Do sperm cells remember?

Spontaneous alternation behavior (SAB) is the universal tendency of animals, including unicellular organisms, to alternate directional choices at consecutive left/right branchings while traversing a maze. Occurrence of SAB implies short-term memory, as a current decision is statistically dependent on previous ones. We developed a procedure to assess SAB in human spermatozoa. A total of 1302 progressively motile spermatozoa from healthy donors were observed as they entered one of two mazes, both fabricated by eximer laser ablation. The control maze was a simple T-maze (width=depth=20 microm, distance between entrance and free choice T-intersection=600 microm). The experimental maze was identical to the control maze except for a forced right-turn 600 microm before the T-intersection. We recorded individual sperm cells' left/right decisions at the T-intersections in both mazes. Of the 714 spermatozoa entering the control maze, 49.1% turned to the left (not significantly different from the chance expectation of 50.0%). Of the 588 spermatozoa entering the experimental maze, 58.6% turned left after the initial forced right turn (significant SAB; P=0.041, Wilcoxon). The statistical dependency of a directional decision on a previous one suggests a physiological 'memory' in human spermatozoa. Among the possible underlying mechanisms are refractory processes in structures responsible for flagellar beating, a postulation which deserves further scrutiny with video-monitored single-cell testing.