Sperm guidance to the egg finds calcium at the helm

Abstract

Sperm respond to multiple cues during guidance to the egg including chemical attractants, temperature, and fluid flow. Of these, sperm chemotaxis has been studied most extensively-over 100 years-but only recently has it started to be understood at the molecular level. The long gestation in this understanding has largely been due to technical limitations that include the detection of calcium signal dynamics in a relatively small structure-the flagellum, measurement of actual chemoattractant gradients, the fact that only subpopulations of sperm respond at any given time, and the diversity in swimming behaviors that sperm exhibit from different species. Today, measurements of flagellar calcium signals on a fast time scale, discovery of the ion channels and organelles that may regulate these signals, and better understanding and quantitation of sperm swimming behaviors involved have given more certainty to our understanding of sperm directional swimming and its control by characteristic, calcium-directed asymmetric flagellar bends. Future research will need to apply these technical advances to other forms of sperm guidance such as thermotaxis and rheotaxis as well as gaining an understanding of how the flagellar apparatus is controlled by calcium.

Sperm selection in the female mammalian reproductive tract. Focus on the oviduct: Hypotheses, mechanisms, and new opportunities

“…Immediate and dramatic control of sperm physiology within the oviduct can be exerted by local concentrations of small molecules such as bicarbonate [103], calcium, [104,105] and nitric oxide [106] as well as by secreted proteins [107]. However, there is little to suggest at present that these effects are preferentially exerted on sperm subpopulations with specific genetic properties.…”

Taste Receptors: New Players in Sperm Biology

“…This drastic depletion, due in part to the many obstacles encountered by sperm during its journey but also to its small size relative to the female genital tract that must be traversed, makes the random encounter of sperm and oocyte unlikely to occur. To explain this phenomenon other than by “luck,” three different mechanisms of sperm guidance have been suggested: (i) thermotaxis or swimming up a temperature gradient, (ii) rheotaxis, that is swimming against a fluid flow and (iii) chemotaxis (Figure 3) [89,90,91,92,93]…”

Human sperm rheotaxis: a passive physical process

“…Similarly, while turning in temperature gradient, sperm bend their flagella and keep the curved flagella immobile for a short period 19 . These asymmetric flagellar beating patterns are results of microtubule sliding movement regulated by calmodulin and calcium 20 21 22 23 ; therefore, active sperm response always comes with calcium signaling and oscillation of intracellular Ca 2+ concentration ([Ca 2+ ] i ) 24 25 26 . For rheotaxis reorientation, neither sperm flagellar behavior or [Ca 2+ ] i is known.…”