The cervix forms a distinct part of the uterus, approximately 3.0 to 3.5 cm long in the adult premenopausal woman. The cervix is open at both ends, the openings termed the external os and internal os. The ectocervix is that portion of the cervix that extends out from the external cervical os to the vaginal fornix. The endocervix extends from the external os to the internal os. The internal os is where the cervical epithelium meets the uterine endometrium (Lichtman).

The cervical epithelium consists of squamous and columnar epithelial cells. The smooth squamous epithelium covers most of the ectocervix. The superficial layer of squamous epithelium varies in thickness depending on the amount of estrogen. The columnar epithelium is darker red and more granular in appearance. It lines the endocervical canal and the endometrium. Where the two types of epithelium meet is termed the squamocolumnar junction (SCJ). This junction may be found anywhere in the ectocervix or endocervix.

The SCJ may vary because of hormonal influences as with pregnancy or with sexual activity. Epidermalization may occur, which is the upward growth of squamous cells replacing columnar cells. Squamous metaplasia is the differentiation of columnar cells into squamous cells. Technically, when the SCJ has been altered, the area between the original and new SCJ is termed the transformation zone (TZ).

Columnar epithelium is sometimes visible on the ectocervix. The appearance is granular and red and is often seen in women who are pregnant or take oral contraceptives. This is termed eversion, ectropion, and sometimes rather inaccurately as erosion. Technically, erosion refers to the pathologic process that damages tissue, usually as a result of trauma.

Embryologic development of the cervix begins with the development of the mullerian or paramesonephric system. The female genital tract develops from the paramesonephric ducts. The paired paramesonephric ducts develop lateral to the mesonephric or wolffian ducts. The fallopian tubes develop from the more cephalad portions of the ducts. The caudal portions are the fused uterovaginal primordium and become the epithelium and glands of the uterus and cervix. (The endometrium develops from the adjacent mesenchyme.)

The mullerian ducts fuse and meet the urogenital sinus at the mullerian tubercle. The wolffian ducts enter the urogenital sinus lateral to the tubercle. The sinovaginal bulb forms as an evagination of endoderm from the urogenital sinus. The bulb develops into the vaginal plate and vaginal cord. The plate advances in a caudal-cranial direction, obliterating the fused mullerian ducts.

The cervix develops from a constriction between the developing uterine corpus and cervix at approximately 11 weeks gestation. The cervix forms from a fusiform thickening of the surrounding mesenchyme. The vaginal lumen also develops at about 11 weeks gestation in a caudal-cranial direction.

The embryologic development of the epithelium is somewhat less clear. The original mullerian epithelium of the vaginal canal is replaced by pseudostratified epithelium just before the vaginal plate proliferates. The pseudostratified epithelium is also of mullerian origin. When the vagina canalizes, the vagina is lined with stratified squamous epithelium of uncertain origin. Hypotheses differ on whether the vaginal lining derives from urogenital sinus epithelium or mullerian epithelum, or even from wolffian origin. The source of endocervical mucosa is also controversial. Some experts propose that the endocervical lining derives from the upward growth of vaginal squamous epithelium of sinus origin with later columnar transformation, while others suggest mullerian origin (Mishell, Ryan).

The vagina forms from the mullerian tubercles and the urogenital sinus. Paired outgrowths of endoderm from the urogenital sinus form the sinovaginal bulbs. These proliferations of cells develop into a cord and vaginal plate. The point where the sinovaginal bulb connects with the urogenital sinus is the hymen, and this is the place where canalization begins. Canalization proceeds cranially to the developing cervix (already canalized) and is complete by about the 21st week (Mishell).