Behavioral mechanism of human sperm in thermotaxis: A role for hyperactivation

STUDY QUESTION
What is the behavioral mechanism underlying the response of human spermatozoa to a temperature gradient in thermotaxis?

SUMMARY ANSWER
Human spermatozoa swim up a temperature gradient by modulating their speed and frequencies of hyperactivation events and turns.

WHAT IS KNOWN ALREADY
Capacitated human spermatozoa are capable of thermotactically responding to a temperature gradient with an outcome of swimming up the gradient. This response occurs even when the gradient is very shallow.

STUDY DESIGN, SIZE, DURATION
Human sperm samples were exposed to a fast temperature change. A quantitative analysis of sperm motility parameters, flagellar wave propagation, and directional changes before, during, and after the temperature change was carried out.

PARTICIPANTS/MATERIALS, SETTING, METHODS
The swimming behavior of 44 human sperm samples from nine healthy donors was recorded under a phase-contrast microscope at 75 and 2000 frames/s. A temperature shift was achieved by using a thermoregulated microscope stage. The tracks made by the cells were analyzed by a homemade computerized motion analysis system and ImageJ software.

MAIN RESULTS AND THE ROLE OF CHANCE
A temperature shift from 31 to 37°C resulted in enhanced speed and a lower frequency of turning events. These were reflected in a 35 ± 1% (mean ± SEM) increase of the straight-line velocity, 33 ± 1% increase of the average path velocity, 11 ± 1% increase of the curvilinear velocity, 20 ± 1% increase of the wobble, and 4 ± 1% increase of the linearity. Qualitatively, the inverse trend was observed in response to a 37-to-31°C shift. In addition, the amplitude of flagellar waves increased close to the sperm head, resulting in higher side-to-side motion of the head and, often, hyperactivation. This increase in the extent of sperm hyperactivation was reflected in an increase in the average (mean ± SEM) fractal dimension from 1.15 ± 0.01 to 1.29 ± 0.01 and in the percentage of hyperactivated spermatozoa from 3 ± 1% to 19 ± 2%. These changes in hyperactivation were observed less often in sperm populations that had not been incubated for capacitation. All these changes partially adapted within 3–10 min, meaning that following the initial change and while being kept at the new temperature, the values of the measured motility parameters slowly and partially returned toward the original values. These results led us to conclude that spermatozoa direct their swimming in a temperature gradient by modulating the frequency of turns (both abrupt turns as in hyperactivation events and subtle turns) and speed in a way that favors swimming in the direction of the gradient.

LIMITATIONS, REASONS FOR CAUTION
The conclusions were made on the basis of results obtained in temporal and steep temperature gradients. The conclusions for spatial, shallow gradients were made by extrapolation.

WIDER IMPLICATIONS OF THE FINDINGS
This is the first study that reveals the behavior of human spermatozoa in thermotaxis. This behavior is very similar to that observed during human sperm chemotaxis, suggesting commonality of guidance mechanisms in mammalian spermatozoa. This study further substantiates the function of hyperactivation as a means to direct spermatozoa in guidance mechanisms.