The squamocolumnar junction is the area in the cervix where the squamous epithelial cells in the vagina give way to the columnar epithelial cells that are found in the uterus, according to the International Agency for Research on Cancer. The exact location of the squamocolumnar junction depends on the age of the woman, whether or not she is menstruating, pregnant, has given birth or is taking oral contraception.
The squamocolumnar junction is a place in the female reproductive anatomy where abnormal cells are likely to exist, says the IARC. Specifically, the human papillomavirus causes changes in the epithelial cells, which can lead to cervical cancer.
During childhood and into a woman's early reproductive years, the squamocolumnar junction is closed to the external os, says the IARC. The external os is a small opening in the place where the vagina meets the cervix. As the girl grows up, estrogen causes the cervix to grow larger and the birth canal to grow longer, and the squamocolumnar junction grows farther away from the external os. During the woman's reproductive years, the type of cells in the area of the cervix and the vagina are constantly changing from columnar epithelium to squamous epithelium. This changes the location of the squamocolumnar junction. By the time the woman enters menopause, it is near the external os again.