitro in the presence of intact PMNL, homogenates of these cells or a low molecular weight (<1500) fraction of the homogenates and after culture appeared similar to the abnormal blastocysts recovered from glycogen-treated uteri. It is proposed that the anti-fertility effect of PMNL infiltrates is due to a low molecular weight component of these cells which acts in utero and is cytotoxic to preimplantation blastocysts before their hatching from the zona pellucida.
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