Sperm navigation in humans: a concerted action of multiple means

For fertilization to occur in mammals, sperm cells must travel a long way through the female genital tract, overcoming numerous obstacles to reach the egg. In recent years it became clear that sperm arrival at the egg is not coincidental but rather that sperm cells must be guided, challenging long-standing beliefs. Three navigation means have been discovered: two active and highly sensitive means-chemotaxis and thermotaxis-and one passive means, rheotaxis. This review critically examines and puts into perspective the data accumulated over the past two decades about these navigation means and their underlying mechanisms. It questions whether these multiple navigation means are redundant or complementary, demonstrates how they work in harmony, and surveys successful attempts to harness them for improving artificial fertilization outcomes.