Cervix
The cervix or uterine cervix is a dynamic fibromuscular sexual organ of the female reproductive system that connects the vagina with the uterine cavity. The human female cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago. The cervix is approximately long with a diameter of approximately and tends to be described as a cylindrical shape, although the front and back walls of the cervix are contiguous. The size of the cervix changes throughout a female's life cycle. For example, females in the fertile years of their reproductive cycle tend to have larger cervixes than postmenopausal females; likewise, females who have produced offspring have a larger cervix than those who have not.
In relation to the vagina, the part of the cervix that opens into the uterus is called the internal os while the opening of the cervix into the vagina is called the external os. Between those extremes is the conduit commonly called the cervical canal. The lower part of the cervix, known as the vaginal portion of the cervix, bulges into the top of the vagina. The endocervix borders the uterus. The cervical conduit has at least two types of epithelium : the endocervical lining is glandular epithelia that lines the endocervix with a single layer of column-shaped cells; while the ectocervical part of the conduit contains squamous epithelium. Squamous epithelia line the conduit with multiple layers of cells topped with flat cells. These two linings converge at the squamocolumnar junction. This junction changes location dynamically throughout a female's life. The cervix is the organ that allows epithelia to flow from a female's uterus and out through their vagina at menstruation. Menstruation releases epithelia from a female’s uterus with every period of their fertile years, unless pregnancy occurs.
Several methods of contraception aim to prevent fertilization by blocking the conduit, including cervical caps and cervical diaphragms, preventing the passage of sperm through the cervix. Other approaches include methods that observe cervical mucus, such as the Creighton Model and Billings method. Cervical mucus's consistency changes during menstrual periods, which may signal ovulation.
During vaginal childbirth, the cervix must flatten and dilate to allow the foetus to progress along the birth canal. Midwives and doctors use the extent of cervical dilation to assist decision-making during childbirth.
Cervical infections with the human papillomavirus can cause changes in the epithelium, which can lead to cancer of the cervix. Cervical cytology tests can detect cervical cancer and its precursors to enable early, successful treatment. Ways to avoid HPV include avoiding heterosexual sex, using penile condoms, and receiving the HPV vaccination. HPV vaccines, developed in the early 21st century, reduce the risk of developing cervical cancer by preventing infections from the main cancer-causing strains of HPV.
Structure
The cervix is part of the female reproductive system. Around in length, it is the lower, narrower part of the uterus, continuous above with the broader upper part—or body—of the uterus. The lower end of the cervix bulges through the anterior wall of the vagina, and is referred to as the vaginal portion of the cervix, while the rest of the cervix above the vagina is called the supravaginal portion of cervix. A central canal, known as the cervical canal, runs along its length and connects the cavity of the body of the uterus with the lumen of the vagina. The openings are known as the internal os and external orifice of the uterus, respectively. The mucosa lining the cervical canal is known as the endocervix, and the mucosa covering the ectocervix is known as the exocervix. The cervix has an inner mucosal layer, a thick layer of smooth muscle, and posteriorly the supravaginal portion has a serosal covering consisting of connective tissue and overlying peritoneum.File:Cervix uteri, breastfeeding woman after 2 births.jpg|thumbnail|alt=A adult woman's cervix viewed through vagina using a vaginal speculum|A normal cervix of an adult viewed through vagina using a bivalved vaginal speculum. The functional squamocolumnar junction surrounds the external os and is visible as the irregular demarcation between the lighter and darker shades of pink mucosa.
In front of the upper part of the cervix lies the bladder, separated from it by cellular connective tissue known as parametrium, which also extends over the sides of the cervix. To the rear, the supravaginal cervix is covered by peritoneum, which runs onto the back of the vaginal wall and then turns upwards and onto the rectum, forming the recto-uterine pouch. The cervix is more tightly connected to surrounding structures than the rest of the uterus.
The cervical canal varies greatly in length and width between women or throughout a woman's life, and it can measure 8 mm at its widest diameter in premenopausal adults. It is wider in the middle and narrower at each end. The anterior and posterior walls of the canal each have a vertical fold, from which ridges run diagonally upwards and laterally. These are known as palmate folds, due to their resemblance to a palm leaf. The anterior and posterior ridges are arranged so that they interlock with each other and close the canal. They are often effaced after pregnancy.
The ectocervix has a convex, elliptical shape and projects into the cervix between the anterior and posterior vaginal fornices. On the rounded part of the ectocervix is a small, depressed external opening, connecting the cervix with the vagina. The size and shape of the ectocervix and the external opening can vary according to age, hormonal state, and whether childbirth has taken place. In women who have not had a vaginal delivery, the external opening is small and circular, and in women who have had a vaginal delivery, it is slit-like. On average, the ectocervix is long and wide.
Blood is supplied to the cervix by the descending branch of the uterine artery and drains into the uterine vein. The pelvic splanchnic nerves, emerging as S2–S3, transmit the sensation of pain from the cervix to the brain. These nerves travel along the uterosacral ligaments, which pass from the uterus to the anterior sacrum.
Three channels facilitate lymphatic drainage from the cervix. The anterior and lateral cervix drains to nodes along the uterine arteries, travelling along the cardinal ligaments at the base of the broad ligament to the external iliac lymph nodes and ultimately the paraaortic lymph nodes. The posterior and lateral cervix drains along the uterine arteries to the internal iliac lymph nodes and ultimately the paraaortic lymph nodes, and the posterior section of the cervix drains to the obturator and presacral lymph nodes. However, there are variations as lymphatic drainage from the cervix travels to different sets of pelvic nodes in some people. This has implications in scanning nodes for involvement in cervical cancer.
After menstruation and directly under the influence of estrogen, the cervix undergoes a series of changes in position and texture. During most of the menstrual cycle, the cervix remains firm and is positioned low and closed. However, as ovulation approaches, the cervix becomes softer and rises to open in response to the higher levels of estrogen present. These changes are also accompanied by changes in cervical mucus, described below.
Development
As a component of the female reproductive system, the cervix is derived from the two paramesonephric ducts, which develop around the sixth week of embryogenesis. During development, the outer parts of the two ducts fuse, forming a single urogenital canal that will become the vagina, cervix and uterus. The cervix grows in size at a smaller rate than the body of the uterus, so the relative size of the cervix over time decreases, decreasing from being much larger than the body of the uterus in fetal life, twice as large during childhood, and decreasing to its adult size, smaller than the uterus, after puberty. Previously, it was thought that during fetal development, the original squamous epithelium of the cervix is derived from the urogenital sinus, and the original columnar epithelium is derived from the paramesonephric duct. The point at which these two original epithelia meet is called the original squamocolumnar junction. New studies show, however, that all the cervical as well as large part of the vaginal epithelium are derived from Müllerian duct tissue and that phenotypic differences might be due to other causes.Histology
The endocervical mucosa is about thick and lined with a single layer of columnar mucous cells. It contains numerous tubular mucous glands, which empty viscous alkaline mucus into the lumen. In contrast, the ectocervix is covered with nonkeratinized stratified squamous epithelium, which resembles the squamous epithelium lining the vagina. The junction between these two types of epithelia is called the squamocolumnar junction. Underlying both types of epithelium is a tough layer of collagen. The mucosa of the endocervix is not shed during menstruation. The cervix has more fibrous tissue, including collagen and elastin, than the rest of the uterus.In prepubertal girls, the functional squamocolumnar junction is just within the cervical canal. Upon entering puberty, due to hormonal influence, and during pregnancy, the columnar epithelium extends outward over the ectocervix as the cervix everts. Hence, this also causes the squamocolumnar junction to move outwards onto the vaginal portion of the cervix, where it is exposed to the acidic vaginal environment. The exposed columnar epithelium can undergo physiological metaplasia and change to tougher metaplastic squamous epithelium in days or weeks, which is very similar to the original squamous epithelium when mature. The new squamocolumnar junction is therefore internal to the original squamocolumnar junction, and the zone of unstable epithelium between the two junctions is called the transformation zone of the cervix. Histologically, the transformation zone is generally defined as surface squamous epithelium with surface columnar epithelium or stromal glands/crypts, or both.
After menopause, the uterine structures involute, and the functional squamocolumnar junction moves into the cervical canal.
Nabothian cysts form in the transformation zone where the lining of metaplastic epithelium has replaced mucous epithelium and caused a strangulation of the outlet of some of the mucous glands. A buildup of mucus in the glands forms Nabothian cysts, usually less than about in diameter, which are considered physiological rather than pathological. Both gland openings and Nabothian cysts are helpful to identify the transformation zone.