Human skin

Source: Wikipedia, the free encyclopedia.
This article is about skin in humans. For other vertebrates, see Skin.

Human skin
subcutis, showing a hair follicle, gland, and sebaceous gland
Details
SystemIntegumentary system
Identifiers
Latincutis
TA98A16.0.00.002
TA27041
THH3.12.00.1.00001
FMA7163
Anatomical terminology]

The human skin is the outer covering of the body and is the largest organ of the

glabrous skin (hairless). The adjective cutaneous
literally means "of the skin" (from Latin cutis, skin).

Skin plays an important

vitamin B folates. Severely damaged skin will try to heal by forming scar tissue
. This is often discoloured and depigmented.

In humans,

dry to non-dry and from oily to non-oily. Such skin variety provides a rich and diverse habitat for bacteria that number roughly 1,000 species from 19 phyla
, present on the human skin.

Structure

Human skin shares anatomical, physiological, biochemical and immunological properties with other mammalian lines, especially pig skin.[1][2] Pig skin shares similar epidermal and dermal thickness ratios to human skin;[1][2] pig and human skin share similar hair follicle and blood vessel patterns;[1][2] biochemically the dermal collagen and elastin content is similar in pig and human skin;[1][2] and pig skin and human skin have similar physical responses to various growth factors.[1][2]

Skin has

spread quickly, and can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has led to the classification of people(s) on the basis of skin colour.[3]

In terms of surface area, the skin is the second largest organ in the human body (the inside of the small intestine is 15 to 20 times larger). For the average adult human, the skin has a surface area of from 1.5–2.0 square metres (16–22 sq ft). The thickness of the skin varies considerably over all parts of the body, and between men and women and the young and the old. An example is the skin on the forearm, which is on average 1.3 mm in the male and 1.26 mm in the female.

micrometres
(μm) in diameter, but there are variants. A skin cell usually ranges from 25 to 40 μm2, depending on a variety of factors.

Skin is composed of three primary layers: the

hypodermis.[4]

Layers, receptors, and appendages of human skin

Epidermis

Main article: Epidermis

The epidermis, "epi" coming from the Greek language meaning "over" or "upon", is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface, which also serves as a barrier to infection and is made up of stratified squamous epithelium with an underlying basal lamina.

The epidermis contains no

"keratinization". This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out, making skin a natural barrier to infection.[7]

2D projection of a 3D OCT-tomogram of the skin at the fingertip, depicting the stratum corneum (≈500 µm thick) with the stratum disjunctum on top and the stratum lucidum in the middle. At the bottom are the superficial parts of the dermis. The sweat ducts are clearly visible. (See also: Rotating 3D Version)

The epidermis contains no blood vessels and is nourished by diffusion from the dermis. The main type of cells that make up the epidermis are keratinocytes, melanocytes, Langerhans cells, and Merkel cells. The epidermis helps the skin regulate body temperature.

Layers

The skin has up to seven layers of ectodermal tissue and guards the underlying

keratinization
and takes place within weeks.

It was previously believed that the

corneocytes that are anucleated, these cells remain alive and metabolically functional until desquamated.[citation needed
]

Sublayers

The epidermis is divided into the following 5 sublayers or strata:

Blood capillaries are found beneath the epidermis and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips.

Genes and proteins expressed in the epidermis

About 70% of all human protein-coding genes are expressed in the skin.

squamous differentiation and cornification
.

Dermis

Main article: Dermis

The dermis is the layer of skin beneath the

blood vessels
. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from the stratum basale of the epidermis.

The dermis is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.

Papillary region

The papillary region is composed of loose

areolar connective tissue
. It is named for its finger-like projections called papillae, which extend toward the epidermis. The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.

In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin's surface. These

epigenetically determined and are therefore unique to the individual, making it possible to use fingerprints or footprints as a means of identification
.

Reticular region

The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of

collagenous, elastic, and reticular fibres that weave throughout it. These protein
fibres give the dermis its properties of strength, extensibility, and elasticity.

Also located within the reticular region are the roots of the hairs, sebaceous glands, sweat glands, receptors, nails, and blood vessels.

Tattoo ink is held in the dermis. Stretch marks, often from pregnancy and obesity, are also located in the dermis.

Subcutaneous tissue

The subcutaneous tissue (also hypodermis and subcutis) is not part of the skin, but lies below the dermis of the cutis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue, adipose tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (subcutaneous tissue contains 50% of body fat). Fat serves as padding and insulation for the body.

Cross-section

Skin layers, of both hairy and hairless skin

Cell count and cell mass

Skin cell table

The below table identifies the skin cell count and aggregate cell mass estimates for a 70 kg adult male (ICRP-23; ICRP-89, ICRP-110).[14][15][16]

Tissue mass is defined at 3.3 kg (ICRP-89, ICRP110) and addresses the skin's epidermis, dermis, hair follicles, and glands. The cell data is extracted from 'The Human Cell Count and Cell Size Distribution',[17][18] Tissue-Table tab in the Supporting Information SO1 Dataset (xlsx). The 1200 record Dataset is supported by extensive references for cell size, cell count, and aggregate cell mass.

Detailed data for below cell groups are further subdivided into all the cell types listed in the above sections and categorized by epidermal, dermal, hair follicle, and glandular subcategories in the dataset and on the dataset's graphical website interface.[19] While adipocytes in the hypodermal adipose tissue are treated separately in the ICRP tissue categories, fat content (minus cell-membrane-lipids) resident in the dermal layer (Table-105, ICRP-23) is addressed by the below interstitial-adipocytes in the dermal layer.

Named Tissue and

Associated Cell Groups

Cell Count Aggregate

Cell Mass (g)

Percent of

Total Mass

Skin Total 6.1E+11 846.7 100%
Adipocyte 7.3E+08 291.9 34.5%
Endothelial Cell (EnCs) 1.5E+10 6.16 0.7%
Epithelial Cells (EpC) 4.1E+11 313.9 37.1%
Eccrine Gland 1.7E+11 105 12.4%
Epidermal Keratinocytes 1.1E+11 85.5 10.1%
Hair Follicle 1.3E+11 119.9 14.2%
Mechanoreceptors 4.9E+09 3.6 0.4%
Epithelial Cells (EpC); Non-Nucleated 7.2E+10 28.2 3.3%
Fibroblasts 4.3E+10 94.6 11.2%
Myocytes 2.6E+07 0.08 0.01%
Neuroglia 8.5E+09 12.8 1.5%
Perivascular Cells / Pericytes / Mural 1.5E+09 0.56 0.07%
Stem Cells; Epithelial (EpSC) 3.6E+09 1.50 0.2%
White Blood Cells 5.4E+10 97.1 11.5%
Granulocytes (Mast Cell) 2.2E+10 32.6 3.8%
Lymphoid 1.3E+10 1.6 0.2%
Monocyte-Macrophage Series 1.9E+10 62.9 7.4%

Development

Further information:
Human embryogenesis § Integumentary system

Skin colour

Main article:
Human skin colour

Human skin shows high skin colour variety from the darkest brown to the lightest pinkish-white hues. Human skin shows higher variation in colour than any other single mammalian species and is the result of

ultraviolet radiation (UVR) penetrating the skin, controlling its biochemical effects.[20]

The actual skin colour of different humans is affected by many substances, although the single most important substance determining human skin colour is the pigment

arterioles dilate.[21]

There are at least five different pigments that determine the colour of the skin.[22][23] These pigments are present at different levels and places.

There is a correlation between the geographic distribution of UV radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that highlight higher amounts of UVR reflect darker-skinned populations, generally located nearer towards the equator. Areas that are far from the tropics and closer to the poles have lower concentration of UVR, which is reflected in lighter-skinned populations.[24]

In the same population it has been observed that adult human females are considerably lighter in skin pigmentation than males. Females need more calcium during pregnancy and lactation, and vitamin D, which is synthesized from sunlight helps in absorbing calcium. For this reason it is thought that females may have evolved to have lighter skin in order to help their bodies absorb more calcium.[25]

The Fitzpatrick scale[26][27] is a numerical classification schema for human skin colour developed in 1975 as a way to classify the typical response of different types of skin to ultraviolet (UV) light:

I Always burns, never tans Pale, Fair, Freckles
II Usually burns, sometimes tans Fair
III May burn, usually tans Light Brown
IV Rarely burns, always tans Olive brown
V Moderate constitutional pigmentation Brown
VI Marked constitutional pigmentation Black

Ageing

Further information: senescence
Further information: Intrinsic and extrinsic ageing
A typical rash
Skin infected with scabies

As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself as a person ages.

Among other things, skin ageing is noted by a decrease in volume and elasticity. There are many internal and external causes to skin ageing. For example, ageing skin receives less blood flow and lower glandular activity.

A validated comprehensive grading scale has categorized the clinical findings of skin ageing as laxity (sagging), rhytids (wrinkles), and the various facets of photoageing, including erythema (redness), and telangiectasia, dyspigmentation (brown discolouration), solar elastosis (yellowing), keratoses (abnormal growths) and poor texture.[28]

Cortisol causes degradation of collagen,[29] accelerating skin ageing.[30]

Anti-ageing supplements are used to treat skin ageing.[citation needed
]

Photoageing

Main article:
Photoageing

Photoageing has two main concerns: an increased risk for skin cancer and the appearance of damaged skin. In younger skin, sun damage will heal faster since the cells in the epidermis have a faster turnover rate, while in the older population the skin becomes thinner and the epidermis turnover rate for cell repair is lower, which may result in the dermis layer being damaged.[31]

UV-induced DNA damage

UV-irradiation of human skin cells generates damages in DNA through direct photochemical reactions at adjacent thymine or cytosine residues on the same strand of DNA.[32] Cyclobutane pyrimidine dimers formed by two adjacent thymine bases, or by two adjacent cytosine bases, in DNA are the most frequent types of DNA damage induced by UV. Humans, as well as other organisms, are capable of repairing such UV-induced damages by the process of nucleotide excision repair.[32] In humans this repair process protects against skin cancer.[32]

Types

Though most human skin is covered with

glabrous skin (hairless).[33] The adjective cutaneous means "of the skin" (from Latin cutis, skin).[34]

Functions

Skin performs the following functions:

  1. Protection: an anatomical barrier from pathogens and damage between the internal and external environment in bodily defence; Langerhans cells in the skin are part of the adaptive immune system.[7][35] Perspiration contains lysozyme that break the bonds within the cell walls of bacteria.[36]
  2. Sensation: contains a variety of nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury; see somatosensory system and haptics.
  3. Heat regulation: the skin contains a blood supply far greater than its requirements, which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat.
  4. Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to fluid loss.
    burns
    .
  5. Aesthetics and communication: others see our skin and can assess our mood, physical state and attractiveness.
  6. Storage and synthesis: acts as a storage centre for lipids and water, as well as a means of synthesis of vitamin D by action of UV on certain parts of the skin.
  7. Excretion:
    sweat contains urea, however its concentration is 1/130th that of urine, hence excretion
    by sweating is at most a secondary function to temperature regulation.
  8. Absorption: the cells comprising the outermost 0.25–0.40 mm of the skin are "almost exclusively supplied by external oxygen", although the "contribution to total respiration is negligible".[6] In addition, medicine can be administered through the skin, by ointments or by means of adhesive patch, such as the nicotine patch or iontophoresis. The skin is an important site of transport in many other organisms.
  9. Water resistance: The skin acts as a water-resistant barrier so essential nutrients are not washed out of the body.[35]

Skin flora

Main article: Skin flora

The human skin is a rich environment for microbes.

nares
(inside the nostril), and on the back.

Reflecting upon the diversity of the human skin researchers on the human skin microbiome have observed: "hairy, moist underarms lie a short distance from smooth dry forearms, but these two niches are likely as ecologically dissimilar as rainforests are to deserts."[37]

The

NIH conducted the Human Microbiome Project to characterize the human microbiota, which includes that on the skin and the role of this microbiome in health and disease.[39]

Microorganisms like Staphylococcus epidermidis colonize the skin surface. The density of skin flora depends on region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings.

Clinical significance

Further information:
skin disease

Diseases of the skin include

skin neoplasms (including skin cancer). Dermatology is the branch of medicine that deals with conditions of the skin.[33]

There are seven cervical, twelve thoracic, five lumbar, and five sacral.[clarification needed] Certain diseases like shingles, caused by varicella-zoster infection, have pain sensations and eruptive rashes involving dermatomal distribution. Dermatomes are helpful in the diagnosis of vertebral spinal injury levels. Aside from the dermatomes, the epidermis cells are susceptible to neoplastic changes, resulting in various cancer types.[40]

The skin is also valuable for diagnosis of other conditions, since many

Skin color affects the visibility of these signs, a source of misdiagnosis in unaware medical personnel.[41][42]

Society and culture

Hygiene and skin care

See also: Exfoliation (cosmetology)

The skin supports its own

epithelial
lining of the body at the orifices, each of which supports its own complement of microbes.

Cosmetics should be used carefully on the skin because these may cause allergic reactions. Each season requires suitable clothing in order to facilitate the evaporation of the sweat. Sunlight, water and air play an important role in keeping the skin healthy.

Oily skin

Oily skin is caused by over-active sebaceous glands, that produce a substance called

sebum, a naturally healthy skin lubricant.[8][44] A high glycemic-index diet and dairy products (except for cheese) consumption increase IGF-1 generation, which in turn increases sebum production.[44] Overwashing the skin does not cause sebum overproduction but may cause dryness.[44]

When the skin produces excessive

blackheads, and buildup of dead skin cells on the surface of the skin.[8] Oily skin can be sallow and rough in texture and tends to have large, clearly visible pores everywhere, except around the eyes and neck.[8]

Permeability

Human skin has a low

nanoparticles can interact directly with cells that have lost their growth-control mechanisms (cancer cells
). Such direct interaction could be used to more accurately diagnose properties of specific tumours or to treat them by delivering drugs with cellular specificity.

Nanoparticles

Nanoparticles 40 nm in diameter and smaller have been successful in penetrating the skin.[47][48][49] Research confirms that nanoparticles larger than 40 nm do not penetrate the skin past the stratum corneum.[47] Most particles that do penetrate will diffuse through skin cells, but some will travel down hair follicles and reach the dermis layer.

The permeability of skin relative to different shapes of nanoparticles has also been studied. Research has shown that spherical particles have a better ability to penetrate the skin compared to

fluorescent
properties but have shown mediocre penetration.

Nanoparticles of different materials have shown skin's permeability limitations. In many experiments, gold nanoparticles 40 nm in diameter or smaller are used and have shown to penetrate to the epidermis.

quantum dots have proven to penetrate very effectively when they have certain properties. Because CdSe is toxic to living organisms, the particle must be covered in a surface group. An experiment comparing the permeability of quantum dots coated in polyethylene glycol (PEG), PEG-amine, and carboxylic acid concluded the PEG and PEG-amine surface groups allowed for the greatest penetration of particles. The carboxylic acid coated particles did not penetrate past the stratum corneum.[49]

Increasing permeability

Scientists previously believed that the skin was an effective barrier to inorganic particles. Damage from mechanical stressors was believed to be the only way to increase its permeability.[51]

Recently, simpler and more effective methods for increasing skin permeability have been developed.

Ultraviolet radiation (UVR) slightly damages the surface of skin and causes a time-dependent defect allowing easier penetration of nanoparticles.[52] The UVR's high energy causes a restructuring of cells, weakening the boundary between the stratum corneum and the epidermal layer.[52][51] The damage of the skin is typically measured by the transepidermal water loss (TEWL), though it may take 3–5 days for the TEWL to reach its peak value. When the TEWL reaches its highest value, the maximum density of nanoparticles is able to permeate the skin. While the effect of increased permeability after UVR exposure can lead to an increase in the number of particles that permeate the skin, the specific permeability of skin after UVR exposure relative to particles of different sizes and materials has not been determined.[52]

There are other methods to increase nanoparticle penetration by skin damage: tape stripping is the process in which tape is applied to skin then lifted to remove the top layer of skin;

skin abrasion is done by shaving the top 5–10 μm off the surface of the skin; chemical enhancement applies chemicals such as polyvinylpyrrolidone (PVP), dimethyl sulfoxide (DMSO), and oleic acid to the surface of the skin to increase permeability;[53][54] electroporation increases skin permeability by the application of short pulses of electric fields. The pulses are high voltage and on the order of milliseconds when applied. Charged molecules penetrate the skin more frequently than neutral molecules after the skin has been exposed to electric field pulses. Results have shown molecules on the order of 100 μm to easily permeate electroporated skin.[54]

Applications

A large area of interest in nanomedicine is the transdermal patch because of the possibility of a painless application of therapeutic agents with very few side effects. Transdermal patches have been limited to administer a small number of drugs, such as nicotine, because of the limitations in permeability of the skin. Development of techniques that increase skin permeability has led to more drugs that can be applied via transdermal patches and more options for patients.[54]

Increasing the permeability of skin allows nanoparticles to penetrate and target

fluorescence microscopy and emit light energy and heat that will destroy cancer cells.[55]

Sunblock and sunscreen

Sunblock and sunscreen are different important skin-care products though both offer full protection from the sun.[56]

Sunblock—Sunblock is opaque and stronger than sunscreen, since it is able to block most of the UVA/UVB rays and radiation from the sun, and does not need to be reapplied several times in a day. Titanium dioxide and zinc oxide are two of the important ingredients in sunblock.[57]

Sunscreen—Sunscreen is more transparent once applied to the skin and also has the ability to protect against UVA/UVB rays, although the sunscreen's ingredients have the ability to break down at a faster rate once exposed to sunlight, and some of the radiation is able to penetrate to the skin. In order for sunscreen to be more effective it is necessary to consistently reapply and use one with a higher sun protection factor.

Diet

sebum production, which contributes to acne, and reversing and treating photodamage, striae, and cellulite
.

Vitamin D and analogues are used to downregulate the cutaneous immune system and epithelial proliferation while promoting differentiation.

Vitamin C is an antioxidant that regulates collagen synthesis, forms barrier lipids, regenerates vitamin E, and provides photoprotection.

Vitamin E is a membrane antioxidant that protects against oxidative damage and also provides protection against harmful UV rays. [58]

Several scientific studies confirmed that changes in baseline nutritional status affects skin condition. [59]

The Mayo Clinic lists foods they state help the skin: fruits and vegetables, whole-grains, dark leafy greens, nuts, and seeds.[60]

See also

References

  1. ^ a b c d e Herron AJ (5 December 2009). "Pigs as Dermatologic Models of Human Skin Disease" (PDF). ivis.org. DVM Center for Comparative Medicine and Department of Pathology Baylor College of Medicine Houston, Texas. Retrieved 27 January 2018. pig skin has been shown to be the most similar to human skin. Pig skin is structurally similar to human epidermal thickness and dermal-epidermal thickness ratios. Pigs and humans have similar hair follicle and blood vessel patterns in the skin. Biochemically pigs contain dermal collagen and elastic content that is more similar to humans than other laboratory animals. Finally pigs have similar physical and molecular responses to various growth factors.
  2. ^ a b c d e Liu J, Kim D, Brown L, Madsen T, Bouchard GF. "Comparison of Human, Porcine and Rodent Wound Healing With New Miniature Swine Study Data" (PDF). sinclairresearch.com. Sinclair Research Centre, Auxvasse, MO, USA; Veterinary Medical Diagnostic Laboratory, Columbia, MO, USA. Archived from the original (PDF) on 27 January 2018. Retrieved 27 January 2018. Pig skin is anatomically, physiologically, biochemically and immunologically similar to human skin
  3. ISBN 978-0-13-981176-0
    .
  4. ^
    ISBN 978-0-521-10681-8
    .
  5. ^ Bennett H (25 May 2014). "Ever wondered about your skin?". The Washington Post. Retrieved 27 October 2014.
  6. ^
    PMID 11826181
    .
  7. ^
    S2CID 31353914
    .
  8. ^ a b c d e f "Skin care" (analysis), Health-Cares.net, 2007, webpage: HCcare Archived 12 December 2007 at the Wayback Machine
  9. PMID 21938268
    .
  10. ISBN 9780429163470
    .
  11. ^ "The human proteome in skin – The Human Protein Atlas". www.proteinatlas.org.
  12. S2CID 802377
    .
  13. PMID 25411189
    .
  14. ^ Snyder WS, Cook M, Nasset E, Karhausen L, Howells GP, Tipton IH (1975). "ICRP Publication 23: report of the task group on reference man". www.icrp.org. Elmsford, NY: International Commission on Radiological Protection. Retrieved 30 October 2023.
  15. S2CID 222552
    . Retrieved 30 October 2023.
  16. doi:10.5453/jhps.45.357
    . Retrieved 30 October 2023.
  17. PMID 37722043
    .
  18. ^ Max Planck Society. "Cellular cartography: Charting the sizes and abundance of our body's cells reveals mathematical order underlying life". medicalxpress.com. Retrieved 30 October 2023.
  19. ^ "Human Cell data". humancelltreemap.mis.mpg.de. Retrieved 30 October 2023.
  20. ISBN 978-1139789004
    .
  21. ISBN 978-0520954816
    .
  22. ISBN 978-81-239-1654-5
  23. ^ "Pigmentation of Skin". Mananatomy.com. Archived from the original on 7 October 2012. Retrieved 3 June 2019.
  24. PMID 16766240
    .
  25. PMID 10896812
    .
  26. ^ "The Fitzpatrick Skin Type Classification Scale". Skin Inc. (November 2007). 28 May 2009. Retrieved 7 January 2014.
  27. ^ "Fitzpatrick Skin Type" (PDF). Australian Radiation Protection and Nuclear Safety Agency. Archived from the original (PDF) on 31 March 2016. Retrieved 7 January 2014.
  28. PMID 18423256
    .
  29. S2CID 24160757
    .(subscription required)
  30. ISSN 0142-5463
    .(subscription required)
  31. PMID 2141531
    .
  32. ^
    PMID 32119110
    .
  33. ^
    ISBN 1-4160-3185-5
    .
  34. ^ "Definition of CUTANEOUS". www.merriam-webster.com. Retrieved 4 March 2022.
  35. ^
    PMID 12880413
    .
  36. ^ Todar K. "Immune Defense against Bacterial Pathogens: Innate Immunity". textbookofbacteriology.net. Retrieved 19 April 2017.
  37. ^
    PMID 19478181
    .
  38. ^ a b Pappas S. (2009). Your Body Is a Wonderland ... of Bacteria. ScienceNOW Daily News Archived 2 June 2009 at the Wayback Machine
  39. ^ "NIH Human Microbiome Project". Hmpdacc.org. Retrieved 3 June 2019.
  40. PMID 29262154
    . Retrieved 28 September 2023.
  41. ^ "Color awareness: A must for patient assessment". American Nurse. 11 January 2011.
  42. ^ McCue D (21 July 2020). "Medical student creates handbook for diagnosing conditions in Black and brown skin". As It Happens. CBC Radio. Retrieved 15 December 2020.
  43. ISBN 0-670-42793-4
  44. ^
    S2CID 2446947
    .
  45. PMID 19670463
    .
  46. ^
    S2CID 25769287
    .
  47. ^
    PMID 16614727
    .
  48. PMID 18499408
    .
  49. ^
    PMID 16443688
    .
  50. PMID 18973786
    .
  51. ^
    PMID 18687009
    .
  52. ^
    S2CID 137060184
    .
  53. PMID 12727808
    .
  54. ^
    S2CID 28888964
    .
  55. S2CID 41561027
    .
  56. ^ "Sunblock vs. sunscreen: Which one should you use?". Baylor Scott & White Health. 17 March 2022.
  57. ^ "Nanotechnology Information Center: Properties, Applications, Research, and Safety Guidelines". American Elements.
  58. ^ Shapiro SS, Saliou C (October 2001). "Role of vitamins in skin care". Nutrition. 17 (10): 839–844.
    PMID 11684391
    .
  59. ^ Boelsma E, van de Vijver LP, Goldbohm RA, Klöpping-Ketelaars IA, Hendriks HF, Roza L (February 2003). "Human skin condition and its associations with nutrient concentrations in serum and diet". The American Journal of Clinical Nutrition. 77 (2): 348–355.
    PMID 12540393
    .
  60. ^ "Foods for healthy skin". Mayo Clinic.

External links

Look up human skin in Wiktionary, the free dictionary.
  • Media related to Human skin at Wikimedia Commons
  • "Skin Conditions". MedlinePlus. U.S. National Library of Medicine. Retrieved 12 November 2013.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Human_skin&oldid=1219521835"