Mucins of the human endocervix

The physical character and amount of mucus secreted by the endocervix changes dramatically at midcycle to facilitate the reproductive process. Mucins expressed by the endocervical epithelium contribute to this all-important physiologic event. This review summarizes work from our laboratory demonstrating the mucin gene expression profile of cervical epithelium and mucin levels in cervical mucus through the menstrual cycle. mRNA levels of the gel-forming mucin MUC5B, the major gel-forming mucin expressed by the endocervical epithelium, peak before midcycle and the amount of MUC5B protein per unit total protein in cervical mucus peaks at midcycle. Message levels for MUC4, a major membrane-spanning mucin of the endocervix, peak at midcycle, but protein levels of MUC4 in human cervical mucus have not been measured. Message for each mucin diminishes dramatically as progesterone levels increase in the blood. These data suggest hormonal regulation of the two mucin genes in the endocervix, but there is no information on their regulation at the biosynthetic level via genomic hormone response elements. Perhaps, through its hydrophilicity, the MUC5B mucin holds water in place at the endocervical canal surface at midcycle, keeping the canal patent for sperm motility. A second potential role of the increased mucins at midcycle is to protect the cervix and uterus at the time when increased water is secreted into the cervical canal to facilitate sperm penetrance. Pathogens and other seminal fluid components may be excluded from entering the uterus by mucin trapping. Studies to determine the mechanism of hormonal regulation of mucins as well as the function of individual mucins are needed.