A nabothian cyst is a mucus-fille cyst on the surface of the cervix. They are most often caused when stratified squamous epithelium of the ectocervix (toward the vagina) grows over the simple columnar epithelium of the endocervix (toward the uterus). This tissue growth can block the cervical crypts (subdermal pockets usually 2-10 mm in diameter), trapping cervical mucus inside the crypts. The transformation of tissue types is called metaplasia.