Before we get into the details of the cells of the mammalian reproductive cycle, let's review what our instrument, the Pap smear, is.
The Pap smear is a diagnostic test named after George Papanicolaou, a Greek physician and zoologist who developed the test in the early 20th century. Although nowadays the test is best known as a screening tool for cervical cancer, its origins actually lie in Papanicolaou's observation of regular vaginal cytological changes over the course of the estrous cycle of the guinea pig. He repeated his observations in a variety of species, for which the pattern of changes was validated, and decided to apply it to humans, using, among others, his wife (who was obviously a very good sport!) as a subject. He was able to demonstrate that the varying proportions of the different types of vaginal epithelial cells in smears on the slides were a reliable indicator of the reproductive stage the subject was in at the time the slide was taken. This proportion came to be known as the maturation index (MI), and is still used as an indicator of reproductive stage in humans and other species.
The technique for the Pap smear is essentially the same one Papanicolaou used in his studies on guinea pigs. Its use as a cancer diagnostic was actually a by-product of this original goal of investigating reproductive cycles. When Papanicolaou began testing his technique on humans, one of the women involved in the tests happened to have cervical cancer, and his observations of her particular cytology in the vaginal smear led to its use as a cancer-screening tool.