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The behavior of golden hamster blastocysts was studied in vitro by continuous time-lapse videomicrography and computer imaging, during and immediately following escape from the zona pellucida. This study revealed numerous small cytoplasmic trophectoderm projections (TEPs) approximately 18 microns long that penetrated the zona pellucida both radically and tangentially and appeared to be actively involved in zona escape in vitro. After escape from their zonae, some blastocysts moved across the culture dish by an endogenous means of locomotion, most likely involving activity of the small TEPs. Several hours after zona escape, embryos expressed large TEPs up to 46 microns long that moved in an undulating manner and showed rapid cycles of extension and retraction; the timing of their appearance suggested that these TEPs are normally involved in attachment to the uterine epithelium. Embryos fixed in utero, during the developmental interval between zona loss and embryo attachment, exhibited large TEPs similar in morphology to those expressed by cultured blastocysts. These observations document for the first time that mammalian blastocysts are capable of endogenous locomotion, confirm TEPs as components of normal blastocyst activity, reveal that there are two kinds of TEPs that differ temporally and morphologically, and extend earlier reports of TEP activity in guinea-pig embryos to the hamster. |
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