ICSI

Great question! Traditionally, sperm are selected visually by the embryologist. That means that they simply eyeball the sperm under the microscope and pick one that looks good. What looks good to an embryologist? A sperm should have a normal sized head- not too big or too small (pinhead sperm). Variations around normal head size suggest that there are chromosomal problems- too much or too little DNA which makes for abnormal embryos. Embryologists also look for normal sized midpiece (the mitochondrial powerhouse part of the sperm) and normal sized single tail. Double tailed or circular tailed sperm are also avoided. Sperm with good forward swimming ability are also preferred. Some labs are trying to add functional tests to the visual selection process. For instance, osmotic swelling can be used to test the sperm membrane response to hypertonic medium. If the sperm membrane responds or swells, it is a live one. This test is useful for men who have a condition in which sperm are alive but incapable of movement. This test lets the embryologist distinguish between live and dead sperm for injection. Some labs combine a preselection binding process to weed out poor quality sperm. Sperm that can bind to a hyaluronan coated slide may be better functional sperm than sperm which can't bind. Binding sperm can then be selected for ICSI. Other tests are trying to get a handle on whether the DNA inside the head is intact. As these newer tests are still under development, by and large, most programs still look at sperm under the microscrope and grab the "prettiest" sperm for injection.