The uterine cervix is the narrow inferior segment of the uterus, which projects into the vaginal cavity. It is a fibromuscular organ lined by a mucous membrane and measures approximately 3cm in length and 2.5cm in diameter. The cervix is continuous at its superior margins with the body of the uterus and at its inferior margins with the vagina.
The cervix can be divided into anatomically distinct regions
- The ectocervix is the external part of the cervix that protrudes into the vaginal cavity. The ectocervix is approximately walnut sized, rounded and convex and covered with a hormone-sensitive non-keratinising stratified squamous epithelium.
- The endocervical canal is the lumen through the cervix that connects the uterine cavity to the lumen of the vagina. The opening of the cervical canal into the vaginal cavity is known as the external os and the opening into the uterine cavity is known as the internal os.
- The endocervix is the tissue surrounding the endocervical canal. The endocervix is lined with simple glandular epithelium, which forms branching crypts into the endocervical stroma to a depth of approximately 0.5-1.0cm.
- The squamocolumnar junction (SCJ) is where the ectocervical epithelium meets the endocervical epithelium. This junction has an abrupt change from stratified non-keratinising squamous cells to simple columnar epithelium, is visible colposcopically and can be demonstrated histologically.
- The location of the SCJ varies throughout a woman’s life due to eversion of columnar epithelium after puberty and during pregnancy followed by metaplasia of basal reserve cells to immature squamous epithelium. The SCJ recedes into the endocervical canal after the menopause.
- The transformation zone (TZ) is the name given to the area of the cervix comprising epithelium that has undergone squamous metaplastic change. It is the area of squamous metaplasia between the squamocolumnar junction and the ectocervix. It is characterised by immature squamous epithelium beneath which endocervical crypts may be seen on histology. This area is where most abnormalities are thought to arise.