Evaluation of cervical mucus is a standard for determining the fertile period in natural family planning. Cervical mucus accepts, filters, prepares, and releases sperm for successful transport to the egg and fertilization. Recent scientific advances provide answers to how the mucus regulates fertility as its physical properties change during the menstrual cycle. Transmission electron microscopy reveals small interstices between mucus macromolecules relative to a sperm head. Thus advancing sperm must push aside or cut through the microstructure. The interstices are largest in the periovulatory phase of the cycle. Small magnetic spheres, comparable with the size of a sperm head, are now being used to study the physical properties of the mucus on the scale of individual sperm.
PIP: The cervical mucus is one of the primary links between the hormone signals and fertility. It provides sperm transport to the upper reproductive tract. In This article, the function, structure, rheologic properties, and pentrability are discussed. From in vitro studies and in vivo studies of primates, it is known that the cervical mucus has 4 main functions related to sperm. 1) sperm are admitted from the hostile vaginal environment. 2) the mucus filters out morphologically abnormal sperm, and seminal plasma constituents are removed from the sperm's surfaces. 3) sperm also are nurtured and supported biochemically. This may be in preparation for fertilization of the egg. 4)the cervical mucus stores the sperm for later release, perhaps to coordinate the timing of insemination with the ovum. Mucus is a gel with solid and liquid phases. There is little knowledge of how the mucus is created or maintained, or about molecular crosslinking. Sperm image analysis, immunobeads, sperm motion analysis, transmission electron microscopy, microheometry, laser light scattering, and other physical means have all contributed to the study of mucus. Mucus has been revealed to be a heterogeneous mosaic like microstructure with interstices between the mucus macromolecules and smaller than a sperm cell. Sperm must thrust through this microstructure. Periovulatory mucus has larger institial spaces, and the water content or hydration distinguishes cyclic changes. Because mucus evaporates quickly on removal from the cervix, it is difficult to culture and analyze. The rheologic properties indicate viscoelasticity and cyclic variation. Sperm penetration is optimal when mucus viscosity and elasticity is minimal, which occurs just before ovulation. However, great variability exists among even normal women. Microrheometer use facilitates the understanding of mucus receptivity to sperm. The sperm surface also has a role in penetrability. 1) The sperm/mucus interaction depends upon the number of sperm that cross the mucus interface and the rate at which sperm migrate through the mucus interior. Morphologically abnormal sperm characteristically have inferior motility. 2) The structure and physical properties of the interface and the interior are different. 3) Penetration is maximal during the periovulatory period. 4) Invitro sperm penetration does not assure in vivo penetration.