Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case focuses on a speculum exam and the area from where the sample is obtained. Can you figure it out?
A 30-year-old woman comes to her primary care physician for an annual examination. The patient feels well and has no complaints. Over the past year, she has been sexually active with three male partners and does not use barrier contraception. A speculum exam is performed, and a specimen is collected using a cytobrush. Pap smear testing reveals cells with enlarged nuclei and perinuclear halos. Which of the following best describes the normal histology of the region of the cervix from where this sample was obtained?
A. Squamous epithelial cells
B. Cuboidal epithelial cells
C. Columnar epithelial cells
D. Transition from squamous to columnar epithelial cells
E. Transition from squamous to cuboidal epithelial cells
Scroll down to find the answer!
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The correct answer to today’s USMLE® Step 1 Question is…
D. Transition from squamous to columnar epithelial cells
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
A. Squamous epithelial cells
Incorrect: Squamous epithelial cells are found in the ectocervix, which is continuous with the vagina. A Pap smear is obtained at the transformation zone, where dysplasia most frequently happens.
B. Cuboidal epithelial cells
Incorrect: Cuboidal epithelial cells are often found in organs with secretory, absorptive, or excretory functions. These include the pancreas, kidneys, and parotid glands. Simple cuboidal epithelium is also present on the outer surface of the ovaries. However, cuboidal epithelial cells are not present in the cervix.
C. Columnar epithelial cells
Incorrect: Columnar epithelial cells are found in the endocervix, which is closer to the uterus. Koilocytes (epithelial cells with enlarged nuclei and perinuclear halos) and cervical dysplasia are more likely to be present at the transformation zone, rather than the endocervix.
E. Transition from squamous to cuboidal epithelial cells
Incorrect: Cuboidal epithelial cells are not present in the cervix. Therefore, there would be no region in the cervix wherein squamous epithelial cells are replaced by cuboidal epithelial cells.
Main Explanation
The cervical canal can be divided into three regions. The endocervix is closer to the uterus, and is lined by columnar epithelial cells. The ectocervix is continuous with the vagina, and it is lined by mature squamous epithelial cells. Where the squamous epithelium of the ectocervix and the columnar epithelium of the endocervix meet, there is a line called the squamocolumnar junction (also termed the transformation zone).
It is at the transformation zone where cellular metaplasia and dysplasia most often take place, and therefore a Pap smear specifically samples tissue from the squamocolumnar junction. Metaplasia occurs when a stressor, such as acidity of the vaginal canal during reproductive years, causes replacement of one cell type by another. In the cervix, columnar epithelium is replaced by squamous epithelium. Metaplasia is reversible once the stressor is removed.
In contrast, dysplasia occurs when fully differentiated cells are replaced by immature cells with varying shape and nuclear morphology, as demonstrated by the koilocytes in this question stem, with their enlarged nuclei and perinuclear halos. Mild dysplasia is reversible. However moderate or severe dysplasia is irreversible, and if not monitored, it can progress to cervical cancer.
Major Takeaway
The cervical canal can be divided into three regions: the ectocervix, the squamocolumnar junction, and the endocervix. The ectocervix contains squamous epithelial cells whereas the endocervix contains columnar epithelial cells. Cellular metaplasia and dysplasia most often occurs at the squamocolumnar junction (i.e. transformation zone).
References
Callahan, T.L., Caughey, A.B. (2018) Blueprints Obstetrics & Gynecology. Wolters Kluwer. ISBN: 978-1975134877.
Sellors, J.W., Sankaranarayanan, R. (2020) Colposcopy and treatment of cervical intraepithelial neoplasia: A beginners’ manual. International Agency for Research on Cancer.
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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.
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