Atlas of visual inspection of the cervix with acetic acid for screening, triage, and assessment for treatment

Naked-eye appearance of squamous epithelium
Normal squamous epithelium appears smooth and covers the ectocervix. The epithelium is transparent. On naked-eye examination, the ectocervix appears pink because the underlying stroma is visible through the transparent epithelium. The cervical stroma appears red, because of the presence of abundant blood vessels. The transparent squamous epithelium acts as a colourless filter for the transmission of light. The squamous epithelium often meets the columnar epithelium around the external os.

Microscopic features of squamous epithelium
Squamous epithelium is composed of about 15–20 layers of squamous cells, neatly arranged in rows and separated from the underlying cervical stroma by a basement membrane. Such multilayered epithelium is known as stratified epithelium. Each cell has a nucleus at the centre and cytoplasm.
The squamous epithelium is divided into the following four layers, starting from the lowest layer of cells:

• Basal layer: A single layer of round cells with large dark-staining nuclei and little cytoplasm, resting on the basement membrane.
• Parabasal layer: A few layers of cells with relatively large dark-staining nuclei, located above the basal layer.
• Intermediate layer: Layers of polygonal cells with small round nuclei and abundant cytoplasm, forming a layer between the parabasal cells and the superficial cells.
• Superficial layer: The topmost layer of cells, which are completely flattened, with very small or absent nuclei.

In mature squamous epithelium, the cytoplasm of the intermediate and superficial layers of cells is replaced by glycogen.

The next section discusses the details of the columnar epithelium.