Foreskin

• 1. Introduction

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  The foreskin, also known as the prepuce, is a double-layered fold of skin, mucous membrane and muscle at the tip of the human penis. It covers both the glans and the urinary opening. In other primates, and in the vast majority of mammals, a comparable structure goes by the name penile sheath. In humans, it is mobile, stretchable and keeps the glans in a moist environment.

  This small piece of anatomy raises surprisingly large questions. How is it built, layer by layer, and what is it wired to feel? How does it form in the womb? Why does it behave so differently in a toddler, a teenager and an adult? Does it do useful work, or is it a leftover of evolution? And what happens when it goes wrong, or when it is removed?

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• 2. Layers and Nerves

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  Two different kinds of tissue meet in the foreskin. The outside continues the skin of the penile shaft and carries a tough, keratinized covering. The inside continues the lining of the glans and is a smooth mucous membrane, much like the inside of an eyelid or a mouth. That inner layer has a notable capacity for repairing itself.

  The outer foreskin covers an area of 7-100 square centimetres, while the inner foreskin covers 18-68. The place where the two surfaces join is called the mucocutaneous junction. On the underside, an elastic and highly vascularized band called the frenulum anchors the inner foreskin to the glans. The World Health Organization describes it as the interface between the outer and inner layers, tightening to narrow the opening when the penis is not erect.

  Beneath the surface sit five layers: outer skin, mucosal epithelium, lamina propria, dartos fascia and dermis. The dartos fascia was once called the peripenic muscle. It is one of two sheaths of smooth muscle under the penile skin, the other being Buck's fascia. Its elastic fibres form a whorl at the tip, the preputial orifice, which stays narrow through infancy and childhood. The dartos reacts to temperature by expanding and contracting. Because it is only loosely tied to the tissue below, the skin can glide freely. Langerhans cells, a type of immature immune cell, appear across the penile lining and sit closest to the surface on the inner foreskin.

  Three nerve sources feed the foreskin. The bilateral dorsal nerve of the penis and branches of the perineal nerve carry sensation, while the pelvic plexus supplies automatic control. The sensory receptors include both pain detectors and touch detectors, with Meissner's corpuscles most common. Blood arrives through the preputial artery, a branch of the axial and dorsal arteries, which give it off before they reach the corona of the glans. The preputial vein, an extension of the superficial dorsal vein, carries blood away and joins the larger dorsal veins that drain the rest of the shaft.

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• 3. Before Birth

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  Everything begins with a bump called the genital tubercle, which forms in the embryo during the early weeks of pregnancy. At first it is undifferentiated. It becomes a penis only when it is exposed to male hormones released by the testicles. The male prepuce corresponds anatomically to the clitoral hood in females. The outer sexual organs show clear differences between twelve and sixteen weeks of gestation, and foreskin development starts at around eleven weeks or earlier and runs to eighteen weeks.

  Since 1866, researchers have disagreed about exactly how the fold takes shape. In that year, Schweigger-Seidel proposed that the prepuce grows from dorsal skin that slowly extends forward until it covers the glans and fuses with its surface. Hunter put forward a similar view in 1935. In 1956, Glenister added a twist. He argued that the fold comes from an ingrowth of cells that rolls outward over the glans, while the resulting layer also grows backward to form a fold at the coronal sulcus.

  By eleven and twelve weeks, a thickening of the epidermis lifts away from the penis as a raised preputial fold. Beneath it forms the preputial lamina, which spreads sideways and upward over the base of the developing glans. At thirteen weeks, the fold covers only part of the glans. By sixteen weeks, the paired folds cover most of it, and their underside edges fuse along the midline. That seam is the penile raphe, a continuation of the perineal raphe. The dorsal nerve, present as early as nine weeks, reaches the far end of the glans and prepuce by sixteen weeks. At nineteen weeks, foreskin development is complete.

  Towards the end of the second trimester, the glans and prepuce become joined by a shared membrane, the preputial or balanopreputial lamina. At birth, this membrane still clings to the glans. The seal begins to give way at varying ages, in childhood, preadolescence or puberty.

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• 4. Coming Apart

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  In a study of three-year-olds, 90% of boys could retract the foreskin to some degree. Even so, 63% of them still had some adhesion between foreskin and glans. Adhesions fade gradually. In younger boys, they often reach the parts of the glans nearest the urinary opening, while older boys tend to keep only isolated thread-like bands.

  The proportion able to retract climbs to 92% at age six and 99% at sixteen. Nearly 100% of adult males can do so. A persistent inability to retract is called phimosis. Most boys lose every adhesion by the age of 10-11, although some keep them until 14-15.

  Two forces do the work of separation: periodic erections and the build-up of smegma. Smegma is an oily secretion found in both sexes that keeps mucous surfaces moist and reduces friction. In boys, it helps dissolve the natural bond between glans and inner foreskin, and it increases around the ages of 12 to 13.

  In the first years of life, forcing the foreskin back can cause pain or microtears, so it is not recommended. Once it starts to loosen, a pediatrician may suggest gentle retraction at home and rinsing with water in the bath. Mild soap is optional if it irritates. If full retraction is hard, a child can simply wash the exposed part of the glans. During puberty, retraction becomes steadily easier, and gentle washing in the shower is enough to prevent smegma from building up.

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• 5. Long, Medium, Short

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  Three categories describe how much of the glans a foreskin covers. A long foreskin has its opening extending beyond the glans. A medium one sits around the urinary opening. A short one leaves most of the glans exposed. Children are usually fully covered, but adults often are not.

  Erection adds more variation. When a foreskin is longer than the erect penis, it does not pull back by itself. It keeps covering all or part of the glans until it is retracted by hand or by sexual activity.

  In 2011, a researcher named Chengzu labelled the long variety a redundant prepuce. Frequent retraction and washing is advised for all adults, especially those with a long foreskin. In 2012, Xianze reported that some men resist exposing the glans because it chafes against clothing. The same report noted that the discomfort fades within one week of continuous exposure. For tight foreskins or those with adhesions, Guochang wrote in 2010 that forcible retraction should be avoided because it can cause injury.

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• 6. Vital or Vestigial

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  Chimpanzees offer a clue about the foreskin's age. Their prepuces partly or completely cover the glans. In primates, the structure exists in both sexes, and it has likely been around for millions of years of evolution.

  In 2007, the World Health Organization reported a "debate about the role of the foreskin". The proposed functions it listed were keeping the glans moist, protecting the penis as it develops in the womb, and adding to pleasure through nerve receptors. Two years later, the organization called it a "myth" that circumcision affects sexual pleasure, a view other major medical organizations share.

  The nerve evidence points in different directions. Meissner's corpuscles are endings involved in fine touch. On a scale where the fingertip scored highest at 0.96, the foreskin scored lowest at 0.28, which suggests it is the least sensitive hairless tissue on the body. Yet 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".

  The protective case goes back to 1949, when the British physician Douglas Gairdner wrote that "it is often stated that the prepuce is a vestigial structure devoid of function". He disagreed. In his words, "it seems to be no accident that during the years when the child is incontinent the glans is completely clothed by the prepuce". Without that cover, he argued, the glans would be exposed to injury from sodden clothes or napkins. Modern accounts add that the foreskin shields infants from ammonia and feces in diapers, lowering the incidence of meatal stenosis. It also guards against abrasions throughout life, supplies extra skin during an erection, and reduces friction during sex.

  In March 2017, Morris and Krieger took the opposite side in the Global Health Journal: Science and Practice. They wrote that "the variability in foreskin size is consistent with the foreskin being a vestigial structure".

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• 7. When It Goes Wrong

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  Four conditions are most often tied to the foreskin: balanitis, phimosis, sexually transmitted infection and penile cancer. A 2012 technical report from the American Academy of Pediatricians, now expired, found that the foreskin can harbor micro-organisms. These may raise the risk of urinary tract infections in some infants and contribute to the spread of certain infections in adults. In recurrent cases, too much soap can irritate the mucous lining, so washing should be gentle.

  In adults, phimosis means the foreskin cannot be pulled back properly. Treatment can involve topical steroid ointments and lubricant during sex, and severe cases may need circumcision. Posthitis is simple inflammation of the foreskin. Frenulum breve describes a frenulum too short to let the foreskin retract fully, which may cause discomfort during intercourse.

  Paraphimosis is the more urgent problem. A tight foreskin becomes trapped behind the glans and swells into a restrictive ring. That ring can cut off the blood supply and leave the glans starved of oxygen, a state called ischemia.

  Lichen sclerosus is a chronic inflammatory skin condition. It most commonly affects adult women but also appears in men and children. Two topical steroids, clobetasol propionate and mometasone furoate, have proven effective against the genital form.

  Three birth defects of the foreskin are known, and all are rare. In aposthia, no foreskin is present at birth. In micropathia, the foreskin fails to cover the glans. In macroposthia, also called congenital megaprepuce, it extends well past the end of the glans.

  One finding concerns infection risk. Larger foreskins have been linked to higher HIV risk in uncircumcised men, most likely because of the bigger inner surface and its dense population of Langerhans cells.

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• 8. Culture and History

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  Circumcision, the partial or complete removal of the foreskin, is most commonly an elective procedure done for preventive, cultural or religious reasons. It can also treat phimosis, balanitis and other conditions. The ethics of performing it on children remain a source of controversy.

  Going the other direction, some men use weights or other devices to stretch penile skin and grow a replacement. The new tissue covers the glans but does not fully copy the features of a real foreskin. Other practices include piercings through the foreskin and slitting it. The most common reconstruction technique is preputioplasty, usually done when a boy is born with a foreskin that is too small. A similar procedure can relieve a tight foreskin without resorting to circumcision.

  Removed foreskins have a second life in the laboratory. Researchers use them to study the structure and proteins of human skin. Tissue from newborns has proven especially useful for manufacturing more human skin. Baby foreskins also serve as skin graft tissue and as an ingredient in drugs based on beta-interferon. Fibroblasts derived from them appear in biomedical research and cosmetic applications.

  In the Greco-Roman world, the foreskin signalled beauty, civility and masculinity. Bareness of the glans was considered vulgar. Some Jews in that world therefore underwent epispasm, a procedure to restore a foreskin that had been removed, so as to avoid being shunned. In ancient Greece, foreskins were prized, and longer ones most of all. The earliest known illustration of the foreskin comes from the Egyptian kingdoms, and later ages left their own depictions in art.

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