The area between the uterus and the vagina in the female reproductive system is called the endocervical canal. At the top end lies the internal orifice of the uterus, or the internal os. This is the opening to the uterine cavity. At the bottom is the external os, which leads to the vagina.
The average length of the canal is about three centimeters, although it can vary from one woman to the next. It is a narrow, flat channel surrounded by thick, smooth muscle and connective tissue. Several folds, called palmate folds, run the length of the structure. The lining of the canal contains numerous mucus-producing cells. The mucus changes throughout the course of a woman’s menstrual cycle, with a more fertile type hospitable to sperm being produced around the time of ovulation.
An area called the squamocolumnar junction marks where the cells of the exterior cervix change to those of the endocervical canal. The region where the change occurs is not typically clearly delineated and can vary from one woman to the next. When a woman has a Pap smear, the doctor will typically try to get a sample of both types. This allows for evaluation of any abnormal cells in either area.
One of the main reasons doctors want to look at both exterior and endocervical cells is that different types of cancer can arise in each area. Squamous cell cancer affects the exterior cells. Adenocarcinomas, which occur much less frequently, arise in the glandular cells of the endocervical canal. If a gynecologist is unable to obtain both types of cells in a Pap smear, this should be noted in the results and discussed with the patient, since it represents a risk factor.
If cancer is suspected, the doctor will often perform an endocervical curettage. This procedure involves scraping a tissue sample from the endocervical canal using a small, scoop-shaped instrument called a curette. The sample can then be viewed under a microscope to confirm if cancer is present.
The endocervical canal plays an important role during both pregnancy and labor. For most of a pregnancy, the canal is a rigid structure, which helps to keep the fetus in the uterus. Later on, a few weeks prior to the due date, the walls begin to thin out and soften in a process called cervical ripening. This prepares the passageway for labor, when it will need to dilate and stretch enough for the baby’s head to pass through.