The cervix contains two kinds of cells: rectangular columnar cells and flat, scale-like squamous cells. Columnar cells are constantly changing into squamous cells in an area of the cervix called the transformation (transitional) zone.
The transformation zone is an area of changing cells and it is the most common place on the cervix for abnormal cells to develop. These abnormal cells can be detected on a Pap smear.
The location of the transformation zone varies among women. In teenage girls, the transformation zone is on the immature cervix's outer surface and is more susceptible to infection than in adult women. In older women, the transformation zone may be higher in the cervical canal.
WebMD Medical Reference from Healthwise