The skin is the largest organ of the body and serves as a complex barrier between the internal environment (organs, muscles, nerves, etc.) and external factors. It is comprised of three main layers –epidermis, dermis, and hypodermis – which play crucial roles in maintaining homeostasis, facilitating sensory experiences, and protecting against pathogens and/or trauma.
The outermost layer of the skin is the epidermis, a stratified epithelium layer with five distinct sublayers – stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum germinativum, or basal cell layer, contains basal cells and melanocytes. Melanocytes produce melanin, a pigment responsible for skin tone and crucial for photoprotection against harmful UV radiation. The stratum spinosum, or squamous cell layer, contains Langerhans cells, or specialized immune cells that contribute to the skin’s immunological defense. The squamous cell layer is also where basal cells produced by the stratum germinativum mature into keratinocytes. As these keratinocytes move upward through layers of the epidermis, they undergo progressive differentiation and keratinization, ultimately forming the protective outermost layer, or stratum corneum. The stratum corneum is composed of numerous layers of flattened, continually cycling keratinocytes which provides a waterproof barrier, prevents water loss and invasion by foreign substances, and protects deeper levels from abrasion.
The dermis, located beneath the epidermis, is a thick, connective tissue layer composed of fibroblasts, blood vessels, nerves, sensory receptors, hair follicles, and glands. Fibroblasts produce collagen and elastin fibers which are crucial for maintaining the structural integrity of the dermis and provide the skin with its tensile strength and elasticity. Blood vessels throughout the dermis are responsible for maintaining homeostasis, supplying nutrients to the skin, and thermoregulation. Sweat glands in the dermis also contribute to thermoregulation through sweat and water secretion. The dermis contains nerve endings such as the Meissner corpuscles, which mediate touch, and the Vater-Pacini corpuscles, which generate a sense of pressure. Pain, temperature, and itch sensations are transmitted by nerve fibers near hair follicles. Sebaceous glands are attached to hair follicles and produce sebum, a lipid-rich substance essential for skin hydration and acts as a barrier against foreign substances.
The hypodermis, or subcutaneous tissue, is predominantly composed of adipose tissue. Adipocytes in this layer function as both an insulator, acting as an energy reservoir and thermoregulator, and a protective cushion against mechanical forces. The blood vessels, nerves, and hair follicles in the hypodermis form a bridge between the skin and deeper tissues. Additionally, the hypodermis plays a role in the absorption and distribution of certain medications.
A comprehensive understanding of the intricate structures and functions of the epidermis, dermis, and hypodermis is essential for identifying and treating various skin disorders, including cancer, acne, etc. Knowledge of skin anatomy allows healthcare professionals to pinpoint the origin and localization of skin disorders as different conditions affect specific layers. Understanding these distinctions aids in accurate diagnosis, interpretation of biopsy results, and the prescription of effective pharmacological interventions. Recognizing the relationship between skin structure and function allows for a holistic approach to patient care in the context of dermatological disorders.
Kolarsick PAJ, Kolarsick MA, Goodwin C. Anatomy and physiology of the skin. Journal of the Dermatology Nurses’ Association. 2011 Jul;3(4):203-13. https://journals.lww.com/jdnaonline/fulltext/2011/07000/anatomy_and_physiology_of_the_skin.3.aspx
Lopez-Ojeda W, Pandey A, Alhajj M, Oakley AM. Anatomy, skin (integument) [updated 2022 Oct 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. 2023 Jan. https://www.ncbi.nlm.nih.gov/books/NBK441980/
The skin, our body's largest organ, serves as a complex and vital protective barrier against environmental factors and helps regulate various physiological functions. Comprising the epidermis, dermis, and hypodermis, the skin's intricate layers and structures contribute to its role in safeguarding our health.
Starting with the epidermis, the outermost layer, it consists of several sub-layers, including the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. Keratinocytes, melanocytes, Langerhans cells, and Merkel cells are key contributors to the epidermal composition. Melanocytes, located in the stratum basale, produce melanin, the pigment responsible for skin color. This adaptation is crucial for protecting against harmful UV radiation, with skin tone variations linked to geographic location and levels of UV exposure.
Moving to the dermis, situated beneath the epidermis, it contains collagen and elastin fibers, blood vessels, nerves, and adnexal structures like hair follicles and sweat glands. The dermis plays a pivotal role in skin strength, flexibility, and various physiological processes, including thermoregulation and sensation.
The hypodermis, the deepest layer, predominantly comprises adipose tissue, offering padding and insulation. This layer contributes to temperature regulation, shock absorption, and energy storage. The skin, with its three layers working together, forms a robust barrier against external threats.
Skin pigmentation, determined by melanin production, is an aspect influenced by genetics, geographic location, and UV exposure. Darker skin tones are an adaptation to high UV exposure, providing protection against skin cancer but posing a risk of vitamin D deficiency. Conversely, lighter skin tones in regions with lower UV exposure facilitate efficient vitamin D synthesis.
Moreover, recent theories propose that diet, particularly during the Neolithic era, played a role in the evolution of skin pigmentation. Neolithic-era farmers, lacking Vitamin D in their diets, may have contributed to the gradual loss of dark-skin pigmentation. This intricate interplay of genetics, environment, and physiological functions underscores the skin's significance beyond its aesthetic aspects.
Human skin, distinct from other mammals, underwent evolutionary changes, such as the reduction of body hair, enhancing thermoregulation as we moved on to wearing clothing. Sweating, an essential function primarily occurring in the dermis, aids in cooling the body, highlighting the adaptive nature of the skin to its environment.
Understanding the skin's anatomy reveals its integral role in maintaining homeostasis, protecting against pathogens, and facilitating sensory perception. Dermatological conditions often stem from disruptions in the skin's various components, emphasizing the importance of proper skincare and protection against environmental stressors. As people age, the loss of collagen and elastin leads to thinner dermis, resulting in sagging skin and wrinkles. Skincare recommendations include daily sunscreen application, avoiding excessive tanning, regular showers, and the use of gentle cleansers.
In summary, the skin, shaped by genetics, environmental influences, and the forces of evolution shows that it is not merely for aesthetics but a dynamic organ vital for our well-being. Learning its complexities allows for refined skincare practices, unveiling the profound role the skin plays in meticulously maintaining our body's delicate balance.
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The largest organ of the integumentary system, the skin, is a protective surface with three layers covering the entire body. The purpose of the skin is to regulate temperature, protect against ultraviolet (UV) light, trauma, pathogens, microorganisms, and toxins. (4) Each layer has its own function with varying properties in thickness, color, and texture. For example, the soles of the feet and the palms of the hands have five layers of skin, also known as thick skin, compared to other locations of the body with only four layers referred to as thin skin.
The three layers of the skin are the epidermis, the dermis and the subcutaneous fat layer with multiple cells and tissue. The deepest portion of the epidermis, stratum basale, contains keratinocytes, melanocytes and merkel cells. Keratinocytes are the building blocks of the fibrous protein, keratin, that gives the hair, skin and nails their hardness and water-resistant properties. (1) Melanocytes synthesize melanin based on the cellular extensions and the amount of melanin they contain. The pigment of your skin does not depend on the number of melanocytes and instead, it’s determined by genetics and sun exposure of your ancestral population (2). Merkel cells are found directly above the basement membrane, producing a sensory experience with mechanoreceptors. The next layer of the epidermis is the stratum spinosum which contains langerhans cells or dendritic cells that originate from the bone marrow and protect from unwanted species and have immune function. (3) Next is the stratum granulosum which contains lamellar granules and glycopeptides that secrete to the surface and act as a glue to keep cells intact. Stratum lucidum or the “clear skin” is only present in thick surfaces and in the second layer. Lastly, the stratum corneum acts as the uppermost layer with dead keratinocytes which regenerate every four to six weeks. (5) Younger cells are in the deepest layer where they can mature and regenerate via mitosis and eventually, reach the top as dead cells that get sloughed off.
The dermis contains two layers, the papillary and reticular layer, where blood vessels, hair follicles, sweat glands, and sensory neurons reside. The papillary layer is composed of dermal papillae and due to friction ridges against the epidermis, the skin is able to grip surfaces. The reticular layer is the deeper layer with dense irregular connective tissue and collagen. Below the dermis is the hypodermis or the subcutis layer where mostly fat is stored along with some hair follicles and blood vessels.
Understanding the location of the skin cells, and what each layer contains is significant in clinical practice. For example, skin cancers are identified once we discover the properties of the cells. Mutated keratinocytes in the squamous cells due to UV damage appear as scaly, flaky patches and even bleeding. Understanding skin physiology is crucial for our patient’s treatment regimen to treat this carcinoma and prevent it from metastasizing.
1. Betts, J. Gordon, et al. “5.1 Layers of the Skin - Anatomy and Physiology 2E.” OpenStax, openstax.org/books/anatomy-and-physiology-2e/pages/5-1-layers-of-the-skin.
2. Cleveland Clinic medical. “Melanin: What Is It, Types & Benefits.” Cleveland Clinic, my.clevelandclinic.org/health/body/22615-melanin.
3. “Default - Stanford Medicine Children’s Health.” Stanford Medicine Children’s Health - Lucile Packard Children’s Hospital Stanford, www.stanfordchildrens.org/en/topic/default?id=anatomy-of-the-skin-85-P01336.
4. Lopez-Ojeda, Wilfredo. Anatomy, Skin (Integument) - Statpearls - NCBI Bookshelf, www.ncbi.nlm.nih.gov/books/NBK441980/.
5. Yousef, Hani Yousef; Mandy. Anatomy, Skin (Integument), Epidermis - StatPearls - NCBI Bookshelf, www.ncbi.nlm.nih.gov/books/NBK470464/.
The skin serves as a protective barrier against environmental toxins and helps maintain the integrity of internal organs. It covers an extensive area of about 2 square meters and consists of three main components: the epidermis, dermis, and subcutis or hypodermis. Some experts further categorize these components into three reactive units: the superficial reactive unit (including the epidermis, dermal-epidermal junction, and superficial dermis), the dermal reactive unit (composed of the reticular dermal layer and dermal microvascular plexus), and the subcutaneous reactive unit (consisting of fat lobules and septae).
The epidermis, the outermost layer of the skin, plays a critical role as a barrier, maintains fluid balance, and prevents infections. The degree of barrier function of the epidermis varies with its thickness, which ranges from 1.5 mm on the palms and soles to 0.1 mm on the eyelids. It consists of four layers: the horny layer (stratum corneum), granular layer (stratum granulosum), spinous layer (stratum spinosum), and basal layer (stratum germinativum), located above the basement membrane zone.
Keratinocytes, derived from ectodermal tissue, make up the majority of epidermal cells. The stratum corneum, composed of differentiated keratinocytes, primarily serves as the physical barrier of the skin. Any disruption or abnormal formation in this layer can compromise the barrier function. The upper spinous and granular layers contain organelles like Odland bodies, which aid in water retention and cell cohesion.
A surface film composed of sebum, sweat, and breakdown products of keratinocytes covers the stratum corneum, acting as an external barrier against bacteria, viruses, and fungi. However, its effectiveness in preventing percutaneous absorption is limited with regard to percutaneous absorption. The major barrier molecules to percutaneous absorption in the skin are lipids called ceramides. Diseases like atopic dermatitis and psoriasis, which are characterized by dry skin, often result from decreased ceramide concentrations, leading to increased penetration of toxins due to barrier degradation.
The basal layer of the epidermis controls epidermal renewal, with stem cells and transient amplifying cells responsible for new epidermal formation approximately every 28 days. Melanocytes in this layer produce melanin, a major skin pigment that protects against ultraviolet radiation and produces skin pigmentation.
The basement membrane zone, comprising three layers (lamina lucida, lamina densa, and sublamina densa), separates the epidermis from the dermis and is associated with various genetic and autoimmune skin conditions.
The dermal-epidermal junction, located just above the basement membrane zone, provides resistance against trauma, supports overlying structures, organizes basal cell cytoskeleton, and acts as a semi-permeable barrier. Beneath the dermal-epidermal junction, the dermis houses adnexal structures, blood vessels, nerves, and appendageal structures. Structural support is provided by collagen and elastin fibers embedded in glycosaminoglycans like chondroitin A and hyaluronic acid. These components collectively ensure the skin's structural integrity and function.
Lewin J.M., & Lewin N.A., & Nelson L.S. (2019). Dermatologic principles. Nelson L.S., & Howland M, & Lewin N.A., & Smith S.W., & Goldfrank L.R., & Hoffman R.S.(Eds.), Goldfrank's Toxicologic Emergencies, 11e. McGraw Hill. https://accesspharmacy-mhmedical-com.jerome.stjohns.edu/content.aspx?bookid=2569§ionid=210268472
Lopez-Ojeda W, Pandey A, Alhajj M, et al. Anatomy, Skin (Integument) [Updated 2022 Oct 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK441980/
Skin Anatomy and Physiology
The skin is the largest organ of the human body accounting for approximately 15% of the total adult body weight. The skin serves to protect the body from the outside environment by acting as a physical barrier. The skin is a component in the integumentary system and is composed of three main layers: the epidermis, dermis, and the hypodermis. The outermost layer of the skin is the epidermis and is comprised of stratified squamous epithelium that contains four to five layers. The stratum basalis is the deepest layer which contains melanocytes, keratinocytes and stem cells. Melanocytes are responsible for producing melanin, which is what provides our skin with color. The next layer is the stratum spinosum that compromises most of the epidermis and contains several layers of cells that are connected by desmosomes, these allow cells to remain tightly bound to one another. The stratum granulosum is several layers of cells that contain lipid-rich granules. The stratum lucidum exists in the thicker areas of the hands and palms and it consists of immortalized cells. Lastly, the stratum corneum is the outermost layer of the epidermis and serves as the protective overcoat. The keratinization and lipid content of this layer allows regulation of water loss from the body by preventing fluid evaporation. The dermis is a thick layer of connective tissue comprised of collagen and elastin allowing for the strength and flexibility of the skin. In the dermis is where the nerve ending, blood vessels, hair shafts, sweat glands, and sebaceous glands are found. The hypodermis is the deepest layer composed mainly of adipose tissue.
The color of our skin is derived from melanocytes, which are dendritic, pigment-synthesizing cells that are derived from the neural crest and confined predominantly to the basal layer of the skin. Melanocytes are responsible for the production of the pigment melanin and its transfer to keratinocytes. Melanin is produced in a rounded, membrane bound organelle known as the melanosome.
Our skin functions to provide sensation, thermoregulation protection, and lastly metabolism. The receptors in the skin provide its ability to sense changes in the outside environment such as pain, temperature, pressure, and touch. The hair and sweat glands found in the layers of skin provide thermoregulation and regulate the internal temperature of our body to maintain homeostasis. It functions as a protective surface by acting as a barrier between our insides and the outside world protecting it from infection, chemical and thermal stress, and UV light. Lastly, the adipose tissue in the hypodermis layer of the skin is vital in the production of Vitamin D and lipid storage.
The high rate of cell proliferation in the epidermis and in the epithelial tissue as well as the tissue being frequently exposed to physical and chemical damage is what results in the exceedingly high rate of skin cancers found in humans as compared with other types of cancer.
1. Agarwal S, Krishnamurthy K. Histology, Skin. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www-ncbi-nlm-nih-gov.jerome.stjohns.edu/books/NBK537325/
3. Jablonski, Nina G. “The Evolution of Human Skin Pigmentation Involved the Interactions of Genetic, Environmental, and Cultural Variables.” Pigment Cell & Melanoma Research, July 2021, www.ncbi.nlm.nih.gov/pmc/articles/PMC8359960/.
The skin is the first physical barrier to the external environment. It is the organ responsible for protecting the rest of the organs in the body making it very crucial. In addition to that tremendous responsibility, it has a series of other functions including temperature regulation, excretion of toxins, homeostasis, and protection against ultraviolet (UV) light, microorganisms, and pathogens.
At a cellular level, the skin consists of several layers, including the epidermis, dermis, and hypodermis, each serving unique purposes. Epidermal cells act as a barrier against pathogens and environmental stressors. Knowledge of this barrier function is crucial in understanding how autoimmune disorders, like psoriasis or eczema, may arise due to dysregulation in the immune response against self-antigens. Additionally, understanding the antigen-presenting cells in the skin helps unravel the intricate immune interactions that lead to disorders such as vitiligo. Furthermore, the dermis houses blood vessels, lymphatic vessels, and nerve endings, which significantly influence immune responses. Understanding the vascular and neural components of the skin aids in comprehending how autoimmune vasculitis or neuropathic conditions may arise when the immune system erroneously targets these structures. The skin also houses specialized immune cells, such as Langerhans cells and various T cell subsets. These cells play critical roles in immune surveillance and regulation. Dysregulation in these immune cell populations can lead to autoimmune conditions like cutaneous lupus erythematosus and dermatomyositis.
The clinical significance of understanding the layers of skin aids in the deeper understanding of various diseases including skin cancer, autoimmune disorders, infections, and inflammatory disorders. A prominent type of skin cancer is melanoma which is when melanocytes are malignant. As previously mentioned melanocytes are found in the epidermis and are responsible for producing melanin, the substance that gives color to the skin, hair, and eyes; however, while melanin serves as a natural defense against harmful UV radiation from the sun, malfunctioning melanocytes can contribute to the formation of skin cancer. Moreover, melanocytes have the ability to migrate throughout the skin and other tissues, allowing melanoma to metastasize or spread to other parts of the body. Once melanoma cells enter the bloodstream or lymphatic system, they can travel to distant organs, such as the lungs, liver, brain, or bones, and form secondary tumors. The prognosis of stage 0 melanoma is 97% 5-year survival rate and its 10% for those with stage 4. This is a vast range and most people are not diagnosed until it is a later stage. Early detection and prompt treatment are crucial in managing melanoma. Regular skin checks and monitoring moles for any changes in size, shape, color, or symmetry can help identify potential skin cancer in its early stages. Additionally, practicing sun safety measures, such as wearing protective clothing, using sunscreen, and avoiding excessive sun exposure during peak hours, can significantly reduce the risk of melanoma development.
Lopez-Ojeda W, Pandey A, Alhajj M, et al. Anatomy, Skin (Integument) [Updated 2022 Oct 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK441980/
Heistein JB, Acharya U, Mukkamalla SKR. Malignant Melanoma. [Updated 2023 May 22]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470409/
Anatomy of the Skin
The skin is the largest organ in the body, made up of the epidermis, the dermis, and the hypodermis. All three have different purposes and work differently, but come together to primarily be the body’s first defense against potentially dangerous substances. The skin also helps to regulate the body’s internal temperature based on the function of the glands such as sweating. The skin varies at different body parts and accounts for how thick or thin each layer will be. For example, areas with no hair (palms of hands) are the thickest, with an extra layer (stratum lucidum). The back is also considered to be the thickest but lacks the extra layer due to the fine hair follicles present. It makes up for this with a thicker dermis layer.
The epidermis alone is made up of 5 layers; the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum, ranging from the deepest layer to the most superficial layer, respectively. The stratum basale is the deepest layer that separates the dermis from the epidermis, producing keratinocytes and melanocytes. The stratum spinosum contains dendritic cells, which are important for the function of our immune system. The stratum granulosum has keratin precursors which can form bundles and hold the cells together. The stratum lucidum, present on the hairless areas of the body, is the thicker layer that produces keratohyalin. The stratum corneum is mostly made of scales, keratin, and dead keratinocytes. This layer is what causes the fluctuation in skin thickness across the body. For example, dead keratinocytes are what makeup calluses.
The dermis only contains two layers, the papillary layer, and the reticular layer. The papillary layer is thinner and made up of loose connective tissue and is the layer that relates to the epidermis. The reticular layer is the deeper layer which has denser connective tissue and is made up of collagen fibers. The reticular layer holds important functions, such as the hair follicles, sensory neurons, and blood vessels.
The hypodermis under the dermis is the deepest layer which also carries the same functions as the reticular layer. This is also known as the subcutaneous fascia, which is where a sat is stored under the skin. This is also the location of subcutaneous injections, with the needle normally being held at a 45-degree angle to result in this layer. Subcutaneous injections need to be in the fatty layer of the skin in order to be absorbed slower in comparison to getting an injection parenterally or intramuscularly.
The skin continues to evolve, influenced by the environment and what is/isn’t available. When looking at what traits were more important from an evolutionary standpoint, those with enhanced sweating and melanin pigmentation, protect the skin from harmful rays and regulate the body’s temperature. Populations that lack these abilities are actually a more recent evolution, indicating people who are more in need of a vitamin D-rich diet since they cannot produce any with their skin alone.
1. Agarwal S, Krishnamurthy K. Histology, Skin. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www-ncbi-nlm-nih-gov.jerome.stjohns.edu/books/NBK537325/
2. Jablonski, Nina G. “The Evolution of Human Skin Pigmentation Involved the Interactions of Genetic, Environmental, and Cultural Variables.” Pigment Cell & Melanoma Research, July 2021, www.ncbi.nlm.nih.gov/pmc/articles/PMC8359960/.
Skin Anatomy, Physiology, & Evolution
As the largest organ of the body, the skin has an extremely important role as a barrier to the surrounding environment. The skin is composed of three layers from outermost to innermost: epidermis, dermis, and hypodermis. Different areas on the body have thinner or thicker skin depending on their function. Thicker skin on the palms of hands and soles of feet has five layers in the epidermis while other parts of the body with thin skin only have four layers. The five layers of the epidermis include the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. Skin with four layers of epidermis lacks the stratum lucidum. Thinner skin is located on the eyelids, axillary, genitals, and mucosal surfaces. The epidermis contains squamous cells, basal cells, and melanocytes. This layer of skin gives it its color, waterproof function, and is constantly shed.
The dermis is divided into two layers: the papillary dermis and the reticular dermis. This middle layer of skin contains blood and lymph vessels, hair follicles, sweat glands, collagen, nerves, and pain and touch receptors. The hypodermis consists of fat and connective tissue serving as a shock absorber and conserving heat.
The skin is important in protecting the body from the sun's UV rays and the sun also helps produce vitamin D in the skin. Skin also has a role in sensation of touch and pain due to the nerves located in the skin. The skin releases sebum and sweat controlling the body's temperature. The skin acts as a barrier against microorganisms and pathogens that could get in the body. The epidermis is embryologically derived from the ectoderm germ layer and the dermis is derived from the mesoderm layer. There are free nerve endings located in the epidermis that help us respond to light touch, pain, and temperature. The arrector pili muscles located in the skin are located where there are hair follicles on the body. When the body is cold or in “fight or flight” the muscles contract and the hairs raise showing goosebumps.
The thickness of skin is dependent on age and gender. Adult males usually have thicker skin than females. Children typically have thin skin that thickens until the fourth decade of life and then starts thinning again in the fifth decade of life. Aging of the skin is also accelerated by UV rays from the sun. It's important to limit time in the sun to prevent premature aging or skin cancer. Many issues can arise in the skin like acne, skin infections, eczema, psoriasis, allergic reactions and many others. Mutations in the epidermis can cause skin diseases. Loss of function mutations in the FLG gene is a major risk factor for atopic dermatitis.
Skin has dramatically evolved over time. A major change was the loss of hair on the body, hypothesized to be as a defense against lice and ectoparasites or as a way to maintain body temperature. Without hair on the body, the skin became more pigmented with exposure to the sun. Human skin has drastically evolved from primate ancestors and can now be differentiated between different ethnicities and races.
Stanford Medicine Children’s health. Stanford Medicine Children’s Health - Lucile Packard Children’s Hospital Stanford. (n.d.). https://www.stanfordchildrens.org/en/topic/default?id=anatomy-of-the-skin-85-P01336
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Anatomy and Evolution of the Skin
The skin is the largest organ of the human body and serves as a protective physical barrier against the environment. It makes up the integumentary system, which consists of the skin in addition to hair, nails, sweat, and oil glands. The skin consists of 3 layers: the epidermis, dermis, and hypodermis, with each layer containing particular types of cells that maintain dermatologic functions; these include temperature regulation, protection against UV light, external trauma, microorganisms, pathogens, and toxins, as well as sensory perception, fluid regulation, and homeostasis.
The epidermis is the visible dermatologic surface made up of stratified squamous epithelial tissue and functions as the physical exterior. Majority of the epidermis is made up of regenerative keratinocytes, building blocks for the protein keratin, which provide the skin’s structure and durability. These cells are replaced every 4 to 6 weeks. Langerhan cells are mono-nuclear phagocytes originating in the bone marrow and migrate to the epidermis to ingest foreign material and uptake debris from dead cells after an infection. In addition, they interact with resident memory T cells, clear apoptotic keratinocytes, and interact with regulatory T cells, all of which are crucial mechanisms in maintaining immune homeostasis. Tight junctions are formed between keratinocytes and langerhans cells necessary for structure preservation. Merkel cells, on the other hand, are located deep within the epidermis at the layer of basal cells. These cells combine with nerve endings to create a sensory receptor for touch and are able to sense pressure changes.
Within the epidermis are 5 layers (from surface level to deep): stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Epidermal thickness varies depending on the location of the body and is thickest in the palms of the hands and soles of the feet, consisting of five epidermal layers. In contrast, thin layers are made up of four. The stratum corneum, the “horny layer,” is the outermost, roughest layer consisting of 20 to 30 sheets of dead keratinocyte cells. The stratum lucidum is the “clear layer” which holds 2-3 rows of clear, flat, dead keratinocytes that are present in the thick skin of the palms and foot soles (this layer is not present in areas of thin skin). The stratum granulosum is the “granular layer” that contains living keratinocytes and are actively forming keratin. Its granular texture is due to the cellular compression and flattening as these cells move up the epidermal layers upon maturation. Regeneration of skin cells occur in the lower layers and mature as it moves up the epidermal layer. The stratum spinosum layer, the “spiny layer” is near the point where cell regeneration/mitosis is most active.
Lastly, the deepest and thinnest epidermal layer is the stratum basale, “basal layer,” made up of a singular layer of columnar cells. This layer connects the epidermis to the dermis. At the basal level is the presence of melanocytes, which are responsible for skin pigmentation and plays an evolutionary role in the correlation between skin pigmentation and geographics with varying intensities of ultraviolet radiation (UVR), in addition to an individual’s genetic composition and cultural behaviors. Pigmentation and the ability to tan are preferable under high ultraviolet radiation conditions (UVR). Depigmented skin, on the contrary, is associated with environments of low or seasonal UVR conditions. Eumelanin is an inert pigment concentrated within keratinocytes in the stratum basale of the epidermis whose role is to absorb UV photons, particularly UVB, upon exposure to the epidermis as a protective mechanism against carcinogenesis and degradation of folate, an essential B-vitamin required for DNA synthesis regulation and repair. Depending on the wavelength of UVR, the location and keratinization of the skin, and the amount of eumelanin, it can penetrate the skin either at the epidermal or dermal level. Generally speaking, the thicker the layers of skin of the stratum corneum, the more protection is available against UVB. There is evidence that darker skin resulted as an adaptation to protect against UVR-induced degradation of folate in the skin, which can lead to fertility complications.
The dermis sits between the epidermis and the hypodermis layer. Collagen and elastin fibers are present at the dermal level, which are responsible for the skin’s strength and elasticity. Most of the skin’s activities occur at the dermis, since it is full of capillaries and blood vessels, and houses hair follicles, oil and sweat glands, and nerve fibers, which register a multitude of sensations, including temperature, pressure, and pain. Fibroblasts, macrophages, adipocytes, mast cells, Schwann cells, and stem cells constitute the dermis. A critical cellular constituent of the dermis are fibroblasts, which synthesize type I and type III collagen, elastic and reticular fibers, and extracellular matrix material. Other cells present in the dermis include histiocytes, which are tissue macrophages that aid the immune system, and mast cells, which are responsible for the secretion of vasoactive and proinflammatory mediators during an allergic and inflammatory response. Within the dermis are two layers: papillary dermis and reticular dermis. The papillary layer is the upper layer and is composed of a thin sheet of areolar connective tissue with peg-like projection, termed “dermal papillae.” In the thick skin of the hands and feet, these protrusions form friction ridges that press up through the epidermis to aid in grip, hence, is the reason for fingerprints. On the other hand, the deeper and thicker layer is the reticular dermis, which makes up 80% of the dermis and consists of dense irregular connective tissue. The reticular dermis is made up of thick elastic fibers, which allow for gliding, stretching, and recoiling of fibers.
Lastly, the hypodermis layer consists of adipose connective tissue that provides insulation, energy storage, shock absorption, assists in hair follicle regeneration, wound healing, and helps anchor the skin. This is where body fat resides. However, a multitude of diseases, whether acquired via viral or bacterial infection, genetic mutations, or drug-induced, can affect the skin’s function.
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(Integument), 17 Oct. 2022, www.ncbi.nlm.nih.gov/books/NBK441980/.
West, Heather C, and Clare L Bennett. “Redefining the Role of Langerhans Cells as Immune
Regulators within the Skin.” Frontiers in Immunology, 5 Jan. 2018, www.ncbi.nlm.nih.gov/pmc/articles/PMC5770803/#:~:text=Langerhans%20cells%20(LC)%20are%20a,key%20role%20as%20immune%20sentinels.
The skin is the body’s largest organ that protects the body from germs and regulates body temperature. There are 3 layers of skin, the epidermis, dermis, and hypodermis. The epidermis is the top layer of the skin that acts as a protective barrier, keeping bacteria and germs out of the body and providing protection from rain, sun, and other elements. Melanin is in the epidermis, which gives the color of the skin, hair, and eyes. The more melanin a person has, the darker their skin is and they may tan more quickly. The dermis is the middle layer that has the collagen and elastin to make the skin cells strong and resilient. Oil glands in the dermis secret oil to keep the skin soft and smooth, as well as preventing the skin from absorbing too much water. There are also sweat glands in the dermis to release sweat to regulate body temperature. The hypodermis is the bottom fatty layer that cushions muscles and bones, and the fat also helps with regulating temperature. There are connective tissues to connect the skin to muscles and bones in the hypodermis as well. As people age, they lose collagen and elastin, causing the dermis to get thinner. The thinner demeris results in sagging skin and wrinkles. To maintain healthier skin, it’s advised to apply sunscreen every day, avoid tanning, shower regularly, and use gentle cleansers.
Human skin is very different from any other known mammal. The loss of the vibrissae hair cover, but still hairy, is what makes human skin unique. Most human hair is miniaturized and the skin appears to be naked. An insulating layer of body hair is crucial to thermoregulatory energetics of most mammals and only the evolution of naked skin is an association of prevention of hyperthermia in hot climates. All non-human primates have apocrine glands over the entire body. Humans have several million eccrine sweat glands, which helps dissipate body heat with an elaborate cutaneous vascular system. There’s a vestiary hypothesis that proposes the hair reduction in humans evolved with a developing intellectual capacity to use artificial insulation. Hairlessness would permit heat dissipation and whole body evaporation, but would sacrifice heat retention. The necessity was met by clothing.
Skin pigmentation exhibits a gradient variation that tracks with altitude. The gradient is thought to reflect selection for lighter skin pigmentation at higher latitudes because of lower UVB exposure that leads to reduction in vitamin D biosynthesis. Genome-wide association studies have identified well over a hundred pigmentation-associated loci and genomic scans in present-day and ancient populations. Studies of present-day and ancient populations have revealed signatures of selection at skin pigmentation loci, and single-nucleotide polymorphism associated with light skin pigmentation at some of these genes exhibit a signal of polygenic selection in Western Eurasians. However, the only documented signal of polygenic selection for skin pigmentation is based on just 4 loci. There are only little evidence of parallel selection on independent haplotypes at skin pigmentation loci, suggesttng that differences in allele frequency across ancestry groups were mostly because of genetic drift.
Skin: Layers, structure and function. Cleveland Clinic. (n.d.). Retrieved March 8, 2022, from https://my.clevelandclinic.org/health/articles/10978-skin
Journal of Human Evolution. JHE | Journal of Human Evolution | Vol 14, Issue 1, Pages 1-105 (January 1985) | ScienceDirect.com by Elsevier. (n.d.). Retrieved March 8, 2022, from https://www.sciencedirect.com/journal/journal-of-human-evolution/vol/14/issue/1
I;, J. D. M. (n.d.). The evolution of skin pigmentation-associated variation in West Eurasia. Proceedings of the National Academy of Sciences of the United States of America. Retrieved March 8, 2022, from https://pubmed.ncbi.nlm.nih.gov/33443182/
Skin: Anatomy, Physiology, & Evolution
The skin is the largest organ in the human body. It acts as the primary defensive layer of the immune system by preventing infectious organisms from entering the body. When we look at our skin we may not realize it but it is actually multiple layers deep with each layer having its own unique components. The most superficial layer of skin is the epidermis which can be broken down into four or five layers depending upon its location on the body. The deepest layer of the epidermis is the stratum basalis. It contains melanocytes, a single row of keratinocytes, and stem cells. This basal cell layer is the site of mitosis, or proliferation of skin cells. The stratum spinosum is the next layer which comprises most of the epidermis with desmosomes attributing to its tightly bound structure. The stratum granulosum contains lipid-rich granules. Cells in this layer begin to lose their nuclei as they become farther from the nutrients of the deeper layers. The stratum lucidum is a layer of the epidermis that only exists in the thick skin located on the soles and palms and consists of immortalized cells. The most superficial layer of the skin is the stratum corneum which serves as a protective layer, preventing loss of internal fluid to evaporation. Beneath the epidermis is the dermis which is a thick layer of connective tissue containing collagen and elastin allowing for the skin’s durability and elasticity. The dermis is also home to nerve endings, blood vessels and glands (sweat glands and sebaceous glands). Finally, the hypodermis is the deepest layer of skin which consists mostly of adipose tissue.
The skin serves four main functions which are sensation, thermoregulation, protection and metabolism. The skin contains different types of receptors which help us to sense pain, temperature, pressure, and touch. The hair and sweat glands in the skin help to maintain proper body temperature. The skin is a barrier that protects our internal organs against infection, chemical stress, thermal stress, and UV light. The deepest layer of the skin plays an important role in the metabolism of Vitamin D (Agarwal 2021). In Nina Jablonski’s lecture on the evolution of skin color, she explains the idea that the primary selective force for evolution of depigmented skin is the promotion of UV radiation-induced vitamin D production. Depigmented skin, skin with less melanin, is able to produce vitamin D when exposed to UV radiation at a much faster rate than those with pigmented skin containing more melanin. On the other hand, the primary selective force for evolution of dark skin is protection against UV radiation-induced changes in folate availability. Folate is essential for DNA production and cell division. Groups of humans living closer to the equator with increased exposure to UV radiation have evolved to have more melanin in their skin to protect themselves from the harmful effects of UV radiation. This evolution of skin color demonstrates the vital role skin plays in the human body and how it has evolved to adapt to its surroundings.
Agarwal S, Krishnamurthy K. Histology, skin. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK537325/. Published May 10, 2021. Accessed March 8, 2022.
The Evolution and Meanings of Human Skin Color . The Leakey Foundation ; 2020. https://www.youtube.com/watch?v=sc4OFcT5m1Y. Accessed March 8, 2022.
Anatomy and Evolution of Skin
The skin is the largest organ of the body- making up about 15% of the total adult body weight. There are three main layers of the skin: the epidermis, the dermis and the hypodermis. The outermost layer is the epidermis which contains four to five layers depending on its location: stratum basale (the deepest portion), stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum (the most superficial portion). In the epidermis, there are keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Then, lies the dermis which consists of two layers, the papillary layer and the reticular layer. It contains collagen, elastin, nerve endings, blood vessels, and adnexal structures such as hair shafts, sweat glands, and sebaceous glands. The deepest layer is the hypodermis which consists mainly of adipose tissue which provides padding and cushioning to protect our internal organs, bones and muscles.
The skin has many functions essential to maintaining homeostasis, protection and social interaction such as protection, thermoregulation, sensation, water storage, absorption, expression and synthesis of vitamin D. The skin serves as the first line of defense against the environment, therefore it must evolve to provide an optimal barrier for the survival of an organism.
The most obvious change to the human skin barrier is skin pigmentation. Melanin is produced by melanocytes, found in the stratum basale, and is responsible for the pigment of the skin. There are two forms of melanin, pheomelanin (yellow-reddish) and eumelanin (black-brown). Pheomelanin is mainly accumulated in lightly-pigmented skin and eumelanin is mostly produced in darkly-pigmented skin. However, the proportion of the two forms of melanin is not the only determinant of skin color, the number and size of melanin particles are also important. Besides melanin, carotene and hemoglobin also affect skin color. Carotene is found in the stratum corneum of the epidermis and the hypodermis and is yellow-orange pigment. The skin may turn this color due to a carotene-rich diet. Hemoglobin is found in the blood vessels of the dermis and is the iron-containing protein pigment of our blood cells. A lack of oxygen-saturated hemoglobin would lead to paler, grayer or bluer color to the skin. Contrarily, oxygen-rich hemoglobin would result in a rosy effect on the skin.
Skin color variation is mainly due to the effects of UV radiation on the skin. Less UV radiation is transmitted through darkly-pigmented skin than lightly-pigmented skin because melanin acts as a built-in sunscreen. Populations closer to the equator tend to have dark skin to protect against UV radiation because overexposure can lead to decrease folic acid levels and skin cancer. Human migration out of Africa into higher latitudes such as Europe and Asia exposed humans to environments with substantially lower UV exposure. To maximize vitamin D synthesis which is a UV-dependent process, these populations evolved lighter skin to absorb more UV radiation. There are many mutations that contributed to the lightening of human skin, such as skin pigment genes, SLC45A2 and SLC24A5 which exhibit higher allele frequencies in Europeans than in Africans and East Asians, and MC1R which plays a key role in controlling the switch from pheomelanin to eumelanin.
1. Agarwal S, Krishnamurthy K. Histology, Skin. [Updated 2021 May 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www-ncbi-nlm-nih-gov.jerome.stjohns.edu/books/NBK537325/
2. Brettmann EA, de Guzman Strong C. Recent evolution of the human skin barrier. Exp Dermatol. 2018;27(8):859-866. doi:10.1111/exd.13689
3. Deng, L., Xu, S. Adaptation of human skin color in various populations. Hereditas 155, 1 (2018). https://doi.org/10.1186/s41065-017-0036-2
4. McKnight G, Shah J, Hargest R. Physiology of the skin, Surgery (Oxford) 2022; 40(1):8-12
5. “The Skin.” Lumen Boundless Anatomy and Physiology, courses.lumenlearning.com/boundless-ap/chapter/the-skin/.
6. Yousef H, Alhajj M, Sharma S. Anatomy, Skin (Integument), Epidermis. In: StatPearls. Treasure Island (FL): StatPearls Publishing; November 19, 2021.
The skin is one of the most important organs for our health, but people often do not think of taking care of the skin as much as other organs. The skin protects our internal organs from foreign particles and pathogens. It serves as a critical barrier, and the structure and function is quite complex. There are layers of the skin: the epidermis, the dermis, and the hypodermis.
The epidermis is the outermost layer of the skin, and it contains the cells that make up the color of our skin. These cells, called melanocytes, produce melanin. Melanin gives our skin color. The more melanin a person has, the darker their skin tone will be. These melanocytes are located at the bottom most part of the epidermis, and these cells also sit close to the dermis. The outermost part of the epidermis is the stratum corneum, and it is a keratinized layer of skin that is responsible for protection and fluid regulation. The stratum corneum keeps our internal fluid from evaporating, and it is critical in maintaining homeostasis. 1
The dermis is the layer of skin that lies just below the epidermis. It contains collagen and elastin, which are two chemicals that are critical in maintaining the stretch and flexibility of the skin. Without these two chemicals, our skin would be very rigid and fragile, and they play a critical role in maintaining skin structure. The dermis also contains nerve endings, blood vessels, hair follicles, sweat and oil glands. These different skin structures are critical in our sensitization, blood flow, and sweat and oil secretion.1
The hypodermis is the layer of skin that contains fat cells. It is mostly adipose tissue, and it represents the deepest level of skin that humans contain.1
Have you ever wondered why people from different parts of the world have different skin tones? Skin color often varies in people in different continents, countries, and even cultures, but why do we care? Skin color has been a major area of scientific research, as there are so many different skin colors. There are two types of melanocytes, which control skin color. Pheomelanin is a chemical that often produces a red or yellow color. Eumelanin produces more brown and darker skin tones. Skin color is often well correlated with the proximity to the equator. The closer populations are to the equator, the darker their skin color. The color is due to the amount of reflectance needed to protect the skin from UV lights. Skin reflectance decreases 8% for every 10 degrees into the Northern hemisphere. Skin color is correlated with distance to the equator due to the level of sun protection that is needed closer to the equator.2
Higher levels of melanin have been linked to increased protection from the dangerous UV rays. Photo damage to the DNA in the skin is one of the major causes of skin cancer. People with higher amounts of melanin are linked to less DNA damage, and decreased incidence of skin cancer. This suggests that people with fair skin and lower amount of melanin are at increased risk for melanoma and other skin cancers.3
Agarwal S, Krishnamurthy K. Histology, Skin. [Updated 2021 May 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537325/
Barsh GS. What controls variation in human skin color? [published correction appears in PLoS Biol. 2003 Dec;1(3):445]. PLoS Biol. 2003;1(1):E27.
Fajuyigbe D, Young AR. The impact of skin colour on human photobiological responses. Pigment Cell Melanoma Res. 2016;29(6):607-618.
A Deeper Look into Our Skin
The skin is the largest organ of the body. It has one of the most important functions for the body, acting as our initial barrier against a myriad of things such as pathogens, UV light and physical injury, etc. Our skin is composed of three primary layers, epidermis, dermis and hypodermis. Starting from the innermost layer, the hypodermis contains a layer fat which acts as a cushion, protecting our internal organs, bones and muscles. Next is the dermis which is made up of two layers, the papillary and reticular layer. The dermis consists of sweat glands, hair follicles, muscles, collagen fibers, and blood vessels. Lastly, we have the epidermis which is comprised of five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. In the stratum basale lies melanocytes which play a critical role in determining our skin color. Melanocytes store a pigment called melanin. There are two types of melanin- eumelanin which is responsible for black/brown pigment and pheomelanin responsible for red/yellow pigment. People with darker skin have more active melanocytes compared to people with lighter skin.
So how exactly do we all have different skin colors? Well, originally we all had dark skin but when people started migrating out of Africa to Europe, our genetics had to acclimate to the surroundings and changes occurred. In areas close to the equator, high levels of UV are able to penetrate dark skin to provide an adequate vitamin D. But those who migrated were not able to absorb enough UV as the rays were not able to penetrate their melanin. Thus, vitamin D levels decreased resulting in compromised health. The evolutionary response was a decrease in pigmentation for individuals populating areas where not much sunlight was available. Research showed early people in Spain and Hungary lacked versions of two genes SLC24A5 and SLC45A2 which were key for pigmentation, therefore leading to the pale skin seen in Europeans today.
Besides melanin, there are other components that can affect our skin color. One is the amount of carotene which is yellow-orange pigment found in the stratum corneum of the epidermis and the hypodermis. Our carotene levels are affected by our diet intake, if the foods are rich in carotene such as carrots. Another element is the amount of oxygen-rich, protein pigment hemoglobin found in blood vessels. Decreased levels of hemoglobin otherwise known as anemia result in paler skin. Also light skinned people, may depict rosier hues due to the
more oxygen-rich hemoglobin in the blood cells circulating their dermis.
Gibbons, Ann, et al. “How Europeans Evolved White Skin.” Science, 10 Dec. 2017
“The Skin.” Lumen Boundless Anatomy and Physiology, courses.lumenlearning.com/boundless-ap/chapter/the-skin/.
Yousef, Hani. “Anatomy, Skin (Integument), Epidermis.” StatPearls [Internet]., U.S. National Library of Medicine, 26 July 2021, www.ncbi.nlm.nih.gov/books/NBK470464/.
Skincare Routines (Products and Ingredients)
Written by Tommy Li and Jerry Lau
Taking care of your skin has been a human habit since the beginning of civilization. With both men and women trying a variety of products to keep discolorations, acne, and wrinkles at bay. Women in ancient Rome used face masks, the ancient Greeks used cold cream, and the ancient Egyptians used an ointment moisturizer (1). Thanks to the power of the internet, the world’s population has been exposed to celebrities and influencers with flawless skin; this has led to an explosion of growth in the beauty industry with a plethora of products to sell. As consumers, it is tough to weed out the products that actually work as advertised and not break the bank at the same time. This piece hopes to give clearer information on what is needed in a skincare routine and what products are available to you.
All skincare routines should have these essential steps: protection, prevention, cleaning, and moisturizing. This routine should be done consistently and for a time before results are revealed. Any product that promises otherwise is not a trustworthy product or is making too bold of a claim. Two of the most important factors of a routine are protection and prevention. Daily use of sunscreen is important whenever you go out, as sun damage results from everyday, incidental ultraviolet exposure. Dermatologists recommend sunscreen that has either the active ingredient zinc oxide or avobenzone for blocking out ultraviolet A and ultraviolet B.
The other factors, cleaning and moisturizing, are also important. Dermatologists recommend products that specify which skin type is formulated for: dry, oily, combination; this information combined with evidence of clinical testing with before and after photos that is readily available to the consumer indicates if a product should be recommended or not (2). Oily skin type requires gel-based and bar cleansers while dry skin type better uses cream or lotion-based ones.
There are other important processes of skin care besides protection, prevention, cleaning, and moisturizing. First, improving texture and tone is the key to youthful skin with radiance. Radiance decreases as people age (2). Toning products can help to remove excess corneocyte buildup by exfoliation. Using toning products can stimulate cell turnover and polish a smoother surface. Then, the aging of skin will nevertheless emerge. Noticeable contour, firmness, wrinkling, and lost of elasticity changes will come out and say hello. Vitamin A related products have been used to redensificate skin by upregulation through collagen and glycosaminoglycans. Lastly, keeping balance of the skin and managing sensitivity are crucial to a perfect skin on your own.
A perfect skin is the most universally desired gift as a human feature. Taking care of skin is a long-term mission. Humans tend to focus on certain aspects of problems, and neglect the overall picture. Skin care is advanced from the basic and expanded to a higher level as human society develops. By enriching our knowledge and using our intelligence, we can help an increased number of patients with skin problems. Solutions are always there to help our patients to maximize their life quality and beauty.
Claudia D. Through the Ages: A Brief History of Skincare. L’Oreal. skincare.com/article/history-of-skin-care. Accessed 31 Aug 2021.
Rodan K, Fields K, Majewski G, Falla T. Skincare Bootcamp: The Evolving Role of Skincare. Plast Reconstr Surg Glob Open. 2016;4(12 Suppl Anatomy and Safety in Cosmetic Medicine: Cosmetic Bootcamp):e1152. Published 2016 Dec 14.
Thank you for your contribution, excellent list of references. Well done!
Natalie Eshaghian & Donna Salib
Anatomy & Evolution of the Skin
Many people do not realize that our skin is considered an organ of our body, let alone one of the largest organs of the human body. Our skin is made up of 3 layers: the epidermis, dermis, and hypodermis. The surface layer of the skin is the epidermis, which consists of hair and the sweat pores. The dermis is the largest layer of the skin, consisting of nerves, hair bulbs, sweat glands, and arteries and veins. Lastly, the hypodermis is the lowest layer of the skin, which has the adipose tissue (also known as the body fat).1,2
Melanocytes are located in the bottom layer of the epidermis and are responsible for producing the protective skin-darkening pigment,melanin.3 Melanocytes take two basic forms: eumelanin and pheomelanin. Eumelanin gives rise to a range of brown skin tones while pheomelanin attributes to freckles and reddish brown hair. Skin tones are correlated with latitude and levels of UV exposure. Through this exposure, our body creates melanin through tyrosine conversion. Melanin is essentially our body's natural sunscreen, protecting us from the natural UV exposure of the sun. The science of skin color has been a natural phenomenon that has been explored since Charles Darwin’s time. Based on his observations and collected data, it was concluded that human skin color was darker at the equator and lighter as people migrated towards the poles of the earth. Humans evolved in Africa, a region saturated by UV rays so to cope with the exposure of UV rays, the type and amount of melanin determined how protected they were from the sun.4 Humans living in the sun-saturated regions of Africa adapted to have higher melanin and eumelanin production giving the skin a darker tone which helped protect them from melanoma. When these sun-adapted humans migrated northward, away from the tropical environment, they were exposed to less sun therefore less melanin was produced. It’s important to realize that humans who had more melanin production not only allowed for less UV penetration but were susceptible to vitamin D-deficiency.5 Vitamin D is associated with strengthening our bones and our immune system. Without it, humans can experience fatigue and osteoporosis.
Our bodies are smart enough to develop and/or mask certain genes to allow us to survive. It was discovered that due to our distance to the equator, our body would develop or lack certain genes to allow us to have certain vitamins.6 Studies have shown that there was a lack of genes known as SLC24A4 and SLC45A2.7 The people who lacked these genes were found to live in areas near the equator, where UV exposure is very high. Their bodies were able to adapt to their environment and allow them to have this lack in gene exposure to cause them to have darker skin tones. With this darker skin tone, it prevented them from absorbing the high UV exposure that they would experience near the equator and naturally protect themselves from the harmful exposure. In addition, it was discovered that people who lived farther from the equator were more pale in skin tone. This was due to their prominence in the SLC24A4 and SLC45A2 genes. These genes allowed their pale skin to absorb the UV exposure more easily since they are less prone to the exposure due to their distance from the equator. Since their bodies can absorb the UV exposure more easily, they are able to naturally create vitamin D in their bodies despite the lack of UV exposure. Although skin tone can be affected based on the location of a person, it can also be determined by their diets as well. Our bodies find different ways to create vitamin D, such as through our diet. Diets rich in vitamin D, such as milk and fish, can allow the body to reach adequate levels of vitamin D despite their lack of UV rays.
Through the discovery of the skin’s anatomy and physiology, you can see how complex our skin is and its importance in our daily lives. The skin not only helps us as a protectant, but it serves as a tool to gain the adequate vitamins and nutrients to survive. In addition, people should be careful to prevent vitamin D deficiency. It is important to keep your skin safe from harmful substances, including excessive amounts of sun. Depending on your residence, it's important to limit your exposure to UV rays depending on your skin tones. In short, recognizing the evolution and adaptive processes of our skin is essential to help us define certain skin conditions and develop treatments suited for all skin types.
1.Agarwal S, Krishnamurthy K. Histology, Skin. In: StatPearls. Treasure Island (FL): StatPearls Publishing; May 10, 2021.https://pubmed.ncbi.nlm.nih.gov/30726010/
2.Yousef H, Alhajj M, Sharma S. Anatomy, Skin (Integument), Epidermis. [Updated 2020 Jul 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470464/
3.Schlessinger DI, Anoruo MD, Schlessinger J. Biochemistry, Melanin. [Updated 2021 May 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459156/
4.Deng, L., Xu, S. Adaptation of human skin color in various populations. Hereditas 155, 1 (2018). https://doi.org/10.1186/s41065-017-0036-2 https://hereditasjournal.biomedcentral.com/articles/10.1186/s41065-017-0036-2
5. Webb AR, Kazantzidis A, Kift RC, Farrar MD, Wilkinson J, Rhodes LE. Colour Counts: Sunlight and Skin Type as Drivers of Vitamin D Deficiency at UK Latitudes. Nutrients. 2018;10(4):457. Published 2018 Apr 7. doi:10.3390/nu10040457 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5946242/
6. The Smithsonian Institution's Human Origins Program. Modern Human Diversity. Skin Color. Date Accessed: July 19, 2017 https://humanorigins.si.edu/evidence/genetics/human-skin-color-variation/modern-human-diversity-skin-color
7. Ann Gibbons. How Europeans evolved white skin. Science. 2017
Have you ever wondered how and why people have different skin tones? Why is it that lighter and darker skin tones and everything in between are associated with different countries and continents as a whole? The Evolution of the Skin. You might have heard of something called melanin which plays a role in the pigment of the skin. There are two different types of melanin; the skin tone is dependent on the different amounts and types of melanin that is produced in the skin. Eumelanin and Phaeomelanin are the two types of melanin. Eumelanin produces a brown or black color while Phaeomelanin produces yellow or red color. High levels of eumelanin will produce a darker skin color while high levels of phaeomelanin produces a lighter skin color. No or little levels of eumelanin or phaeomelanin is the result of albinism (a skin condition in which a person has no or little melanin levels which results in a pale skin color).
Now that we know the different melanin types that are responsible for the skin tone, let’s discuss the adaptation of skin tone in different parts of the world. There are a few factors that contribute to the variation in skin tone. We will start with the effect of Ultraviolet (UV) radiation on different populations. The closer you are to the equator, the more you are exposed to the UV rays of the sun. People closer to the equator tend to have darker skin in order to protect them against the UV rays of the sun and the way this works is due to the excess production of eumelanin. Eumelanin acts as natural form of sunscreen to protect from the harmful effects that the UV rays can cause to a person especially those living near the equator. You are more likely to notice countries in South America, Africa and South Asia populated with people with darker skin pigmentation. In more European and North American countries are more populated with people with lighter skin pigmentation. The UV rays in these countries are not as strong as the ones closer to the equator therefore, there is less melanin produced.
Vitamin D also plays a role in the skin. The sun is a great source for Vitamin D therefore people who live in countries that are not exposed to the UV rays of the sun as much may become Vitamin D deficient and would need supplementation of Vitamin D in their diet.
The evolution of the skin to adapt to the UV rays in different parts of the world is not solely due to melanin production, it is also due to certain genes related to the evolution of skin color. Although not well studied, there are genes such as the SLC24A5 and SLC45A2 that were found in European lighter skin population and it was not found in the darker skin population. Gene expression is part of evolution and it can help with adaption to the environment that is being inhabited.
It is important to understand that those who lack pigmentation and move to areas with high UV rays, need to protect their skin from absorbing too much of the radiation. Similarly, those who have high pigmentation and move to an area with low UV rays need to protect themselves against any deficiencies such as Vitamin D and calcium. Overall migration and adaptation have played a role in the evolution of the skin.
Deng, Lian, and Shuhua Xu. “Adaptation of Human Skin Color in Various Populations.” Hereditas, BioMed Central, 15 June 2017, www.ncbi.nlm.nih.gov/pmc/articles/PMC5502412/.
“Guns Germs & Steel. The World - The Equator.” PBS, Public Broadcasting Service, www.pbs.org/gunsgermssteel/world/pup/world_equator.html.
“Hair and Skin Colour.” Hair and Skin Colour | DermNet NZ, dermnetnz.org/topics/hair-and-skin-colour/.
The functions of the skin are part of everyone’s makeup – protection, thermoregulation, osmoregulation, excretion, senses, and absorption. However, skin color is an aspect that is different for every human and can range from a very dark brown to a near yellow-pink. One’s skin can also vary in the thickness, temperature and the degree to which it is vascularized. Melanin, which is produced by melanocytes in the epidermis, is responsible for skin color. In places on the body that are lighter and thicker, like the palms of the hands and soles of the feet, melanin is lacking. Eumelanin is responsible for black/brown pigment, while pheomelanin is responsible for red/yellow pigment, and genes determine which type of melanin will dominate.
When looking at the evolution of skin color, it has been proposed that lighter skin arose in Europe when humans left Africa. They believed that lighter skin was needed in higher latitudes to synthesize Vitamin D in places where UV light was lower, but newer studies suggest latitude was not the only factor to drive the evolution of skin color. Diet is thought to have played a role, contributing the lack of Vitamin D to the loss of dark skin pigmentation. Genetic mutations in fairer-skinned Europeans showed a compromised skin barrier that allowed them to live at higher latitudes. They studied a skin barrier protein that showed a higher rate in this population, and it is called filaggrin. Filaggrin is broken down into urocanic acid, which is the most potent absorber of UVB light. The strongest hypothesis for the evolution of darker skin, is that is allowed protection against photodegradation of cutaneous and systemic folate under high UVR conditions for early humans in Africa.
Depigmented skin has evolved more times than pigmented skin in human history. There are various genetic mutations that contributed, and it’s not fully understood why mutations for things like blue eyes developed (the HERC2/OCA2 gene). The same gene that contributes to blue eyes is believed to contribute to pale skin and blond hair, combinations seen in hunter gatherers in Sweden. These traits swept over Europe as more breeding happened over time. Overall the skin color has evolved into two processes: one favoring dark pigmentation and photoprotection against high loads of UVA and UVB near the equator, the other emphasizing depigmented skin to promote seasonal, UVB-induced photosynthesis of vitamin D3 nearer the poles (Jablonski).
Due to a multitude of discoveries about skin color evolution, the color can be predicted based on location. When humans migrated further from the equator, skin color adapted in order for skin to absorb UV light better. This was necessary because melanin is able to block some UV light (not all) and was lacking in people at a greater distance from the equator. It was helpful to learn how skin color has evolved and become what it is today.
Jablonski Nina G. and Chaplin George. The colours of humanity: the evolution of pigmentation in the human lineage. Phil. Trans. R. Soc. 2017. B37220160349