Uterine cervix protrudes in upper vagina and contains endocervical canal, linking uterine cavity to vagina. Endocervical canal is lined by a single layer of tall columnar cells. Where the cervix is exposed to more hostile environment of vagina (ectocervix), it is lined by thick stratified squamous epithelium, as in rest of vagina. Junction between vaginal epithelium and endocervical epithelium is quite marked and abrupt. It is normally located at external os.
During each menstrual cycle, there is eversion of columnar epithelium near squamo-columnar junction, exposing it to hostile vaginal environment. This induces the growth of stratified squamous epithelium over the exposed area (squamous metaplasia). It is considered normal variant in women of reproductive age.
The importance of this transformational zone is that it may undergo malignant change causing cancer of cervix.