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In male human anatomy, the foreskin is a generally retractable double-layered fold of skin and mucous membrane that covers the glans penis and protects the urinary meatus (pronounced /miːˈeɪtəs/) when the penis is not erect. It is also described as the prepuce, a technically broader term that also includes the clitoral hood in women, to which the foreskin is embryonically homologous.
The outside of the foreskin is a continuation of the skin on the shaft of the penis, but the inner foreskin is a mucous membrane like the inside of the eyelid or the mouth. The mucocutaneous zone occurs where the outer and inner foreskin meet. Like the eyelid, the foreskin is free to move after it separates from the glans, usually by puberty. Smooth muscle fibres keep it close to the glans but make it highly elastic. The foreskin is attached to the glans with a frenulum, which helps return the foreskin over the glans. At the end of foreskin, there is a band of tissue called the ridged band, which, according to one study, is rich in nerve endings called Meissner's corpuscles. According to a study by Sorrells et al., the five most sensitive areas of the penis are on the foreskin.
In children, the foreskin covers the glans completely, but, in adults, this need not be so. Schöberlein  found that about 50% of young men had full coverage of the glans, 42% had partial coverage, and, in the remaining 8%, the glans was uncovered. After adjusting for circumcision, he stated that, in 4% of the young men, the foreskin had spontaneously atrophied (shrunk). There is considerable variation in the degree to which the foreskin retracts during erection; in some adults the foreskin remains covering the glans until retracted by sexual activity.
Eight weeks after fertilization, the foreskin begins to grow over the head of the penis, covering it completely by 16 weeks. At this stage, the foreskin and glans share an epithelium (mucous layer) that fuses the two together. It remains this way until the foreskin separates from the glans.
At birth, the foreskin is usually still fused with the glans. As childhood progresses the foreskin and the glans gradually separate, a process that may not be complete until late puberty. Thorvaldsen and Meyhoff reported that average age of first foreskin retraction in Denmark is 10.4 years. Wright argues that forcible retraction of the foreskin should be avoided and that the child himself should be the first one to retract his own foreskin. Premature retraction may be painful, and may result in infection.
The World Health Organization state that there is "debate about the role of the foreskin, with possible functions including keeping the glans moist, protecting the developing penis in utero, or to enhance sexual pleasure due to the presence of nerve receptors".
Taylor et al. described the foreskin in detail, documenting a ridged band of mucosal tissue. They stated "This ridged band contains more Meissner's corpuscles than does the smooth mucosa and exhibits features of specialized sensory mucosa." In 1999, Cold and Taylor stated "The prepuce is primary, erogenous tissue necessary for normal sexual function." Boyle et al., state that "The complex innervation of the foreskin and frenulum has been well-documented, and the genitally intact male has thousands of fine touch receptors and other highly erogenous nerve endings—many of which are lost to circumcision, with an inevitable reduction in sexual sensation experienced by circumcised males." The AAP noted that the work of Taylor et al. "suggests that there may be a concentration of specialized sensory cells in specific ridged areas of the foreskin."
Moses and Bailey (1998), describe the evidence of sensory function as "indirect," and state that, "aside from anecdotal reports, it has not been demonstrated that this is associated with increased male sexual pleasure." The World Health Organization states that there is little evidence for diminished sexual function, adding that studies have been inconsistent. Fink et al. reported "[a]lthough many have speculated about the effect of a foreskin on sexual function, the current state of knowledge is based on anecdote rather than scientific evidence." Masood et al. state that "[c]urrently no consensus exists about the role of the foreskin or the effect circumcision has on penile sensitivity and overall sexual satisfaction." Schoen states that "[a]necdotally, some have claimed that the foreskin is important for normal sexual activity and improves sexual sensitivity. Objective published studies over the past decade have shown no substantial difference in sexual function between circumcised and uncircumcised men."
The term 'gliding action' is used in some papers to describe the way the foreskin moves during sexual intercourse. This mechanism was described by Lakshamanan & Prakash in 1980, stating that "[t]he outer layer of the prepuce in common with the skin of the shaft of the penis glides freely in a to and fro fashion..." Several opponents of circumcision have argued that the gliding movement of the foreskin is important during sexual intercourse. Warren & Bigelow claim that gliding action would help to reduce the effects of vaginal dryness and that restoration of the gliding action is an important advantage of foreskin restoration. O'Hara describes the gliding action, stating that it reduces friction during sexual intercourse, and suggesting that it adds "immeasurably to the comfort and pleasure of both parties". Taylor suggests that the gliding action, where it occurs, may stimulate the nerves of the ridged band, and speculates that the stretching of the frenulum by the rearward gliding action during penetration triggers ejaculation.
Whiddon (1953) and Foley (1966) also believed that the presence of the foreskin made sexual penetration easier.
Gairdner (1949) states that the foreskin protects the glans but some studies show that inflammation of the glans is more common when the foreskin is present.
The fold of the prepuce maintains sub-preputial wetness, which mixes with exfoliated skin to form smegma. Some authors believe that smegma contains antibacterial enzymes, though their theory has been challenged. The American Academy of Pediatrics state that "No controlled scientific data are available regarding differing immune function in a penis with or without a foreskin." Inferior hygiene has been associated with balanitis, though excessive washing can cause non-specific dermatitis.
Frenulum breve is where the frenulum is insufficiently long to allow the foreskin to fully retract, which may lead to discomfort during intercourse. The frenulum may also tear during intercourse. Phimosis is a condition when the foreskin of an adult cannot be retracted properly. (Before adulthood, the foreskin may still be separating from the glans.) Phimosis can be treated by gently stretching the foreskin, by changing masturbation habits, using topical steroid ointments, preputioplasty, or by circumcision.
Non-retractable foreskin is not always indicative of a clinical condition. The first time the foreskin retracts (say during masturbation or sex) can be a slightly painful experience, as the glans penis is not used to friction and air contact. However the problem resolves itself after a few days of acclimatization, which may include consciously retracting the foreskin and exposing it to air, water, or friction. A condition called paraphimosis may occur if a tight foreskin becomes trapped behind the glans and swells as a restrictive ring. This can cut off the blood supply, resulting in ischaemia of the glans penis.
Korean doctors claim that South Korean men have a gene that causes “abundant foreskin,” believing that at least 90% of Korean men have “too much” foreskin. 
Although there is no scientific evidence of this alleged genotype and it was found that most Korean physicians could not define phimosis, almost all of South Korean physicians recommend universal circumcision because they believe it eliminates tight foreskins and brings many benefits. 
Aposthia is a rare condition in which the foreskin is not present at birth.
Surgical and other modifications of the foreskin
Circumcision is the removal of the foreskin, either partially or completely. It may be done for religious, aesthetic, health, or hygiene reasons, or to treat disease.
Preputioplasty is a procedure to relieve a tight foreskin without resorting to circumcision.
Other practices include genital piercings involving the foreskin and slitting the foreskin.
Langerhans cells are immature dendritic cells that are found in all areas of the penile epithelium, but are most superficial in the inner surface of the foreskin. The recent Szabo and Short (2000) study targets Langerhans cells as receptors of HIV, and states that these cells "must be regarded as the most probable sites for viral entry in primary HIV infection in men." Langerhans cells are also known to express the c-type lectin langerin, which may play a role in transmission of HIV to nearby lymph nodes. However, de Witte et al. (2007) reported that langerin, produced by Langerhans cells, blocks the transmission of HIV to T cells. However, overall transmission rates of HIV are higher with a foreskin than without a foreskin.
A penis and foreskin flaccid (left) and semi-erect (right) with part of the glans penis uncovered
foreskin covering over glans
glans penis almost covered by foreskin; external urethral orifice left open
A penis with a short foreskin, with part of the glans uncovered
A short foreskin (partial circumcision) retracted to expose the glans penis
An erect penis with short foreskin fully retracted and the glans penis fully uncovered.
Foreskin in non-human species
In koalas, the foreskin contains naturally-occurring bacteria that play an important role in fertilization. Almost all mammal penises have foreskins, although in non-human cases the foreskin is usually a sheath into which the whole penis is retracted. Only monotremes (the platypus and the echidna) lack foreskins.
Foreskin-based medical and consumer products
Foreskins obtained from circumcision procedures are frequently used by biochemical and micro-anatomical researchers to study the structure and proteins of human skin. In particular, foreskins obtained from newborns have been found to be useful in the manufacturing of more human skin.
Foreskins from circumcised babies are used to make a commercial anti-wrinkle skin cream. A six-week supply costs US$130.
Foreskins of babies are also used for skin graft tissue, and for β-interferon-based drugs.
Foreskin fibroblasts have been used in biomedical research.
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- ^ Sorrels, Morris; James L. Snyder, Mark D. Reiss, Christopher Eden, Marilyn F. Milos, Norma Wilcox and Robert S. Van Howe. (2007). "Fine-touch pressure thresholds in the adult penis" (PDF). Bjuinternational 99: 864–869. http://www.doctorsopposingcircumcision.org/pdf/sorrells_2007.pdf.
- ^ Schöberlein circumcision taboos. Phimosis frenulum and foreskin conditions, phimosis and male initiation
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- ^ Øster, J (1968). "Further fate of the foreskin: incidence of preputial adhesions, phimosis, and smegma among Danish schoolboys". Arch Dis Child 43 (228): 200–3. doi:10.1136/adc.43.228.200. PMID 5689532. PMC 2019851. http://www.cirp.org/library/general/oster/.
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- ^ Wright JE (February 1994). "Further to "the further fate of the foreskin". Update on the natural history of the foreskin". Med. J. Aust. 160 (3): 134–5. PMID 8295581. http://www.cirp.org/library/normal/wright2/.
- ^ "Male circumcision: Global trends and determinants of prevalence, safety and acceptability" (PDF). World Health Organization. pp. 18. http://whqlibdoc.who.int/publications/2007/9789241596169_eng.pdf.
- ^ Taylor, JR; Lockwood, AP; Taylor, AJ (1996). "The prepuce: specialized mucosa of the penis and its loss to circumcision". Br J Urol 77 (2): 291–5. doi:10.1046/j.1464-410X.1996.85023.x. PMID 8800902. http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1464-410X.1996.85023.x.
- ^ Boyle, G; Goldman, R; Svoboda, J; Fernandez E (2002). "Male Circumcision: Pain, Trauma and Psychosexual Sequelae". Journal of Health Psychology 7: 329–343. doi:10.1177/1359105302007003225. http://www.cirp.org/library/psych/boyle6/.
- ^ "American Academy of Pediatrics: Circumcision Policy Statement". Pediatrics 103 (3): 686–693. March 1999. doi:10.1542/peds.103.3.686. PMID 10049981. http://aappolicy.aappublications.org/cgi/content/full/pediatrics%3b103/3/686.
- ^ Moses S; Bailey RC, Ronald AR (1998). "Male circumcision: assessment of health benefits and risks". Sexually Transmitted Infections Vol 74 (Issue 5): 368–373. doi:10.1136/sti.74.5.368 (inactive 2010-01-08). http://sti.bmj.com/cgi/reprint/74/5/368. Retrieved 2007-04-28. "There is indirect evidence suggesting that the foreskin may have an important sensory function, although aside from anecdotal reports, it has not been demonstrated that this is associated with increased male sexual pleasure.".
- ^ "Male circumcision: Global trends and determinants of prevalence, safety and acceptability" (PDF). World Health Organization. pp. 22. http://www.who.int/entity/hiv/topics/malecircumcision/JC1320_MaleCircumcision_Final_UNAIDS.pdf. "Although it has been argued that sexual function may diminish following circumcision due to the removal of the nerve endings in the foreskin and subsequent thickening of the epithelia of the glans, there is little evidence for this and studies are inconsistent."
- ^ Fink KS, Carson CC, DeVellis RF (May 2002). "Adult circumcision outcomes study: effect on erectile function, penile sensitivity, sexual activity and satisfaction". J. Urol. 167 (5): 2113–6. doi:10.1016/S0022-5347(05)65098-7. PMID 11956453. http://linkinghub.elsevier.com/retrieve/pii/S0022-5347(05)65098-7.
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- ^ Kigozi G, Watya S, Polis CB, et al. (January 2008). "The effect of male circumcision on sexual satisfaction and function, results from a randomized trial of male circumcision for human immunodeficiency virus prevention, Rakai, Uganda". BJU Int. 101 (1): 65–70. doi:10.1111/j.1464-410X.2007.07369.x (inactive 2008-06-26). PMID 18086100. "Opponents of circumcision, using results from selected observational studies, have argued that the procedure impairs sexual function, and reduces sexual pleasure and satisfaction through keratinization of the glans, removal of the most sensitive preputial tissues, and loss of the 'gliding' mechanism provided by the foreskin".
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Friction is not entirely eliminated during natural intercourse but it is largely eliminated. Friction can take place in the lower vagina, but only if the man uses a stroke that exceeds the (forward and backward) gliding range of the shaft's extra skin. And in such a case, there will be friction only to the extent that the shaft exceeded its extra skin, which is uncommon since the natural penis has a propensity for short strokes. Primarily, it is the penis head that makes frictional contact with the vaginal walls, usually in the upper vagina where there is ample lubrication. [...] The gliding principle of natural intercourse is a two-way street—the vagina glides on the shaft skin while the shaft skin massages the penis shaft as it glides over it."
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