Using rat, mouse and rabbit animal systems we have investigated a possible role for lysosomal enzymes in the embryo attachment reaction. Specific histochemical methods were used to study the activity patterns of lysosomal enzymes, acid phosphatase, leucylnaphthylamidase and non-specific esterase, as well as lipid content in blastocysts collected from rats and mice just prior to nidation. In all three species, the cellular distribution and activity patterns of these enzymes in uterine regions bearing implanting blastocysts were shown to differ considerably from those in non-implant bearing uterine sites. From results obtained with oestrogen antagonists, nafoxidine and CI-628 citrate, we suggest that local oestrogen action, probably arising from the blastocyst, is essential for the initiation of increased cellular permeability and associated changes in lysosomal enzyme patterns necessary for the formation of the implantation chamber in the rat and mouse and for the embryo attachment reaction in the rabbit.
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