The cervix is the lower, cylindrical part of the uterus that opens into the vagina. Histologically the cervix is different from the rest of the uterus and also has a mucosa that doesn’t shed during menstruation. The cervical mucosa can be separated into three distinct regions: the endocervix, transformation zone, and ectocervix, which is also called the exocervix. The endocervical mucosa forms the wall of the cervical canal and will have a mucus-secreting simple columnar epithelium. The ectocervical mucosa is present in the part of the cervix that protrudes into the upper vagina and surrounds the external os, which is the lower opening of the uterus. This mucosa will have a non-keratinized stratified squamous epithelium that’s continuous with the vagina. The transformation zone is the region between the endo- and ectocervix. There’s an abrupt change from columnar cells to squamous cells within the transformation zone called the squamocolumnar junction. The exact location of the squamocolumnar junction will change or shift depending on the age of the individual as well as changes in the size of the uterus during menstruation. The transformation zone is also the ideal site to collect cells from during a Pap smear or Pap test. Underneath all regions of the cervical mucosa is a thick cervical wall that consists of dense connective tissue. Unlike the uterus, it only has a small amount of smooth muscle present.
The vagina connects the cervix to the external opening of the vagina called the vaginal vestibule. The wall of the vagina doesn’t contain glands and is composed of three main layers: the inner mucosa, which is typically about 150-200 µm in adults; a muscular layer called the muscularis; and an adventitia of connective tissue. The adventitia also contains a large amount of elastic fibers, which allow the vaginal wall to be both strong and flexible, which is particularly beneficial during childbirth.
Let’s first take a closer look at the mucosa of the endocervix, which is continuous with the endometrium of the uterus and is lined with a similar epithelium of mucus-secreting simple columnar cells. Underlying the epithelium is a thick lamina propria, which contains the large, branched cervical glands that produce about 20-60 mg of cervical mucus per day. But during the time around ovulation, the increasing level of estrogen will cause the glands to produce significantly more cervical mucus, which can be as much as 600 mg per day. The consistency of the mucus will change as well and become watery. This allows the sperm to move more easily into the uterus.
At the transformation zone, the simple columnar cells of the endocervical mucosa suddenly change to the stratified squamous cells of the ectocervical mucosa. Pap smears ideally include the transformation zone because cells in this zone are frequently undergoing metaplasia, changing from columnar cells to squamous cells. The frequent changing of cell-type increases the chance of an abnormal change to occur, which can potentially be a precancerous change. Squamous cell carcinoma is the most common type of cervical cancer and is most often found in the transformation zone. Human Papillomavirus or HPV is also a predisposing factor for squamous cell carcinoma, particularly the strains HPV-16 and HPV-18.
