The normal endocervical mucosa consists of mucus-producing tubules and clefts (mucosal infoldings, usually called glands), loosely arranged in a fibrous stroma. A single layer of tall, columnar epithelial cells covers the mucosal surface and lines the intricate folds, clefts, and tubules. The small nuclei are basally placed during the early proliferative phase. The clear cytoplasm contains abundant mucus, especially in the late proliferative phase (Fig. 14). Where the endocervical mucosa merges with the isthmic mucosa, endometrial-type glands intermingle with endocervical glands (Figs. 15,16).
Beneath the endocervical columnar epithelium a small single layer of reserve cells can often be detected (Figs. 17,18). Immunohistochemically, these reserve cells differ in their cytoskeleton from the columnar cells. Although both cell types stain positively with broad-reacting cytokeratin antibodies, reserve cells remain unstained with antibodies against cytokeratin 18 (Fig. 17), but do react positively with antibodies against KA 1, detecting the complex of cytokeratin 5 with cytokeratin 14 (Gould et al. 1990), a reaction characteristic of squamous epithelium (Fig. 18). In contrast, the columnar cells stain with antibodies against cytokeratin 18 (Fig. 17), and 8, but not with antibodies against KA 1.
Consequently, the reserve cells of the endocervical epithelium differ immunohistochemically from the columnar cells covering them, much like the basal cells of the ecto-cervix differ from the cells overlying them, but in different ways. The basal layer of the ectocervix expresses cytokeratins characteristic for single (glandular) epithelial cells, yet is covered by squamous epithelium. The reserve cells of the endocervix contain cy-tokeratins characteristic for epithelial cells with squamous differentiation and are covered by a simple, glandular epithelium. Although the reserve cells are bipotential and capable of producing either keratin or mucin, they are not essentially precursors of the columnar cells, which can themselves proliferate by mitotic activity (Hiersche and Nagl 1980).
This distinctive endowment of cytokeratins of the basal and reserve cells and their bipotential capacities to differentiate in two different directions may explain how and why both epithelia at the squamocolumnar junction respond so characteristically to regenerative and metaplastic influences initiated by the eversion of the endocervix during the reproductive years.
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