The Biology of the Cervix
The cells on the surface of the cervix facing the vagina (the ectocervix) are, by and large, squamous cells. The cells on the inside of the cervix, that line the passage from the vagina to the uterus (the endocervix), are made up a different type of cell. These are called columnar epithelial cells. The space in between where these two cell types are clearly established is known as the transformation zone. The transformation zone is the area of the cervix where the single layer of columnar cells is being replaced by multiple layers of squamous cells. The vast majority of cervical cancers start in the transformation zone. This is because the transformation zone is the area where the squamous cells that HPV prefers to infect are most vulnerable.
Cervical ectopy is how doctors describe the condition when columnar cells from the endocervix are present on the ectocervix, and thus more susceptible to infection. In particular, columnar cells are more likely to be infected by chlamydia, gonorrhea, and certain forms of HPV (most HPV strains preferentially infect squamous cells). Some degree of ectopy is normal during puberty, but the amount of ectopy usually decreases over time as a natural consequence of aging.
Why Younger Women Are At Increased Risk of Infection
Because younger post-pubertal women have more ectopy than older women, their cervixes are more susceptible to infection by numerous STDs. They also have larger transformation zones, which results in an increased risk of abnormal pap smears and cervical cancer.
Although other factors, such as pregnancy and oral contraceptive use, can also affect the degree of cervical ectopy and the size of the transformation zone, younger women are at increased risk of numerous sexually transmitted infections simply because of their age.