The cervix is the lower, fibromuscular part of the uterus that connects to the vagina. It is a cylindrical or conical structure, measuring about 3 to 4 centimeters long and 2.5 centimeters in diameter. The size and shape of the cervix can vary depending on a woman’s age, hormonal status, and whether she has given birth.
Differentiating the Ectocervix and Endocervix
The cervix has two primary regions: the ectocervix and the endocervix. The ectocervix is the outer portion that extends into the vagina and is visible during a pelvic exam. This region is covered by a durable, multi-layered tissue called stratified squamous epithelium, which is similar to the lining of the vagina and has a smooth, pink appearance. These cells contain glycogen, a form of stored glucose.
The endocervix is the inner canal connecting the external opening (external os) with the uterus. This canal is lined with a single layer of mucus-secreting cells called simple columnar epithelium, giving it a reddish, glandular appearance. The alkaline mucus produced by these cells fills the endocervical canal. The endocervix also contains extensive sensory nerve endings, unlike the ectocervix.
The Transformation Zone
The area where the squamous cells of the ectocervix and the columnar cells of the endocervix meet is the transformation zone. This junction, called the squamocolumnar junction (SCJ), is not fixed and shifts its location on the cervix throughout a woman’s life. This zone is the site of a process called squamous metaplasia, where one mature cell type is replaced by another.
In the cervix, columnar cells exposed to the vagina’s acidic environment are gradually replaced by more resilient squamous cells. This replacement happens as underlying “reserve cells” differentiate into squamous epithelium.
Clinical Significance in Cervical Health Screening
The transformation zone is important for cervical health because it is the area most susceptible to infection with the human papillomavirus (HPV). Most cases of cervical cancer and its precancerous precursors originate within the cells of this zone. For this reason, cervical cancer screening tests, like the Pap smear and HPV tests, are designed to collect cells from this region.
During a screening, a clinician uses a small brush or spatula to gently scrape cells from the cervix. The goal is to obtain a sample including cells from the ectocervix and endocervix, ensuring the transformation zone is represented.
This sample is examined in a lab for abnormal cellular changes (dysplasia) or high-risk HPV types. Regular screening allows for the detection and treatment of precancerous conditions, preventing invasive cancer.
How Hormones and Age Affect the Cervix
The structure of the cervix and the location of the transformation zone are influenced by hormonal changes throughout a woman’s life. During puberty and the reproductive years, increased estrogen levels cause the endocervical columnar epithelium to evert, or turn outwards, onto the ectocervix. This process, known as ectropion, makes the transformation zone larger and more visible. Pregnancy also brings about changes, including a softening of the cervix and an increase in mucus production.
With the onset of menopause, declining estrogen levels cause the cervix to shrink, prompting the transformation zone to recede into the endocervical canal. As a result, the squamocolumnar junction may move to a position where it is not visible during a pelvic exam. This change can make it more challenging to collect an adequate cell sample from the transformation zone during a Pap test in postmenopausal women.