Cervix

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Cervix
Scheme female reproductive system-en.svg
Schematic frontal view of female anatomy
Female reproductive system lateral nolabel.png
1: Fallopian tube, 2: bladder, 3: pubic bone, 4: vaginaG-Spot, 5: clitoris, 6: urethra, 7: vagina, 8: ovary, 9: sigmoid colon, 10: uterus, 11: fornix, 12: cervix, 13: rectum, 14: anus
Latin Cervix uteri
Gray's p.1259
Artery Vaginal artery and uterine artery
Precursor Müllerian duct
MeSH Cervix+uteri
Dorlands
/Elsevier
cervix uteri

The cervix (Latin: neck) is one of the parts of the female reproductive system that lies between the uterus and vagina. The cervix has a central canal and an internal and external opening, and is between two and three centimetres long. The ectocervix refers to the outer part of the cervix, and has an epithelia that changes from a layer of column-type cells to multiple layers of flat cells over time. Because of this change, known as metaplasia, this part of the cervix is at increased risk of cancer.

The cervix is also important in fertility and childbirth, with the cervical mucous used in several methods of fertility awareness, such as the Creighton Model and Billings Method, due to its changes in consistency throughout the menstrual period. The size of the cervix is also used to divide labour into stages. The shape of the cervix can change, particularly after normal childbirth, changing from a slit to a more rounded opening.

The cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago.

Structure[edit]

Diagram of the uterus, cervix and vagina, with the external os and internal os of the uterus labelled.
The uterus, cervix and vagina, with the external os and internal os of the uterus labelled.

The cervix is one of the organs in the female reproductive system between the vagina and uterus. The cervix has a central canal, which has an external opening to the vagina, and an internal opening to the uterus. These structures are also known as the internal and externa os. The part of the cervix outside of the external opening in the vagina is called the ectocervix, and the part within the vagina known as the endocervix.[1] The cervix is about 2–3 cm long.[2]

The cervical canal varies greatly in length and width,[2] and can measure 7–8 mm at its widest diameter in pre-menopausal adults. The ectocervix has a convex, elliptical surface and is divided into anterior and posterior labia (lip–shaped structures). The size and shape of the external opening and the ectocervix can vary according to age, hormonal state, and whether natural, or normal childbirth has taken place. Where no natural childbirth has taken place, the external orifice appears as a small, circular opening of about 8mm. On average, the ectocervix is 3 cm long and 2.5 cm wide.[2] In women who have given birth naturally (vaginal childbirth), the ectocervix appears bulkier and the external os appears a lot wider.

Position[edit]

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, with approaching ovulation, the cervix becomes softer, and rises to open, in response to the higher levels of oestrogen present.[3] These changes are accompanied by the production of cervical mucus, supportive of the survival and movement of sperm.

Blood supply[edit]

The cervix is supplied by the descending cervical branch of the uterine artery.[4]

Lymphatic drainage[edit]

The lymphatic drainage of the cervix is along the uterine arteries and cardinal ligaments to the parametrial, external iliac vein, internal iliac vein, and obturator and presacral lymph nodes.[2] From these pelvic lymph nodes, drainage proceeds to the paraaortic lymph nodes. In some instances, the lymphatics drain directly to the paraaortic nodes.

Innervation[edit]

Nerves supplying the cervix pass through uterosacral ligaments.[4]

Development[edit]

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.[5]

Histology[edit]

The epithelium of the cervix varies. The transformation zone, also referred to as the squamocolumnar junction is adjacent to the borders of the ectocervix and the endocervix of the canal; and refers to the area where the change occurs between the squamous epithelium lining the ectocervix to the columnar epithelium that lines the endocervix.[4] The squamous epithelium of the ectocervix is without keratin, and is continuous with the adjacent vagina. Underlying both types of epithelium is a tough layer of collagen.[6] The transformation zone undergoes physiological changes at different times. At puberty, the columnar epithelium extends outwards, and cover portions of the ectocervix. The cervix also undergoes changes related to the menstrual cycle and pregnancy[clarification needed]. Additionally, the transformation zone may retreat post-menopause, such that the ectocervix is again covered with stratified squamous epithelium. Also, when the endocervix is exposed to the harsh acidic environment of the vagina it undergoes metaplasia to squamous epithelium.[7]

Function[edit]

The cervix is part of the female reproductive system, lying between the vagina and the uterus.

Dilation[edit]

During menstruation, the cervix stretches open slightly to allow the endometrium to be shed. This stretching is believed to be part of the cramping pain that many women experience. Evidence for this is given by the fact that some women's cramps subside or disappear after their first vaginal birth because the cervical opening has widened.

Cervical mucus[edit]

A mucous plug
Mucus plug

After a menstrual period ends, the external os is blocked by mucus that is thick and acidic. This "infertile" mucus blocks spermatozoa from entering the uterus.[8] For several days around the time of ovulation, "fertile" types of mucus are produced; they have a higher water content, and are less acidic and higher in electrolytes. These electrolytes cause the 'ferning' pattern that can be observed in drying mucus under low magnification; as the mucus dries, the salts crystallize, resembling the leaves of a fern.[3]

Cervical mucus is produced by glands in the endocervix and is composed 90% of water. Depending on the water content which varies during the menstrual cycle, the mucus functions as a barrier or a transport medium to spermatozoa: during the proliferative phase, the mucus is thin and serous to allow sperm to enter the uterus while during the secretory phase, the mucus is thick to prevent sperm from interfering with the already fertilized egg. Thick mucus also prevents pathogens from interfering with a nascent pregnancy. Cervical mucus also contains electrolytes (calcium, sodium and potassium), organic components such as glucose, amino acids and soluble proteins.[9] Cervical mucus may contain trace elements including zinc, copper, iron, mangenese and selenium, the levels of which vary dependant on cyclical hormone variation during different phases of the menstrual cycle.[10][11] Various enzymes have been identified in human cervical mucus. Glycerol is a natural ingredient of human cervical fluid.[12][13][14][15] Studies have shown that the amount of glycerol in cervical fluid increases during sexual excitement.[13]

Some methods of fertility awareness such as the Creighton Model and the Billings Method involve estimating a woman's periods of fertility and infertility by observing changes in her body. Among these changes are several involving the quality of her cervical mucus: the sensation it causes at the vulva, its elasticity (Spinnbarkeit), its transparency, and the presence of ferning.[3]

Most methods of hormonal contraception work primarily by preventing ovulation, but their effectiveness is increased because they prevent the fertile types of cervical mucus from being produced. Conversely, methods of thinning the mucus may help to achieve pregnancy. One suggested method is to take guaifenesin in the few days before ovulation.[3]

During pregnancy, a cervical mucus plug, forms to provide a protective seal for the uterus against the entry of pathogens and against leakage of uterine fluids. The nature of this mucus has antibacterial properties. This plug comes out as the cervix dilates, either during labour or shortly before.

Labour[edit]

Cervix dilation sequence in labour
Cervix dilation sequence in labour

A main function of the cervix is its role in childbirth. As the foetus descends in preparation for birth, the presenting part, usually the head, rests on and is supported by the cervix. This support begins to give way as the cervix starts to dilate when the uterus begins its contractions. Any other presentation is termed a malpresentation which is also termed a breech birth. During childbirth, the cervix must dilate to a diameter of more than 10 cm to accommodate the head of the fetus as it is pushed from the uterus to the vagina. In becoming wider, the cervix also becomes shorter, known as effacement. When this happens an area between the cervix and the vagina becomes exposed and is known as the birth canal.[16]

Along with other factors, cervical dilation is used to divide delivery into stages. Generally, the active first stage of labour is defined with a cervical dilation of more than 3–4 cm,[17][18] with the second phase of labor defined when the cervix is dilated to more than 10 cm, regarded as its fullest dilation.[16]

Cervix as reservoir[edit]

The cervix acts as a reservoir of sperms after coitus.[4] This one statement of fact counters a weakly founded theory put forward that the convulsing cervix, during orgasm, draws semen into the uterus. This upsuck theory has generally accepted for some years, although there does not appear to be any evidence.[19] In 2005, science historian Elisabeth Lloyd queried the logic of this theory and the methodology used to support it. Her findings were that over 70% of the data came from one woman.[20]

Clinical significance[edit]

alt = The cervix viewed from the vagina side through a Vaginal speculum

Cervical incompetence is a condition in which there is shortening of the cervix due to dilation and thinning, before term pregnancy, and is the strongest predictor of preterm birth.[21][22][23][24] Some treatments to prevent the spread of cervical cancer, such as LEEP, cold-knife conization, or cryotherapy may shorten the cervix.

Cervical cancer nearly always implicates papillomavirus infection. Vaccines against this virus, greatly reduce the chances of developing cervical cancer, especially if administered before a female becomes sexually active. Potentially pre-cancerous changes in the cervix can be detected by a Pap test, (a cervical smear) in which epithelial cells are scraped from the surface of the cervix and examined under a microscope. Most cervical cancer is detected in this way, and without any other symptoms. When symptoms occur, they may include vaginal bleeding, discharge, or discomfort.[25] With appropriate treatment of detected abnormalities, cervical cancer can be prevented. Most women who develop cervical cancer have never had a Pap test, or have not had one within the last ten years. Worldwide, cervical cancer is the fifth most deadly cancer in women.[26] It affects about 16 per 100,000 women per year and kills about 9 per 100,000 per year.[27] Pap test screening has greatly reduced cervical cancer incidence and mortality in nations with regular screening programs.

Metaplasia, which can take place in the transformation zone, can cause the glands underlying the columnar epithelia to become blocked, leading to the formation of Nabothian cysts.[3] So that metaplasia increases the risk of cancer forming in the transformation zone, which is the most common area for cervical cancer to occur.[7]

Labour difficulties can necessitate varying techniques of intervention. Reporting systems such as the Bishop score, (also known as a cervix score), used to recommend interventions, also incorporate cervical dilation and effacement as a factor. The time taken for the cervix to dilate and efface in labour may affect a decision to intervene, such as a forceps delivery, induction, or Caesarean section (C-section). A history of past vaginal deliveries, parity, is a strong factor in influencing how rapidly the cervix ought to be able to dilate in labour.[16]

History[edit]

Cervix comes from Latin: Cervix (the neck) from the Proto-Indo-European root "ker-", referring to a "structure that projects". Thus the word cervix is linguistically related to the English word "horn", "head" (Sanskrit: sar), "head" (Greek: koryphe), and "deer" (Welsh: carw).[28][29]

The cervix was documented in anatomical literature in at least the time of Hippocrates, although there was some variation in early writers, who used the term to refer to both the cervix and the internal uterine orifice.[30] The first attested use of the word to refer to the cervix of the uterus was in 1702.[28]

The colposcope, used in a colposcopy to visualise the cervix, was invented in 1925. The Pap test, (previously known as a Pap smear) was developed by Georgios Papanikolaou in 1928.[31] An alternate approach, a LEEP procedure using a heated loop of platinum to excise a patch of cervical tissue, was independently developed by Aurel Babes in 1927.[32]

Cervical cancer has been described for over 2,000 years, with descriptions provided by both Hippocrates and Aretaeus,[31] although the causal role played by human papillomavirus (HPV), in the majority of cases was only elucidated by Harald zur Hausen, who published a hypothesis in 1976, and whose hypothesis was confirmed in 1983 and 1984.[33] Based on work done by Jian Zhou and Ian Fraser, a vaccine for four strains of HPV was released in 2006.[34]

References[edit]

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