<p>(a). The simple columnar epithelium of the endocervix: (Left) as mid-cycle progresses into luteal phase, a small number of virions are able to enter the cervical canal to associate with the simple columnar epithelium of the endocervix. (Middle) During the luteal phase and/or with the use of progesterone contraceptives (e.g., DMPA), virions are more readily able to enter the cervical canal to interact with the simple columnar epithelium of the endocervix. (Right) During the follicular phase, virions rarely enter the cervical canal to interact with the endocervical simple columnar. Tissue layers are labeled as follows: C = simple columnar, LP = lamina propria. (b). The stratified squamous epithelium of the ectocervix and vagina: (Left) At the midcycle phase, the squamous epithelium is thickest and virions may penetrate the non-viable stratum corneum. However, target cells are primarily located below the epidermis, in the lamina propria; therefore, the likelihood of virus interacting with a target cell is minimal. (Middle) Although few virions are visualized penetrating the stratified squamous epithelium due to the lack of stratum corneum during the luteal phase (or with exogenous progesterone treatments; e.g., DMPA), there is a greater propensity for virus to interact with an infiltrating intraepithelial target cell near the luminal surface. (Right) Like the mid-cycle phase, the squamous epithelium during the follicular phase is thick, and virions are associated with the non-viable stratum corneum. However, target cells are primarily located below the epidermis, in the lamina propria; therefore, the likelihood of virus interacting with a target cell is minimal. Tissue layers are labeled as follows: SC = stratum corneum, G = granulosum, S = spinosum, and B = basal layer, LP = lamina propria.</p
