An introduction to the anatomy of the cervix
The cervix is a very important structure in the female reproductive system, connecting the uterus to the vagina. The cervix plays a vital role in fertility and maintenance of pregnancy, from conception to delivery [1]. Additionally, the cervix protects the uterus from potential pathogens, and has roles in immune response and inflammation control [2].
Anatomically, the cervix can be devided into two regions:
- the ectocervix, which is located proximally to the vagina, and
- the endocervix which is located distally to the uterus.
These structures are composed of cervical epithelial cells, which communicate with immune cells to initiate, maintain and regulate innate and adaptive immune responses [3]. We can further divide these cells into:
- squamous epithelial cervical cells, which populate the ectocervix as well as the endocervix,
- endocervical cells in the endocervix,
- tubal type epithelial cells in the endocervix [4].
Additionally, reserve cells line the whole cervical canal, with the highest concentration in the upper third of the cervix. These are thought to be the progenitor cells for the squamous and columnar epithelial cells, which then differentiate if a replacement cell is needed [4, 5].
Endocervical and Squamous metaplastic cells on a pap smear
A pap smear is a very important screening test that detects the presence of abnormal cells in the cervix. These abnormal cells can progress to cervical cancer if left untreated, so it is important to conduct the test at regular intervals as indicated by your doctor. When you obtain a report from your pap smear, it will contain information about the type of cells detected.
What are Endocervical Cells on a pap smear?
Endocervical cells are glandular cells located within the endocervix (inner cervix) that produce mucous. The presence of endocervical cells on a pap smear only indicates that cells from inside the cervix were taken during the sampling. A sample of these cells may be taken, but not always. It is standard to have endocervical cells on a pap smear, and their presence does not indicate cancer or a pre-cancerous condition [6].
What are Squamous Metaplastic Cells?
Squamous cells are a type of cells found throughout various tissues throughout the body, including the skin and the linings of the organs, including the cervix. Squamous cells have a variety in functions in maintaining tissue homeostasis, including protection, functions in absorption and filtration of fluids, secretion of mucus, sensory functions and regeneration.
Structurally, they appear as thin, flat shaped cells. Squamous cells can often grow at a high rate, and when evidence of this is seen under the microscope, they are described as metaplastic. Therefore, the presence of squamous metaplastic cells on a pap smear indicates that there are cells which are growing, suggesting a normal function [6].
Squamous cells can, however, undergo malignant transformation, giving rise to squamous cell carcinoma. This is a common type of skin tumour, associated with UV exposure. However, squamous cell carcinoma can occur in the cervix as well.
Understanding Pap Smear Results
On a high level, there are two outcomes of a pap smear test. A negative result means that no abnormalities were detected, and you should continue with the recommended schedule of future tests [7]. This includes presence of endocervical cells, as well as squamous metaplastic cells.
A positive (or abnormal) result indicates that there were cells present that were different from other cells. This doesn't necessarily mean that you have cervical cancer.
We can further classify these abnormalities into two categories: low-grade and high-grade changes [7]. Low-grade changes often mean presence of the human papillomavirus (HPV). You may need further testing, such as a coploscopy to further verify these changes. High-grade changes can suggest precancerous or cancerous cells. In this case, another pap smear test is required after six months, followed by a colposcopy or cone biopsy if necessary [7].
A very common abnormal finding is Atypical Squamous Cells of Undetermined Significance (ASCUS). These are squamous cells, which are however different from the squamous metaplastic cells. With ASCUS, further testing is needed - this usually involves testing for HPV and colposcopy. If this HPV test is negative, an additional pap smear may be required [7].
In terms of prevalence, in a study of 37.5 thousand pap smears, ASCUS were reported for 0.3% of Pap smears, while other epithelial abnormalities included atypical glandular cells of unknown significance (AGUS) in 0.05% of cases, low-grade squamous intraepithelial lesion (LSIL) in 0.02% of cases, atypical squamous cells with possible high-grade lesion (ASC-H) in 0.02% of cases, and high-grade squamous intraepithelial lesion (HSIL) in 0.008% of cases [8]. Despite low prevalence of these findings, it is very important to attend scheduled pap smears and action all findings as soon as possible.