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The vagina connects the cervix to the outside. The wall of the vagina consists of, from inside to outside, stratified squamous epithelium, folded mucosa without glands, fibrous lamina propria, thin muscularis composed of inner circular and outer longitudinal smooth muscle layers, and serosa (adventitia). Characteristics of the stratified squamous epithelium, in particular its number of layers and differentiation pattern, change in response to hormone levels, which vary in the estrous cycle stages. In proestrus the stratified squamous epithelium thickens and is not yet fully cornified (for micrographs of the vagina in proestrus at several magnifications, see Vagina). In estrus the stratified squamous epithelium is thick and fully cornified; superficial epithelial cells detach into the lumen. In metestrus (metaestrus) the stratified squamous epithelium is regressed, having detached cornified cells, while leukocytes are in the stroma and infiltrate the epithelium. In diestrus the epithelium is thin and mucified; many leukocytes are in stroma and migrate via the epithelium to the lumen.
The micrograph of the vagina in proestrus shows a relatively thin layer of stratified squamous epithelium with cornification in a few places. The micrograph of the vagina in estrus displays a much thicker and fully cornified stratified squamous epithelium. The micrograph of the vagina in metestrus presents a regressed stratified squamous epithelium as well as infiltrating leukocytes. In the micrograph of the vagina in diestrus, the epithelium is mucified columnar and the lumen contains mucus and desquamated cells.
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