Ultrastructure of blastocyst attachment in the mouse

Summary Mouse embryos have been examined with light and electron microscopy after fixation by perfusion with glutar aldehyde, and embedding in plastic.The Zona pellucida is dissolved gradually around the blastocyst just prior to attachment, and Zona free blastocysts exist only for a very short time.Blastocyst attachment is established when the trophoblast and uterine cell surface membranes lie within 150 Å apart over wide areas. The uterine epithelium does not show any signs of degeneration.Trophoblast attachment probably precedes decidual cell reaction.This work was supported by the Swedish Government Funds for Supporting Medical Research and the Swedish Medical Research Council (Project No. 12 X-70-02).