1. Which embryos: Collapse any embryo that has stretching of the zona. That is, if the cavity is sufficiently expanded to cause the zona to stretch and thin, the embryo needs to be collapsed. If the embryo has a small cavity, that has not caused any stretching of the zona (typically <50% of the volume of the embryo), collapsing is unnecessary.
2. When to collapse: This procedure is performed just prior to vitrification. In general, embryos will need a few minutes to collapse (up to 10 minutes maximum) after trophectoderm disruption, so collapsing is usually performed prior to setting up the solutions and straws for the procedure. Identify the embryos for collapsing and perform the collapsing procedure on a heated stage. Return the embryos to the culture incubator. Then set up thevitrification solutions, straws, labels and other items needed to perform vitrification. This usually takes 5-10 minutes, during which time the majority of embryos will have collapsed nicely.
3. How to collapse using a laser: Identify 2 trophectoderm cells that are located well away from the inner cell mass (ICM), and which can be visualized cleanly on the edge of the embryo (see figure 1). Cells that are nicely stretched out are ideal. Position the laser such that it will fire at the junction between the 2 cells (see figure 2). The laser should be set on its minimum setting, which is 50mW for the Research Instruments device. Fire the laser at the targeted spot once and only once (this is very important). Typically, the embryo will not begin to collapse immediately, and there is no need to fire the laser again. In fact, firing the laser a second time can actually seal up the hole made by the first shot.
It is not necessary to breach the zona for successful collapsing. If the zona is thick at the chosen spot, it will likely not be completely breached. If the zona has thinned considerably, the hole may cut through the entire zona. Usually, embryos with the complete breach will collapse more quickly, but even without the breach, the vast majority will be nicely collapsed after 5 minutes back in the incubator. Occasionally, an embryo will not collapse even after time back in the incubator, but subsequently will collapse in either the equilibration solution (ES) or vitrification solution (VS).