Sperm cells are guided to the egg by chemical cues in a process termed
chemotaxis. Chemotactic navigation requires a tight feedback loop of sensing and motility.
The sperm flagellum combines sensing and motility in one:
Chemoattractant receptors on the flagellar surface signal to an
intraflagellar signaling cascade that controls the shape of the
flagellar beat.
We put forward a theory of how sampling a concentration gradient along
helical paths allows
sperm
of marine species to steer up-gradient. This navigation principle is
confirmed by recent high-speed
tracking of sea urchin sperm in three space dimensions. A simple
steering feedback explains how helical swimming paths bend to align
with a concentration gradient. This deterministic navigation strategy
is fundamentally different from those employed by most bacteria (biased
random walk) or immune cells (spatial comparison), and can be shown to
be particularly
well suited for fast swimmers operating at the limits of chemical
detection.