The Known and Unknown About Female Reproductive Tract Mucus Rheological Properties

Abstract

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.

Keywords: cervix; female reproductive tract; mucus; uterus; viscosity.

FIGURE 1

Mammalian female reproductive tracts exhibit similar anatomy with species‐specific adaptions. (a) Human, (b) bovine, and (c) mouse. The female reproductive tract mucus, secreted by goblet cells, regulates spermatozoa travel by varying estradiol and progesterone levels. Cervices crypts are shown as zig‐zag lines.

FIGURE 2

The literature has limited and variable information on the viscosity along the female tract. However, most studied species data support a viscosity decrease along the female reproductive tract. Human cervix viscosity (in magenta) from four papers is 200–50 Pa·s (within the magenta square). Bovine (in orange) cervix and oviduct data from four papers suggest a viscosity decrease along the female reproductive tract (orange square connected by a dotted line). Bovine data from the same publication are marked with a solid orange line. Seven species of rodents (in blue) report simultaneous viscosity analysis (line) in the uterus and oviduct. Five species had high‐to‐low viscosity changes (filled stars), and two had low to high (empty stars). Circles indicate nonspecific times in the menstrual cycle. Triangles indicate at or near the beginning of the follicular phase or the end or near the end of the luteal cycle. Stars indicate during ovulation. The human data were collected from the papers [, –63]. The bovine data were collected from the papers [34, 50, 64]. The rodent data were collected from the papers [33, 65]. The measurements from papers [34, 50, 64] were taken at body temperature, whereas the other measurements were taken at room temperature.

FIGURE 3

The hypothetical viscosity decreases along the female reproductive tract might interact with increasing spermatozoa capacitation along the female reproductive tract to facilitate spermatozoa's travel to the egg.

FIGURE 4

Spermatozoa motility in water and viscous media differs depending on whether it is progressive (a) or hyperactive (b) motility. Progressive motility spermatozoa in watery media (ai) have an increased flagellar amplitude than viscous media (aii). Hyperactivated spermatozoa in watery media exhibit large, whip‐like flagellar patterns (bi). In contrast, in viscous media, only the latter half of the principal piece and the entire endpiece move because of the lack of accessory structures in those parts (bii).