Anatomy

The cervix is the canal between a woman’s internal reproductive system and the external environment.  Located at the entrance to the uterus, and forming a narrow junction between the uterus and the vagina, the cervix helps prevent infections from reaching the uterus.  On average, a woman’s cervix measures 3-4 cm in length and approximately 2.5 cm in diameter, although there is great variation in size and shape throughout a woman’s life.

The cervix is composed of distinct anatomical regions:

• Internal os: The opening of the cervix into the uterus, this is the uppermost portion of the cervix, at the upper end of the endocervical canal.
  
• Endocervix/endocervical canal: The part of the cervical canal that does not protrude into the vagina.  This interior portion of the cervix varies widely in length and width, and is lined with mucus-secreting glandular cells.  It is more difficult to visualise or sample by cytology than the exterior ectocervix.
  
• Ectocervix (or exocervix): This is the external portion of the cervix that protrudes into the vagina.  It is also the most readily visible portion of the cervix and is lined with squamous epithelium. 
  
• External os: The opening of the cervix into the vagina, at the lower end of the endocervical canal.
  
• Transformation zone: An area of the ectocervix where transformation between glandular and squamous cell types takes place.  The precise location and size of the transformation zone varies throughout a woman’s life.

The most common site of cervical cancer is the transformation zone. This is the area where transformation naturally occurs between cell types.  At the transformation zone there are fewer cell layers above the basal layer than elsewhere on the ectocervix.  As the basal cells are the targets of HPV infection, and can be reached more easily in the transformation zone, this makes them vulnerable to infection.  For this reason, the transformation zone is the area examined most closely in the process of cervical screening.

The cervix is a dynamic structure; its shape and size vary throughout a woman’s lifetime, largely due to the influences of reproductive hormones and the menarche.  The onset of menarche leads to changes in the histology of the cervix, as replacement of the external epithelial cells occurs over time.

Parity also has a profound effect on the basic anatomy of the cervix:

• Prior to childbirth, the external os is round
  
• After childbirth, the external os becomes elongated