In humans, a few hundred million sperm enter the female reproductive tract, but only one in a million sperm reaches the fertilization site in the oviduct. It is unknown whether or how the hundreds of sperm that reach the fertilization site are molecularly different from those that lag behind and never reach the fertilization site. During their journey, mammalian sperm gradually obtain the fertilizing ability through extensive biochemical and functional changes – a process called “capacitation.” Reconstituting capacitation in the test tube made possible in vitro fertilization (IVF), which has produced nearly 5 million babies. However, the way that sperm interact with the egg differs significantly between in vitro and in vivo. Because most previous studies are based on population averages of millions of in vitro capacitated sperm, it is unclear which changes are physiologically relevant to sperm motility and fertilizing capability. 

We are investigating whether individual spermatozoa display unique molecular signatures during in vivo capacitation that determine fertility. To directly assess the small number of in vivo capacitated sperm in the oviduct that actually reach the egg, we have developed in situ molecular imaging of individual sperm cells in intact tissue.

Male factor diagnosis account for more than 30% of infertility in assisted reproductive technology (ART) users. By identifying the molecular signatures of sperm that reach and fertilize the egg in vivo and in situ, we can dramatically improve ART by enriching sperm with the fertile molecular signatures.