Binding of proteins to mouse blastocysts after the attachment stage of implantation

Experimentally delayed mouse blastocysts were activated to implant by exogenous estradiol and after varying duration of estrogen influence, the blastocysts were flushed out from the uterine horns. Then they were incubated in the cold with different, 125I-conjugated proteins and the amount of protein bound to the blastocysts was determined by radioassay. Three radiolabelled proteins: human serum albumin (125I-HSA), human serum transferrin (125I-HST) and normal rabbit IgG (125I-RIgG) were tested and it was found that the uptake of each protein markedly increased between 14 and 24 hours of estrogen-activation. It was possible to partially block the binding of 125I-HSA and 125I-RIgG with the respective unlabelled protein. Unlabelled RIgG could also partially block the uptake of 125I-HSA whereas HSA did not impede the binding of 125I-RIgG. During delay of implantation the protein binding was low and approximately the same as after 14 hours of activation. However, at 18 and 24 hours of activation protein uptake increased gradually. Raising the incubation temperature from 0 to 37 degrees C did not significantly influence the protein binding capacity of the 24-hour-activated blastocyts. Whole blastocyst autoradiography indicated that the labelled protein was heavily bound in patches, preferentially located in the abembryonic half of the postattachment blastocysts. It is assumed that the binding of protein to abembryonic trophoblast cells of the implanting blastocyst can be attributed to the presence of protein receptors on the surface of these cells.