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Foreskin

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This article is about human foreskin. For foreskin in other mammals, see Preputial sheath.
Foreskin
Foreskin partially retracted over the glans penis, with a ridged band visible at the end of the foreskin
Details
PrecursorGenital tubercle, urogenital folds
ArteryDorsal artery of the penis
VeinSuperficial dorsal vein of the penis
NerveDorsal nerve of the penis
Identifiers
LatinPraeputium
MeSHD052816
TA98A09.4.01.011
TA23675
FMA19639
Anatomical terminology

In male human anatomy, the foreskin is a double-layered fold of 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 foreskin is fairly strechable, and acts as a natural lubricant.

The foreskin is typically retractable over the glans, and depending on its length, which varies between males, it may remain covering the glans whether the penis is flaccid or erect. Research found that 95% of males were able to fully retract their foreskin by adulthood.[1]

The World Health Organization states that functions of the foreskin 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 conditions, treatments for which can include circumcision, whereby some or all of the foreskin is surgically removed. This procedure is most commonly elected for religious reasons. Regular washing under the foreskin was found to reduce the risk of numerous penile disorders.[2]

Description

File:Male anatomy.png
Diagram of a portion of the male anatomy.

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.[3] The foreskin is attached to the glans by a frenulum, which helps return the foreskin over the glans. The tapering end of the foreskin beyond the glans has been called the acroposthion.

Taylor et al. (1996) reported the presence of Krause end-bulbs and a type of nerve ending called Meissner's corpuscles.[4] 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.[4] They are affected by age: their incidence decreases after adolescence.[5] Meissner's corpuscles could not be identified in all individuals.[6] 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".[7] In the late 1950s, Winkelmann suggested that some receptors had been wrongly identified as Meissner's corpuscles.[8][9]

The College of Physicians and Surgeons of British Columbia have 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."[10]

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.[11]

At birth, the foreskin is often still fused with the glans.[11] As childhood progresses the foreskin and the glans gradually separate, a process that may not be complete until late puberty.[12] Thorvaldsen and Meyhoff (2005) reported that 21% of 7-year-old boys had non-retractable foreskins, and this number dropped to 7% at puberty, with first retraction at an average age of 10.4 years.[13] 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.[14] Attempts to forcibly retract it can be painful and may injure the foreskin.[15]

In children, the foreskin usually covers the glans completely, but not always in adults. Schöberlein (1966) conducted a study among 3000 young men from Southern Germany, found that 49.6% had the glans fully covered by foreskin; 41.9% partially covered; 8.5% uncovered - around half of which (4%) have the foreskin atrophied spontaneously without previous surgery.[16] During erection, the degree of foreskin retraction varies considerably; in some adults, the foreskin remains covering the glans until retracted by sexual activity.

Functions

The foreskin typically covers the glans when the penis is flaccid (top image), but generally retracts upon erection (bottom image).

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".[17]

Sexual

Whiddon (1953), Foley (1966), and Morgan (1967) all believed that the presence of the foreskin made sexual penetration easier.[18][19][20]

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."[4] In 1999, Cold and Taylor stated: "The prepuce is primary, erogenous tissue necessary for normal sexual function."[21] 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."[22] 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."[23]

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."[24] 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."[25] 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."[26] Masood et al. (2005) state that "currently no consensus exists about the role of the foreskin."[27] Schoen (2007) states that "anecdotally, 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."[28]

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..."[29] Several people have argued that the gliding movement of the foreskin is important during sexual intercourse.[30] 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.[31] 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".[32] Taylor (2000) suggests that the gliding action, where it occurs, may stimulate the nerves of the ridged band,[33] and speculates (2003) that the stretching of the frenulum by the rearward gliding action during penetration triggers ejaculation.[34]

The Royal Australasian College of Physicians has stated that the foreskin protects the glans, and that "the foreskin is a primary sensory part of the penis, containing some of the most sensitive areas of the penis. The effects of circumcision on sexual sensation however 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."[35] 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’."[36]

Protective and immunological

Gairdner (1949) states that the foreskin protects the glans.[11] 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."[37] Inferior hygiene has been associated with balanitis,[38] though excessive washing can cause non-specific dermatitis.[39][40]

Evolution

In primates, the foreskin is present in the genitalia of both sexes and likely has been present for millions of years of evolution.[41] The evolution of complex penile morphologies like the foreskin may have been influenced by females.[42][43][44]

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."[45]

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.[46] Phimosis can be treated by gently stretching the foreskin, by changing masturbation habits,[47] 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.[48]

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.[49]

Other practices include genital piercings involving the foreskin and slitting the foreskin.[50]

Langerhans cells

Langerhans cells are immature dendritic cells that are found in all areas of the penile epithelium,[51] but are most superficial in the inner surface of the foreskin.[51] A study by Szabo and Short (2000) 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."[52] 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.[51] However, de Witte et al. (2007) argued that langerin, produced by Langerhans cells, blocks the transmission of HIV to T cells.[53]

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.[54]

Human growth factors derived from newborns' foreskins are used to make a commercial anti-wrinkle skin cream, TNS Recovery Complex.[55](subscription required)

Foreskins of babies are also used for skin graft tissue,[56][57][58] and for β-interferon-based drugs.[59]

Foreskin fibroblasts have been used in biomedical research.[60]

See also

References

  1. ^ Øster, Jakob (April 1968). "Further Fate of the Foreskin Incidence of Preputial Adhesions, Phimosis, and Smegma among Danish Schoolboys". Archives of Disease in Childhood. 43. Department of Paediatrics, Central Hospital, Randers, Denmark: 200–202. doi:10.1136/adc.43.228.200. PMC 2019851. PMID 5689532. Retrieved November 14, 2011.
  2. ^ Krueger, H; Osborn, L (1986). "Effects of hygiene among the uncircumcised". The Journal of family practice. 22 (4): 353–5. PMID 3958682.
  3. ^ Lakshmanan S, Prakash S. Human prepuce - some aspects of structure & function. Indian J Surg. 1980;44:134–7.
  4. ^ a b c Taylor, JR (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  5. ^ Dong G, Sheng-mei X, Hai-yang J, et al. Observation of Meissner's corpuscle on fused phimosis. J Guangdong Medical College. 2007.
  6. ^ Haiyang J, Guxin W, Guo Dong G, Mingbo T et al.. Observation of Meissner's corpuscle in abundant prepuce and phimosis. J Modern Urol. 2005.
  7. ^ Bhat GM, Bhat MA, Kour K, Shah BA. Density and structural variations of Meissner's corpuscle at different sites in human glabrous skin. J Anat Soc India. 2008;57(1):30–33. | doi= | pmid= | pmc= }}
  8. ^ Winkelmann RK. The cutaneous innervation of human newborn prepuce. J Investigative Dermatol. 1956;26(1):53–67. doi:10.1038/jid.1956.5. PMID 13295637.
  9. ^ Winkelmann RK. The mucocutaneous end-organ: the primary organized sensory ending in human skin. AMA Arch Dermatol. 1957;76(2):225–35. PMID 13443512.
  10. ^ College of Physicians and Surgeons of British Columbia (2009). "Circumcision (Infant Male)" (PDF). Retrieved April 22, 2012.
  11. ^ a b c Gairdner, D (1949). "Fate of the Foreskin". BMJ. 2 (4642): 1433–7. doi:10.1136/bmj.2.4642.1433. PMC 2051968. PMID 15408299.
  12. ^ Øster J. Further fate of the foreskin: incidence of preputial adhesions, phimosis, and smegma among Danish Schoolboys. Arch Dis Child. 1968;43:200–3. doi:10.1136/adc.43.228.200. PMID 5689532. PMC 2019851.
  13. ^ Thorvaldsen MA, Meyhoff H (2005). "Phimosis: Pathological or Physiological?". Ugeskr Læger. 167 (17): 1858–62.
  14. ^ Wright JE. Further to "the further fate of the foreskin". Update on the natural history of the foreskin. Med J Aust. 1994;160(3):134–5. PMID 8295581.
  15. ^ "Circumcision of infant males" (PDF). RACP. p. 7.
  16. ^ "The Significance and Frequency of Phimosis and Smegma". Male-initiation.net. 1966. Retrieved 2013-04-16.
  17. ^ "Male circumcision: Global trends and determinants of prevalence, safety and acceptability" (PDF). World Health Organization. p. 13.
  18. ^ Whiddon D (1953). "I. Should baby be circumcised?". Lancet. 265 (6781): 337–8. PMID 13085774. {{cite journal}}: Unknown parameter |month= ignored (help)
  19. ^ Foley. The unkindest cut of all. Fact. 1967;2(4):2–9.
  20. ^ Morgan WKC. Penile plunder. Med J Aust. 1967;1:1102–03.
  21. ^ Cold CJ, Taylor JR. The prepuce. BJU Int. 1999;83 Suppl. 1:34–44. doi:10.1046/j.1464-410x.1999.0830s1034.x. PMID 10349413.
  22. ^ Boyle, G; Goldman, R; Svoboda, J; Fernandez E (2002). "Male Circumcision: Pain, Trauma and Psychosexual Sequelae". Journal of Health Psychology. 7 (3): 329–343. doi:10.1177/1359105302007003225. PMID 22114254.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  23. ^ "American Academy of Pediatrics: Circumcision Policy Statement". Pediatrics. 103 (3): 686–693. 1999. doi:10.1542/peds.103.3.686. PMID 10049981. {{cite journal}}: Unknown parameter |month= ignored (help)
  24. ^ Moses S (1998). "Male circumcision: assessment of health benefits and risks". Sexually Transmitted Infections. 74 (5): 368–373. doi:10.1136/sti.74.5.368. PMC 1758146. PMID 10195035. 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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  25. ^ "Male circumcision: Global trends and determinants of prevalence, safety and acceptability" (PDF). World Health Organization. p. 16.
  26. ^ Fink KS, Carson CC, DeVellis RF (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. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  27. ^ Masood S, Patel HR, Himpson RC, Palmer JH, Mufti GR, Sheriff MK (2005). "Penile sensitivity and sexual satisfaction after circumcision: are we informing men correctly?". Urol. Int. 75 (1): 62–6. doi:10.1159/000085930. PMID 16037710.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  28. ^ Schoen EJ (2007). "Should newborns be circumcised?: YES". Can Fam Physician. 53 (12): 2096–8, 2100–2. PMC 2231533. PMID 18077736. {{cite journal}}: Unknown parameter |month= ignored (help)
  29. ^ Lakshmanan S; Prakash S (1980). "Human prepuce: some aspects of structure and function". Indian Journal of Surgery. 44: 134–137. The outer layer of the prepuce in common with the skin of the shaft of the penis glides freely in a to and fro fashion and has to be delicate and thin, as was observed in this study. [...] The inner lining of the projecting tubular part has the structure of the outer layer and adds to the thin gliding skin when retracted.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  30. ^ Kigozi G, Watya S, Polis CB; et al. (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. 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 {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  31. ^ Warren, J; Bigelow J (September/October 1994). "The case against circumcision". Br J Sex Med: 6–8. {{cite journal}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
  32. ^ O'Hara K (2002). Sex as Nature Intended It: The Most Important Thing You Need to Know about Making Love, but No One Could Tell You Until Now. Turning Point Publications. p. 72. During intercourse, the natural penis shaft actually glides within its own shaft skin covering. This minimizes friction to the vaginal walls and opening, and to the shaft skin itself, adding immeasurably to the comfort and pleasure of both parties.
    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.
  33. ^ Taylor, J (2000). "Back and Forth". Pediatrics News. 34 (10): 50.
  34. ^ Taylor JR (2003). "Evidence sketchy on circumcision and cervical cancer link". Can Fam Physician. 49: 1592. PMC 2214164. PMID 14708921. {{cite journal}}: Unknown parameter |month= ignored (help)
  35. ^ Paediatrics & Child Health Division, The Royal Australasian College of Physicians (2010). "Circumcision of Infant Males" (PDF). Retrieved April 22, 2012. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |month= ignored (help)
  36. ^ "Non-therapeutic circumcision of male minors (2010)". KNMG. 12 June 2010.
  37. ^ "Circumcision policy statement. American Academy of Pediatrics. Task Force on Circumcision". Pediatrics. 103 (3): 686–93. 1999. doi:10.1542/peds.103.3.686. PMID 10049981. {{cite journal}}: Unknown parameter |month= ignored (help)
  38. ^ O'Farrell N, Quigley M, Fox P. Association between the intact foreskin and inferior standards of male genital hygiene behaviour: a cross-sectional study. Int J STD AIDS. 2005;16(8):556–9. doi:10.1258/0956462054679151. PMID 16105191. O'Farrell N, Quigley M, Fox P (2005). "Association between the intact foreskin and inferior standards of male genital hygiene behaviour: a cross-sectional study". Int J STD AIDS. 16 (8): 556–9. doi:10.1258/0956462054679151. PMID 16105191.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  39. ^ Birley HDL, Luzzi GA, Bell R. [http://www.cirp.org/library/disease/balanitis/birley/ Clinical features and management of recurrent balanitis; association with atopy and genital washing]. Genitourin Med. 1993;69(5):400–3. PMID 8244363. PMC 1195128.
  40. ^ Dunn HP. Non-surgical management of phimosis. Aust N Z J Surg. 1989;59(12):963. PMID 2597103.
  41. ^ Martin, Robert D. (1990). Primate Origins and Evolution: A Phylogenetic Reconstruction. New Jersey: Princeton University Press. ISBN 978-0-691-08565-4.
  42. ^ Diamond, Jared M. (1997). Why Sex is Fun: The Evolution of Human Sexuality. London: Weidenfeld & Nicolson. ISBN 0-465-03126-9.
  43. ^ Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex. London: Murray. ISBN 1-148-75093-2.
  44. ^ Short, RV (1981). "Sexual selection in man and the great apes". Graham CE, ed. Reproductive Biology of the Great Apes: Comparative and Biomedical Perspectives. New York: Academic Press.
  45. ^ Simmons MN, Jones JS. Male genital morphology and function: an evolutionary perspective. J Urol. 2007;177(5):1625–31. doi:10.1016/j.juro.2007.01.011. PMID 17437774. Simmons MN, Jones JS (2007). "Male genital morphology and function: an evolutionary perspective". J. Urol. 177 (5): 1625–31. doi:10.1016/j.juro.2007.01.011. PMID 17437774. {{cite journal}}: Unknown parameter |month= ignored (help)
  46. ^ Kayaba H, Tamura H, Kitajima S, et al. Analysis of shape and retractability of the prepuce in 603 Japanese boys. J Urol. 1996;156(5):1813–5. PMID 8863623.
  47. ^ Beaugé M. The causes of adolescent phimosis. Br J Sex Med. 1997;(September–October):26.
  48. ^ "Ching-Chi Chi1 et al: Topical interventions for genital lichen sclerosus". Onlinelibrary.wiley.com. Retrieved 2012-07-16.
  49. ^ "Dunn HP. Non-surgical management of phimosis. ''Aust N Z J Surg'' 1989;59(12):963". Cirp.org. Retrieved 2012-07-16.
  50. ^ "Paraphimosis : Article by Jong M Choe, MD, FACS". eMedicine. Retrieved 2012-07-16.
  51. ^ a b c McCoombe SG, Short RV. Potential HIV-1 target cells in the human penis. AIDS. 2006;20(11):1491–5. doi:10.1097/01.aids.0000237364.11123.98. PMID 16847403.
  52. ^ "How does male circumcision protect against HIV infection?". Retrieved 2007-11-30.
  53. ^ de Witte L, Nabatov A, Pion M, et al.. Langerin is a natural barrier to HIV-1 transmission by Langerhans cells. Nat. Med. 2007;13(3):367–71. doi:10.1038/nm1541. PMID 17334373.
  54. ^ McKie, Robin (1999-04-04). "Foreskins for Skin Grafts". The Toronto Star.
  55. ^ "SkinMedica Seeks Niche in Skin-Care Drugs, Products". Orange County Business Journal. August 5, 2002. Retrieved February 11, 2011. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  56. ^ High-Tech Skinny on Skin Grafts; 1999-02-16 [Retrieved 2008-08-20].
  57. ^ Grand DJ. Medscape. Skin Grafting; August 15, 2011 [Retrieved August 18. 2012].
  58. ^ Amst, Catherine (July 27, 1998). "Biotech Bodies". www.businessweek.com. The McGraw-Hill Companies Inc. Retrieved 2008-08-20. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  59. ^ Cowan, Alison Leigh (April 19, 1992). "Wall Street; A Swiss Firm Makes Babies Its Bet". New York Times:Business. New York Times. Retrieved 2008-08-20.
  60. ^ Hovatta O, Mikkola M, Gertow A, et al.. A culture system using human foreskin fibroblasts as feeder cells allows production of human embryonic stem cells. Hum Reprod. 2003;18(7):1404–9. doi:10.1093/humrep/deg290. PMID 12832363.
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