Trophectoderm projections: a potential means for locomotion, attachment and implantation of bovine, equine and human blastocysts.

The behaviour of bovine, equine and human blastocysts was studied in vitro by time-lapse videomicrography and computer imaging. This study revealed that cytoplasmic extensions of the trophectoderm ['trophectoderm projections' (TEP)] were expressed by embryos of all three species, prior to or during zona escape. Bovine and human blastocysts escaped their zonae with a combination of blastocoele expansion, collapse and re-expansion coupled with the penetration of the zona pellucida by TEP. In equine embryos, after several cycles of blastocoele expansion and collapse, trophectoderm ruptured the zona with the concomitant appearance of TEP. This study provides documentation that TEP are expressed by a diverse range of mammalian species, bringing the total number of species in which this phenomenon is found to six, since TEP are also known to be expressed by guinea-pig, hamster and rhesus monkey blastocysts, representing rodents, ungulates and primates. In all species studied, the dynamic nature (extension, retraction, and angular movement) of the TEP was similar, moving in an undulating manner with rapid cycles of extension and retraction. Because TEP appear to be a general feature of mammalian blastocysts, they are implicated in one or more key events in early development, namely zona escape, attachment and/or implantation.