The bottom section of the uterus that opens into the vagina is known as the cervix. The cervix extends roughly 4 inches (10 centimeters) during labor to allow the infant to pass from the uterus via the vaginal opening. The cervix protects the uterus from bacterial infiltration through lying at the base of it. It’s where an essential epithelial transition originates. A simple columnar epithelium lines the upper cervix (endocervix), which contains mucous-secreting cells. The lower cervix (ectocervix), on the other hand, is lined with a stratified squamous epithelium. The external os is the transition point between these two epithelia. The underlying layers of the cervix, rather than smooth muscle fibers, are predominantly made up of collagenous and elastic connective tissue.

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University of Oklahoma Health Sciences Center. (n.d.). Female reproductive system. Retrieved from https://www.ouhsc.edu/histology/text%20sections/female%20reprodcutive.html