The superficial layer includes five to eight rows of ﬂattened cells with small uniform nuclei and a cytoplasm filled with glycogen. The nucleus becomes pyknotic, and the cells detach from the surface (exfoliation). These cells form the basis for Papanicolaou (Pap) testing.
Figure 19.4 Different locations of the transformation zone and the squamocolumnar junction during a woman’s lifetime. The arrows mark the active transformation zone.
Columnar Epithelium Columnar epithelium has a single layer of columnar cells with mucus at the top and a round nucleus at the base. The glandular epithelium is composed of numerous ridges, clefts, and infoldings and, when covered by squamous metaplasia, leads to the appearance of gland openings. Technically, the endocervix is not a gland, but often the term gland openings is used.
Metaplastic Epithelium Metaplastic epithelium, found at the SCJ, begins in the subcolumnar reserve cells (Fig. 19.4). Under stimulation of lower vaginal acidity, the reserve cells proliferate, lifting the columnar epithelium. The immature metaplastic cells have large nuclei and a small amount of cytoplasm without glycogen. As the cells mature normally, they produce glycogen, eventually forming the four layers of epithelium. The metaplastic process begins at the tips of the columnar villi, which are exposed first to the acid vaginal environment. As the metaplasia replaces the columnar epithelium, the central capillary of the villus regresses, and the epithelium ﬂattens out, leaving the epithelium with its typical vascular network. As metaplasia proceeds into the cervical clefts, it replaces columnar epithelium and similarly ﬂattens the epithelium. The deeper clefts may not be completely replaced by the metaplastic epithelium, leaving mucus-secreting columnar epithelium trapped under the squamous epithelium. Some of these glands open onto the surface; others are completely encased, with mucus collecting in nabothian cysts.
CHAPTER 19 Intraepithelial Disease of the Cervix, Vagina, and Vulva
Figure 19.5 A: Active metaplasia in the transformation zone. B: Maturing metaplasia in the transformation zone.
Gland openings and nabothian cysts mark the original SCJ and the outer edge of the original transformation zone (5) (Fig. 19.5A and B).
Figure 19.6 Cervical intraepithelial neoplasia grade 1 (CIN 1) with koilocytosis. The normal maturation process and differentiation from the basal and parabasal layers to the intermediate and superficial layers are maintained. In the upper layers, koilocytes are characterized by perinuclear halos, well-defined cell borders, and nuclear hyperchromasia, irregularity, and enlargement.