Somewhere on the cervix the two covering cell types, squamous and columnar, meet at a place called the squamo-columnar junction. This junction may be on the outside of the cervix or it may be within the cervical canal where it is difficult to see. The squamo-columnar junction is also called the transformation zone because the tall columnar cells are constantly being transformed into flat squamous cells. When a Pap smear is taken some of these cells are scraped off and examined under a microscope. It is in this transformation zone that abnormal growth, or dysplasia, develops.
Dysplasia means disordered growth.
It is easier to understand dysplasia if we first examine the normal cervix. When we look at the lining of the normal cervix under a microscope we see layers of cells. The normal distribution is that the bottom layer is made of round young cells. As the cells mature they rise to the surface and flatten out, so that on the surface the cells are flat. In dysplasia there is a lack of this organized growth process.