The anatomy of the uterine cervix

The cervix is the narrow inferior segment of the uterus which projects into the vaginal vault.
It is a fibromuscular organ lined by mucous membrane and measures 3 cm in length and 2.5cm in diameter. In the adult, it is angled downwards and backwards.
In the nulliparous female it is barrel shaped but it changes shape in pregnancy and the menopause.
About half the length of the cervix (the portio vaginalis) extends into the vagina.

Diagram of the female reproductive system… (Ross and Wilson: Anatomy and Physiology in health and illness; figure 19.3)

Lateral view of the female reproductive organs to show relationship to rectum and bladder… (Ross and Wilson: Anatomy and Physiology in health and illness; figure 19.4)

The cervix is traversed by the endocervical canal which is continuous with the body of the uterus at the isthmus above and opens into the vaginal cavity at the cervical os.
The shape of the external os varies. In the nulliparous woman it is small and circular. After pregnancy it is slit like. After the menopause it may narrow almost to a pin point.

The nulliparous cervix: note the small round os

The multiparous cervix

The cervix after menopause: note narrow os

Main anatomical regions of the cervix

The Endocervix extends from the isthmus (internal os) to the ectocervix and contains the endocervical canal. It is lined by mucous secreting columnar epithelium which is thrown into folds and projects into the underlying stroma forming complex glands or crypts
The Ectocervix which extends from the squamo columnar junction to the vaginal fornices and is covered by non keratinising stratified squamous epithelium which is hormone sensitive.
The Squamocolumnar Junction (SCJ) is located at the point where the columnar epithelium and the squamous epithelium meet. Its location varies throughout life as a result of metaplastic changes in the columnar epithelium of the cervix. Before puberty the SCJ is usually located at the external os; in the parous women it may be on the ectocervix; after the menopause the SCJ is usually within the endocervical canal.
The Transformation zone (TZ) This zone incorporates the area of metaplastic change in the cervix. It is important in view of the fact that the cells of the transformation zone are extremely susceptible to carcinogens and most cancers arise in the TZ.

Original squamocolumnar junction at external os in prepubertal cervix.

Adolescence produces eversion of endocervical epithelium and exposure to the vaginal environment- note relocation of squamocolumnar junction.

Squamocolumnar junction within endocervical canal after the menopause.

Histological section showing squamocolumnar junction: this region is susceptable to hormonal changes and its location changes during reproductive life.