The enigma of egg rotation — ASN Events

The enigma of egg rotation (#55)

Kenta Ishimoto 1 , Masahito Ikawa 2 , Masaru Okabe 2
  1. Research Institute for Mathematical Sciences, Kyoto University, Kyoto, Japan
  2. Research Institute for Microbial Diseases, Osaka University, Suita, Japan

  When cumulus-free mouse eggs are mixed with spermatozoa, many start to rotate by zona binding spermatozoa. Interestingly, the eggs tend to rotate anti-clockwise when viewed from above.  Some researchers have noted this phenomenon, but the mechanism of this enigmatic anti-clockwise rotation remains unexplained. 

  In the current study, we confirmed this tendency by observing eggs begin to rotate as spermatozoa, introduced from a distance, began to bind to zona pellucida. Under our experimental conditions, anti-clockwise rotation occurred ten times more frequently than clockwise rotation. 

  It was assumed that the helical tail movement exerted by spermatozoa pushing the surrounding medium with their flagellar motion might provide the necessary force to rotate eggs. However, the magnitude of this force has been estimated as approximately 11 pN for each single spermatozoon, using a detailed hydrodynamic numerical simulation. In our computer simulation model, a single spermatozoon is not sufficient to rotate eggs and symmetry break is not possible when many spermatozoa are on the zona. However, by introducing a condition of three-dimensional hindrance of sperm binding from the direction of dish bottom, anti-clockwise rotation occurred​ and the computer-simulated egg rotation movies realistically mimic real egg rotations. 

  We conclude that the strong tendency for anti-clockwise egg rotation to occur was attributable to the basic structure of sperm flagellum and a hindrance of sperm binding from the direction of dish bottom