The Known and Unknown About Female Reproductive Tract Mucus Rheological Properties

Spermatozoa reach the fallopian tube during ovulation by traveling through the female reproductive tract mucus. This non-Newtonian viscoelastic medium facilitates spermatozoon movement to accomplish fertilization or, in some cases, blocks spermatozoon movement, leading to infertility. While rheological properties are known to affect spermatozoon motility with in vitro models using synthetic polymers, their precise effects in vivo are understudied. This paper reviews the rheological measurements of reproductive tract mucus during ovulation in humans and model animals, focusing on viscosity and its potential effect on spermatozoa. Mucus viscosity in the female reproductive tract's different compartments is poorly understood. While information on this subject is incomplete, most mammals appear to have a viscosity decrease along their female reproductive tracts. Based on this sparse information, we hypothesize that viscosity changes in female reproductive tracts may guide spermatozoa to eggs, a novel concept that could improve our understanding of reproductive biology.