Cervix

Cervix uteri

Also known as: uterine cervix, transformation zone, external os, internal os

The narrow lower third of the uterus (≈2.5–3 cm), mostly dense connective tissue, that projects into the vagina; its mucus gates sperm entry, its canal stays closed in pregnancy, and its transformation zone is the site of cervical cancer.

Show on body map Female Reproductive Organs

Overview

Length≈2.5–3 cm

Smooth muscle content≈10–15 %

Full dilatation in labour10 cm

The cervix has a supravaginal part (above the vaginal attachment; related to the bladder in front and the ureters ≈2 cm laterally) and a vaginal part (portio vaginalis, ectocervix) that protrudes into the vault of the vagina, surrounded by the fornices. The spindle-shaped cervical canal runs from the internal os (junction with the uterine cavity) to the external os — small and round in a nulliparous woman, a transverse slit after vaginal birth. The canal's mucosa forms branching folds (plicae palmatae) and deep mucus-secreting crypts. Unlike the body, the cervix is composed mainly of collagen-rich connective tissue with only ≈10–15 % smooth muscle.

The endocervix is lined by simple columnar mucus-secreting epithelium; the ectocervix by non-keratinised stratified squamous epithelium. They meet at the squamocolumnar junction, whose position changes with estrogen exposure — everted onto the ectocervix in puberty, pregnancy and with the combined pill (ectropion), receding into the canal after menopause. The zone of squamous metaplasia between the original and current junctions is the transformation zone, where HPV-related neoplasia develops. Blocked crypts form Nabothian cysts.

Cervical mucus is a hormonal sensor: under estrogen around ovulation it is abundant, clear, watery and stretchy (spinnbarkeit), and forms a fern pattern on drying, allowing sperm to pass; under progesterone it becomes scant, thick and impenetrable. In pregnancy a mucus plug seals the canal, and before labour the cervix ripens (collagen remodelling, hyaluronan, prostaglandins) so that it can efface and dilate to 10 cm.

Location: Lower third of the uterus; supravaginal part in the pelvis, vaginal part projecting into the upper vagina.

Parts & subdivisions

Internal os
Junction with the isthmus/uterine cavity.
Cervical canal (endocervix)
Columnar mucus-secreting mucosa with crypts.
External os and ectocervix
Stratified squamous epithelium facing the vagina.
Transformation zone
Metaplastic area between original and current squamocolumnar junctions; sampled in screening.

Anatomy

Innervation: Uterovaginal plexus; visceral pain fibres travel with the pelvic splanchnic nerves (S2–S4) — the cervix is relatively insensitive to cutting but sensitive to dilatation.
Arterial supply: Cervical (descending) branches of the uterine artery, running at ≈3 and 9 o'clock, and the vaginal arteries.
Lymphatic drainage: Parametrial, obturator, internal and external iliac and sacral nodes → common iliac and para-aortic nodes.
Histology: Endocervix: simple columnar mucinous epithelium with crypts. Ectocervix: non-keratinised stratified squamous epithelium (glycogen-rich, stains with iodine). Stroma: dense collagen with little smooth muscle.

Function

Regulates sperm entry via cyclic changes in mucus.
Barrier against ascending infection (mucus, plug).
Retains the pregnancy, then ripens, effaces and dilates in labour.

Clinical significance

Cervical cancer and screening

Persistent high-risk HPV (16, 18) infection of the transformation zone causes CIN and squamous-cell carcinoma or adenocarcinoma. Screening by HPV testing with cytology triage, colposcopy and LLETZ excision prevents most cancers; HPV vaccination prevents infection.

Cervical insufficiency and preterm birth

Painless second-trimester dilatation leading to pregnancy loss; a short cervix on transvaginal scan (<25 mm) predicts preterm birth and is managed with vaginal progesterone or cervical cerclage.

Cervicitis and ectropion

Mucopurulent discharge and contact bleeding from Chlamydia or gonorrhoea; ectropion is a benign cause of post-coital bleeding.

Test yourself

During a hysterectomy, the ureter is most at risk where it passes:

Over the pelvic brim anterior to the common iliac artery
Inferior to the uterine artery about 2 cm lateral to the cervix
Through the inguinal canal
Behind the round ligament at the deep ring

Answer: Inferior to the uterine artery about 2 cm lateral to the cervix

"Water under the bridge": the uterine artery crosses superior to the ureter in the base of the broad ligament, close to the supravaginal cervix, where a clamp placed too laterally can injure the ureter.

Cervical cancer most often arises in which region?

Endocervical crypts near the internal os
Transformation zone
Vaginal fornix
Isthmus of the uterus

Answer: Transformation zone

The transformation zone, where columnar epithelium undergoes squamous metaplasia, is vulnerable to persistent high-risk HPV infection and is the site of nearly all CIN and cervical carcinoma — it is the area sampled in screening.