Foreskin

In male human anatomy, the foreskin is a double-layered fold of smooth muscle tissue, blood vessels, neurons, skin, and mucous membrane that covers the glans penis and protects the urinary meatus 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 highly innervated mucocutaneous zone of the penis occurs near the tip of the foreskin. The foreskin is mobile, fairly stretchable, and acts as a natural lubricant.

The foreskin is typically retractable over the glans. Coverage of the glans in a flaccid and erect state varies depending on foreskin length. The foreskin is attached to the glans at birth and is generally not retractable in infancy. The age at which a boy can retract his foreskin varies, but research found that 95% of males were able to fully retract their foreskin by adulthood. Inability to retract the foreskin in childhood should not be considered a problem unless there are other symptoms.

The World Health Organization debates the precise functions of the foreskin, which may include "keeping the glans moist, protecting the developing penis in utero, or enhancing sexual pleasure due to the presence of nerve receptors".

The foreskin may become subject to a number of pathological conditions. Most conditions are rare, and easily treated. In some cases, particularly with chronic conditions, treatment may include circumcision, a procedure where the foreskin is partially or completely removed.

Description
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. The ridged band of highly innervated tissue is located just inside the tip of the foreskin. Like the eyelid, the foreskin is free to move after it separates from the glans, which usually occurs before or during puberty. The foreskin is attached to the glans by a frenulum.

Taylor et al. (1996) reported the presence of Krause end-bulbs and a type of nerve ending called Meissner's corpuscles. Their density is reportedly greater in the ridged band (a region of ridged mucosa at the tip of the foreskin) than in the larger area of smooth mucosa. They are affected by age: their incidence decreases after adolescence. Meissner's corpuscles could not be identified in all individuals. Bhat et al studied Meissner's corpuscles at a number of different sites, including the "finger tips, palm, front of forearm, sole, lips, prepuce of penis, dorsum of hand and dorsum of foot". They found the lowest Meissner's Index (density) in the foreskin, and also reported that corpuscles at this site were physically smaller. Differences in shape were also noted. They concluded that these characteristics were found in "less sensitive areas of the body". In the late 1950s, Winkelmann suggested that some receptors had been wrongly identified as Meissner's corpuscles.

The College of Physicians and Surgeons of British Columbia has written that the foreskin is "composed of an outer skin and an inner mucosa that is rich in specialized sensory nerve endings and erogenous tissue."

Development
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.

According to a 1949 study by Gairdner, the foreskin is usually still fused with the glans at birth. As childhood progresses, they gradually separate. There are differing reports on the age at which the foreskin can be retracted. Thorvaldsen and Meyhoff (2005) reported that 21% of 7-year-old boys in their study had non-retractable foreskins and this proportion dropped to 7% at puberty, with first retraction at an average age of 10.4 years but Gairdner (1949) reported that only 10% of 3-year-old boys had non-retractable foreskins, however, Gairdner was wrong about development of foreskin retraction. Wright (1994) 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. Attempts to forcibly retract it can be painful and may cause injury.

In children, the foreskin usually covers the glans completely but in adults it may not. Schöberlein (1966) conducted a study of 3,000 young men from Germany and found that 49.6% had the glans fully covered by foreskin, 41.9% were partially covered and 8.5% were uncovered - around half of which (4%) had the foreskin atrophied spontaneously without previous surgery. During erection, the degree of automatic foreskin retraction varies considerably; in some adults, the foreskin remains covering all or some of the glans until retracted manually or by sexual activity. This variation was regarded by Chengzu (2011) as an abnormal condition named 'prepuce redundant'. Frequent retraction and washing under the foreskin is suggested for all adults but particularly for those with a long, or 'redundant' foreskin. When the foreskin is longer than the erect penis, it will not spontaneously retract upon erection.

It is shown that manual foreskin retraction during childhood or even adulthood serves as a stimulant to normal development and automatic retraction of the foreskin, which suggests that many conditions affecting the foreskin may be prevented or cured behaviorally. Some males, according to Xianze (2012), may be reluctant for their glans to be exposed because of discomfort when it chafes against clothing, although the discomfort on the glans was reported to diminish within one week of continuous exposure. Guochang (2010) states that for those whose foreskin are too tight to retract or have some adhesions, forcible retraction should be avoided since it may cause injury.

Functions
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 enhancing sexual pleasure due to the presence of nerve receptors".

Sexual
Some argue that the foreskin is specialist tissue that is packed with nerves and contains stretch receptors.

The Royal Australasian College of Physicians has stated that the effects of circumcision on sexual sensation are not clear, with reports of both enhanced and diminished sexual pleasure following the procedure in adults and little awareness of advantage or disadvantage in those circumcised in infancy." The Royal Dutch Medical Association (2010) states that many sexologists view the foreskin as "a complex, erotogenic structure that plays an important role 'in the mechanical function of the penis during sexual acts, such as penetrative intercourse and masturbation'."

Some early studies showed that the presence of the foreskin made sexual penetration easier.

Taylor et al. (1996) 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. (2002) 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." The AAP noted that the work of Taylor et al. (1996) "suggests that there may be a concentration of specialized sensory cells in specific ridged areas of the foreskin."

The World Health Organization (2007) states that "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 et al. (2002) reported "although 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. (2005) state that "currently no consensus exists about the role of the foreskin." Schoen (2007) states that "anecdotally, some have claimed that the foreskin is important for normal sexual activity and improves sexual sensitivity.

The term 'gliding action' is used in some medical literature 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 people have argued that the gliding movement of the foreskin is important during sexual intercourse. Warren & Bigelow (1994) state 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 (2002) 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 (2000) suggests that the gliding action, where it occurs, may stimulate the nerves of the ridged band, and speculates (2003) that the stretching of the frenulum by the rearward gliding action during penetration triggers ejaculation. It is argued that removal of the foreskin results in a thickening of the glans because of chafing and abrasion from clothing, leading to loss of sensation. It is also thought that removal of the foreskin can lead to trauma of the penis during masturbation due to the loss of the gliding action of the foreskin and greater friction, requiring the need of artificial lubrication. During sex, the loss of gliding action is also thought to cause pain, dryness and trauma of the vagina. The trauma and abrasions of the vagina can lead to easier entry of sexually transmitted diseases. One study showed that the loss of the foreskin resulted in decreased masturbatory pleasure and sexual enjoyment.

Protective and immunological
Gairdner (1949) states that the foreskin protects the glans. The fold of the prepuce maintains sub-preputial wetness, which mixes with exfoliated skin to form smegma. The American Academy of Pediatrics (1999) 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.

Evolution
In primates, the foreskin is present in the genitalia of both sexes and likely has been present for millions of years of evolution. The evolution of complex penile morphologies like the foreskin may have been influenced by females.

The foreskin is also suggested to aid in the absorption of vaginal secretions, which contain hormones like vasopressin, that helps induce pair bonding and protective behaviors in the male.

Conditions
Simmons et al. (2007) report that the foreskin's presence "frequently predisposes to medical problems, including balanitis, phimosis, venereal disease and penile cancer", and additionally state that "because we now are able to effectively treat foreskin related maladies, some societies are shifting toward foreskin preservation."

Frenulum breve is a frenulum that is insufficiently long to allow the foreskin to fully retract, which may lead to discomfort during intercourse. Phimosis is a condition where 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 the more radical option of circumcision. Posthitis is an inflammation of the foreskin.

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.

Lichen sclerosus is a chronic, inflammatory skin condition that most commonly occurs in adult women, although it may also be seen in men and children. Topical clobetasol propionate and mometasone furoate were proven effective in treating genital lichen sclerosus.

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 requirements, health reasons such as to treat a medical disorder, or personal preferences surrounding hygiene and aesthetics. Preputioplasty is a minor procedure designed to relieve a tight foreskin without resorting to circumcision.

Foreskin restoration techniques (developed to help circumcised men 'regrow' a skin covering for the glans by tissue expansion) can be used by men with short foreskins to lengthen the natural foreskin so that it covers the glans. A narrow foreskin may also be widened by tissue expansion.

Other practices include genital piercings involving the foreskin and slitting the foreskin.

Langerhans cells
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. 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.

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.

Human growth factors derived from newborns' foreskins are used to make a commercial anti-wrinkle skin cream, TNS Recovery Complex.

Foreskins of babies are also used for skin graft tissue,  and for β-interferon-based drugs.

Foreskin fibroblasts have been used in biomedical research.