![\"10.3.1\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Sunburn-2.jpg\")
Figure 10.3.1 Feel the burn!<\/em>[\/caption]\r\n\r\nWhat is the\u00a0Epidermis?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5991\"]epidermis[\/pb_glossary]<\/strong>\u00a0is the outer of the two main layers of the [pb_glossary id=\"3521\"]skin[\/pb_glossary]. The inner layer\u00a0is\u00a0the\u00a0[pb_glossary id=\"5967\"]dermis[\/pb_glossary]. It averages about 0.10 mm thick, and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with \u2014 but structurally distinct from \u2014 the [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary]\u00a0that line the mouth, anus, urethra, and vagina.\r\n
What is the\u00a0Epidermis?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5991\"]epidermis[\/pb_glossary]<\/strong>\u00a0is the outer of the two main layers of the [pb_glossary id=\"3521\"]skin[\/pb_glossary]. The inner layer\u00a0is\u00a0the\u00a0[pb_glossary id=\"5967\"]dermis[\/pb_glossary]. It averages about 0.10 mm thick, and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with \u2014 but structurally distinct from \u2014 the [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary]\u00a0that line the mouth, anus, urethra, and vagina.\r\n\r\nStructure of the Epidermis<\/h1>\r\n<\/div>\r\nThere are no blood vessels and very few nerve cells in the epidermis. Without blood to bring epidermal cells oxygen and nutrients, the cells must absorb oxygen directly from the air and obtain nutrients via [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] of fluids from the dermis below. However, as thin as it is, the epidermis still has a complex structure. It has a variety of cell types and multiple layers.\r\nCells\u00a0of the Epidermis<\/h2>\r\nThere are several different types of\u00a0cells\u00a0in the epidermis. All of the cells are necessary for the important functions of the epidermis.\r\n\r\n \t- The epidermis consists mainly of stacks of [pb_glossary id=\"3525\"]keratin[\/pb_glossary]-producing epithelial cells called\u00a0[pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary]<\/strong>. These cells make up at least 90 per cent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.<\/li>\r\n \t
- Another eight per cent of epidermal cells are [pb_glossary id=\"3526\"]melanocytes[\/pb_glossary]<\/strong>. These cells produce the pigment melanin that protects the\u00a0dermis\u00a0from UV light.<\/li>\r\n \t
- About one per cent of epidermal cells are [pb_glossary id=\"3540\"]Langerhans cells[\/pb_glossary]<\/strong>. These are immune system cells that detect and fight pathogens entering the skin.<\/li>\r\n \t
- Less than one per cent of epidermal cells are [pb_glossary id=\"3527\"]Merkel cells[\/pb_glossary]<\/strong>, which respond to light touch and connect to nerve endings in the dermis.<\/li>\r\n<\/ul>\r\n
Layers of the Epidermis<\/h2>\r\nThe epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis is thicker than in the rest of the body. The layers of the epidermis are shown in Figure 10.3.2, and described in the following text.\r\n\r\n[caption id=\"attachment_3541\" align=\"aligncenter\" width=\"1024\"]![\"Diagram](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0353_Epidermis-2.png\")
Figure 10.3.2 The epidermis has multiple layers, and structures (such as hairs from the dermis below it) pass through them. This diagram illustrates the five layers that exist on the palms and soles of the feet.<\/em>[\/caption]\r\nStratum Basale<\/h3>\r\nThe\u00a0[pb_glossary id=\"3542\"]stratum basale[\/pb_glossary]<\/strong>\u00a0is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the\u00a0[pb_glossary id=\"5907\"]basement membrane[\/pb_glossary]<\/strong>. The stratum basale contains stem cells \u2014 called\u00a0[pb_glossary id=\"5903\"]basal cells[\/pb_glossary]<\/strong>\u00a0\u2014 which divide to form all the [pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary] of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basale. [pb_glossary id=\"3545\"]Melanocytes[\/pb_glossary]\u00a0and [pb_glossary id=\"3527\"]Merkel cells[\/pb_glossary]\u00a0are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.\r\nStratum Spinosum<\/h3>\r\nJust above the stratum basale is the\u00a0[pb_glossary id=\"3546\"]stratum spinosum[\/pb_glossary]<\/strong>. This is the\u00a0thickest\u00a0of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active [pb_glossary id=\"3540\"]Langerhans cells[\/pb_glossary].\r\nStratum Granulosum<\/h3>\r\nThe next layer above the stratum spinosum is the\u00a0[pb_glossary id=\"3547\"]stratum granulosum[\/pb_glossary]<\/strong>. In this layer, keratinocytes have become nearly filled with [pb_glossary id=\"3525\"]keratin[\/pb_glossary], giving their cytoplasm a granular appearance.\u00a0[pb_glossary id=\"5651\"]Lipids[\/pb_glossary]\u00a0are released by keratinocytes in this layer to form a\u00a0lipid\u00a0barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from\u00a0blood vessels\u00a0in the dermis to receive\u00a0nutrients. Each dying cell digests its own\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]\u00a0and\u00a0[pb_glossary id=\"5557\"]organelles[\/pb_glossary], leaving behind only a tough, keratin-filled shell.\r\nStratum Lucidum<\/h3>\r\nOnly on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the\u00a0[pb_glossary id=\"3548\"]stratum lucidum[\/pb_glossary]<\/strong>. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.\r\nStratum Corneum<\/h3>\r\nThe uppermost layer of the epidermis everywhere on the body is the\u00a0[pb_glossary id=\"3549\"]stratum corneum[\/pb_glossary]<\/strong>. This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basale to make their way to the top of the stratum corneum to replace shed cells.\r\n\r\nFunctions of the Epidermis<\/h1>\r\n<\/div>\r\nThe epidermis has several crucial functions in the body. These functions include protection,\u00a0water\u00a0retention, and vitamin D synthesis.\r\nProtective Functions<\/h2>\r\nThe epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.\r\nProtection from Physical Damage<\/h3>\r\nMost of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.\r\nProtection from Pathogens<\/h3>\r\n[caption id=\"attachment_3550\" align=\"alignright\" width=\"337\"]![\"Skin:](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Scraped-knee-by-Alpha-on-flickr-2.jpg\")
Figure 10.3.3 This scrape on the knee provides an opportunity for bacteria to enter the body through the broken skin.<\/em>[\/caption]\r\n\r\nWhen pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape (like the one pictured in Figure 10.3.3). That\u2019s why it is important to clean and cover even a minor<\/em> wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH\u00a0of about 5.0), low amounts of\u00a0water, the presence of antimicrobial substances produced by epidermal cells, and\u00a0competition\u00a0with non-pathogenic microorganisms that normally live on the epidermis.\r\n\r\n \r\nProtection from UV Light<\/h3>\r\n[pb_glossary id=\"3537\"]UV light[\/pb_glossary]\u00a0that penetrates the epidermis can damage epidermal cells. In particular, it can cause\u00a0mutations\u00a0in\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]\u00a0that lead to the\u00a0development\u00a0of\u00a0skin [pb_glossary id=\"5605\"]cancer[\/pb_glossary], in which epidermal cells grow out of control. UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful\u00a0reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body\u2019s vitamin B9 level by 50 per cent.<\/em>\r\n\r\n[pb_glossary id=\"3526\"]Melanocyte[\/pb_glossary]s in the stratum basale of the epidermis contain small\u00a0organelles\u00a0called\u00a0[pb_glossary id=\"5851\"]melanosomes[\/pb_glossary]<\/strong>, which produce, store, and transport the dark brown pigment [pb_glossary id=\"5845\"]melanin[\/pb_glossary]. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to [pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary]\u00a0in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin, where it can cause damage. The more melanin there is in the skin, the more UV light can be absorbed.\r\nWater\u00a0Retention<\/h2>\r\nSkin's ability\u00a0to hold water and not lose it to the surrounding environment is due mainly to the [pb_glossary id=\"3549\"]stratum corneum[\/pb_glossary].\u00a0[pb_glossary id=\"5651\"]Lipids[\/pb_glossary]\u00a0arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.\r\n\r\nAlthough the skin is impermeable to water, it is not impermeable to\u00a0all <\/em>substances. Instead, the skin is\u00a0[pb_glossary id=\"5859\"]selectively<\/em>\u00a0permeable[\/pb_glossary], allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.\r\n\r\n \t- Selective permeability allows certain medications to enter the bloodstream through the capillaries in the [pb_glossary id=\"5967\"]dermis[\/pb_glossary]. This is the basis of medications that are delivered using topical ointments, or patches (see Figure 10.3.4) that are applied to the skin. These include steroid hormones, such as [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).<\/li>\r\n \t
- Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead, as well as many pesticides.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_3554\" align=\"aligncenter\" width=\"394\"]
![\"Skin](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Nicoderm-2.jpg\")
Figure 10.3.4 This skin patch delivers small amounts of nicotine through the skin of a person in a smoking cessation program.<\/em>[\/caption]\r\nVitamin D Synthesis<\/h3>\r\nVitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basale and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.\r\n\r\nWhat Gives Skin Its Colour?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5845\"]Melanin[\/pb_glossary]<\/strong> in the epidermis is the main substance that determines the colour of human skin. It explains most of the variation in skin colour in people around the world. Two other substances also contribute to skin colour, however, especially in light-skinned people: carotene and hemoglobin.\r\n\r\n \t- The pigment\u00a0[pb_glossary id=\"5841\"]carotene[\/pb_glossary]<\/strong>\u00a0is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.<\/li>\r\n \t
- [pb_glossary id=\"3556\"]Hemoglobin[\/pb_glossary]<\/strong> is a red pigment found in red\u00a0blood\u00a0cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink colour is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.<\/li>\r\n<\/ul>\r\nHear what Bill Nye has to say about the subject of skin colour in the video here<\/a>.\r\n\r\n
Bacteria\u00a0on Skin<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_3557\" align=\"alignleft\" width=\"251\"]![\"Skin](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Staphylococcus-aureus-bacteria-by-Microbe-World-on-flickr-2.jpg\")
Figure 10.3.5 The bacterium Staphylococcus epidermidis is a common microorganism living on healthy human skin.<\/em>[\/caption]\r\n\r\nThe surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly one thousand bacterial species (including the one in Figure 10.3.5) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.\r\n\r\nIn general, the normal microorganisms living on the skin keep one another in check, and thereby play an important role in keeping the skin healthy.\u00a0If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain\u00a0species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas (such as\u00a0hair\u00a0follicles) and in adjacent areas of the skin.\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\nBecause of the negative health effects of excessive UV light exposure, it is important to know the facts about protecting the skin from UV light.\r\n
Structure of the Epidermis<\/h1>\r\n<\/div>\r\nThere are no blood vessels and very few nerve cells in the epidermis. Without blood to bring epidermal cells oxygen and nutrients, the cells must absorb oxygen directly from the air and obtain nutrients via [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] of fluids from the dermis below. However, as thin as it is, the epidermis still has a complex structure. It has a variety of cell types and multiple layers.\r\nCells\u00a0of the Epidermis<\/h2>\r\nThere are several different types of\u00a0cells\u00a0in the epidermis. All of the cells are necessary for the important functions of the epidermis.\r\n\r\n \t- The epidermis consists mainly of stacks of [pb_glossary id=\"3525\"]keratin[\/pb_glossary]-producing epithelial cells called\u00a0[pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary]<\/strong>. These cells make up at least 90 per cent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.<\/li>\r\n \t
- Another eight per cent of epidermal cells are [pb_glossary id=\"3526\"]melanocytes[\/pb_glossary]<\/strong>. These cells produce the pigment melanin that protects the\u00a0dermis\u00a0from UV light.<\/li>\r\n \t
- About one per cent of epidermal cells are [pb_glossary id=\"3540\"]Langerhans cells[\/pb_glossary]<\/strong>. These are immune system cells that detect and fight pathogens entering the skin.<\/li>\r\n \t
- Less than one per cent of epidermal cells are [pb_glossary id=\"3527\"]Merkel cells[\/pb_glossary]<\/strong>, which respond to light touch and connect to nerve endings in the dermis.<\/li>\r\n<\/ul>\r\n
Layers of the Epidermis<\/h2>\r\nThe epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis is thicker than in the rest of the body. The layers of the epidermis are shown in Figure 10.3.2, and described in the following text.\r\n\r\n[caption id=\"attachment_3541\" align=\"aligncenter\" width=\"1024\"] Figure 10.3.2 The epidermis has multiple layers, and structures (such as hairs from the dermis below it) pass through them. This diagram illustrates the five layers that exist on the palms and soles of the feet.<\/em>[\/caption]\r\nStratum Basale<\/h3>\r\nThe\u00a0[pb_glossary id=\"3542\"]stratum basale[\/pb_glossary]<\/strong>\u00a0is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the\u00a0[pb_glossary id=\"5907\"]basement membrane[\/pb_glossary]<\/strong>. The stratum basale contains stem cells \u2014 called\u00a0[pb_glossary id=\"5903\"]basal cells[\/pb_glossary]<\/strong>\u00a0\u2014 which divide to form all the [pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary] of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basale. [pb_glossary id=\"3545\"]Melanocytes[\/pb_glossary]\u00a0and [pb_glossary id=\"3527\"]Merkel cells[\/pb_glossary]\u00a0are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.\r\nStratum Spinosum<\/h3>\r\nJust above the stratum basale is the\u00a0[pb_glossary id=\"3546\"]stratum spinosum[\/pb_glossary]<\/strong>. This is the\u00a0thickest\u00a0of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active [pb_glossary id=\"3540\"]Langerhans cells[\/pb_glossary].\r\nStratum Granulosum<\/h3>\r\nThe next layer above the stratum spinosum is the\u00a0[pb_glossary id=\"3547\"]stratum granulosum[\/pb_glossary]<\/strong>. In this layer, keratinocytes have become nearly filled with [pb_glossary id=\"3525\"]keratin[\/pb_glossary], giving their cytoplasm a granular appearance.\u00a0[pb_glossary id=\"5651\"]Lipids[\/pb_glossary]\u00a0are released by keratinocytes in this layer to form a\u00a0lipid\u00a0barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from\u00a0blood vessels\u00a0in the dermis to receive\u00a0nutrients. Each dying cell digests its own\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]\u00a0and\u00a0[pb_glossary id=\"5557\"]organelles[\/pb_glossary], leaving behind only a tough, keratin-filled shell.\r\nStratum Lucidum<\/h3>\r\nOnly on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the\u00a0[pb_glossary id=\"3548\"]stratum lucidum[\/pb_glossary]<\/strong>. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.\r\nStratum Corneum<\/h3>\r\nThe uppermost layer of the epidermis everywhere on the body is the\u00a0[pb_glossary id=\"3549\"]stratum corneum[\/pb_glossary]<\/strong>. This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basale to make their way to the top of the stratum corneum to replace shed cells.\r\n\r\nFunctions of the Epidermis<\/h1>\r\n<\/div>\r\nThe epidermis has several crucial functions in the body. These functions include protection,\u00a0water\u00a0retention, and vitamin D synthesis.\r\nProtective Functions<\/h2>\r\nThe epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.\r\nProtection from Physical Damage<\/h3>\r\nMost of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.\r\nProtection from Pathogens<\/h3>\r\n[caption id=\"attachment_3550\" align=\"alignright\" width=\"337\"] Figure 10.3.3 This scrape on the knee provides an opportunity for bacteria to enter the body through the broken skin.<\/em>[\/caption]\r\n\r\nWhen pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape (like the one pictured in Figure 10.3.3). That\u2019s why it is important to clean and cover even a minor<\/em> wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH\u00a0of about 5.0), low amounts of\u00a0water, the presence of antimicrobial substances produced by epidermal cells, and\u00a0competition\u00a0with non-pathogenic microorganisms that normally live on the epidermis.\r\n\r\n \r\nProtection from UV Light<\/h3>\r\n[pb_glossary id=\"3537\"]UV light[\/pb_glossary]\u00a0that penetrates the epidermis can damage epidermal cells. In particular, it can cause\u00a0mutations\u00a0in\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]\u00a0that lead to the\u00a0development\u00a0of\u00a0skin [pb_glossary id=\"5605\"]cancer[\/pb_glossary], in which epidermal cells grow out of control. UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful\u00a0reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body\u2019s vitamin B9 level by 50 per cent.<\/em>\r\n\r\n[pb_glossary id=\"3526\"]Melanocyte[\/pb_glossary]s in the stratum basale of the epidermis contain small\u00a0organelles\u00a0called\u00a0[pb_glossary id=\"5851\"]melanosomes[\/pb_glossary]<\/strong>, which produce, store, and transport the dark brown pigment [pb_glossary id=\"5845\"]melanin[\/pb_glossary]. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to [pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary]\u00a0in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin, where it can cause damage. The more melanin there is in the skin, the more UV light can be absorbed.\r\nWater\u00a0Retention<\/h2>\r\nSkin's ability\u00a0to hold water and not lose it to the surrounding environment is due mainly to the [pb_glossary id=\"3549\"]stratum corneum[\/pb_glossary].\u00a0[pb_glossary id=\"5651\"]Lipids[\/pb_glossary]\u00a0arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.\r\n\r\nAlthough the skin is impermeable to water, it is not impermeable to\u00a0all <\/em>substances. Instead, the skin is\u00a0[pb_glossary id=\"5859\"]selectively<\/em>\u00a0permeable[\/pb_glossary], allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.\r\n\r\n \t- Selective permeability allows certain medications to enter the bloodstream through the capillaries in the [pb_glossary id=\"5967\"]dermis[\/pb_glossary]. This is the basis of medications that are delivered using topical ointments, or patches (see Figure 10.3.4) that are applied to the skin. These include steroid hormones, such as [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).<\/li>\r\n \t
- Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead, as well as many pesticides.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_3554\" align=\"aligncenter\" width=\"394\"] Figure 10.3.4 This skin patch delivers small amounts of nicotine through the skin of a person in a smoking cessation program.<\/em>[\/caption]\r\n
Vitamin D Synthesis<\/h3>\r\nVitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basale and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.\r\n\r\nWhat Gives Skin Its Colour?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5845\"]Melanin[\/pb_glossary]<\/strong> in the epidermis is the main substance that determines the colour of human skin. It explains most of the variation in skin colour in people around the world. Two other substances also contribute to skin colour, however, especially in light-skinned people: carotene and hemoglobin.\r\n\r\n \t- The pigment\u00a0[pb_glossary id=\"5841\"]carotene[\/pb_glossary]<\/strong>\u00a0is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.<\/li>\r\n \t
- [pb_glossary id=\"3556\"]Hemoglobin[\/pb_glossary]<\/strong> is a red pigment found in red\u00a0blood\u00a0cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink colour is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.<\/li>\r\n<\/ul>\r\nHear what Bill Nye has to say about the subject of skin colour in the video here<\/a>.\r\n\r\n
Bacteria\u00a0on Skin<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_3557\" align=\"alignleft\" width=\"251\"] Figure 10.3.5 The bacterium Staphylococcus epidermidis is a common microorganism living on healthy human skin.<\/em>[\/caption]\r\n\r\nThe surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly one thousand bacterial species (including the one in Figure 10.3.5) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.\r\n\r\nIn general, the normal microorganisms living on the skin keep one another in check, and thereby play an important role in keeping the skin healthy.\u00a0If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain\u00a0species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas (such as\u00a0hair\u00a0follicles) and in adjacent areas of the skin.\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\nBecause of the negative health effects of excessive UV light exposure, it is important to know the facts about protecting the skin from UV light.\r\n
- \r\n \t
- The epidermis consists mainly of stacks of [pb_glossary id=\"3525\"]keratin[\/pb_glossary]-producing epithelial cells called\u00a0[pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary]<\/strong>. These cells make up at least 90 per cent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.<\/li>\r\n \t
- Another eight per cent of epidermal cells are [pb_glossary id=\"3526\"]melanocytes[\/pb_glossary]<\/strong>. These cells produce the pigment melanin that protects the\u00a0dermis\u00a0from UV light.<\/li>\r\n \t
- About one per cent of epidermal cells are [pb_glossary id=\"3540\"]Langerhans cells[\/pb_glossary]<\/strong>. These are immune system cells that detect and fight pathogens entering the skin.<\/li>\r\n \t
- Less than one per cent of epidermal cells are [pb_glossary id=\"3527\"]Merkel cells[\/pb_glossary]<\/strong>, which respond to light touch and connect to nerve endings in the dermis.<\/li>\r\n<\/ul>\r\n
Layers of the Epidermis<\/h2>\r\nThe epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis is thicker than in the rest of the body. The layers of the epidermis are shown in Figure 10.3.2, and described in the following text.\r\n\r\n[caption id=\"attachment_3541\" align=\"aligncenter\" width=\"1024\"]
Figure 10.3.2 The epidermis has multiple layers, and structures (such as hairs from the dermis below it) pass through them. This diagram illustrates the five layers that exist on the palms and soles of the feet.<\/em>[\/caption]\r\nStratum Basale<\/h3>\r\nThe\u00a0[pb_glossary id=\"3542\"]stratum basale[\/pb_glossary]<\/strong>\u00a0is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the\u00a0[pb_glossary id=\"5907\"]basement membrane[\/pb_glossary]<\/strong>. The stratum basale contains stem cells \u2014 called\u00a0[pb_glossary id=\"5903\"]basal cells[\/pb_glossary]<\/strong>\u00a0\u2014 which divide to form all the [pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary] of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basale. [pb_glossary id=\"3545\"]Melanocytes[\/pb_glossary]\u00a0and [pb_glossary id=\"3527\"]Merkel cells[\/pb_glossary]\u00a0are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.\r\n
Stratum Spinosum<\/h3>\r\nJust above the stratum basale is the\u00a0[pb_glossary id=\"3546\"]stratum spinosum[\/pb_glossary]<\/strong>. This is the\u00a0thickest\u00a0of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active [pb_glossary id=\"3540\"]Langerhans cells[\/pb_glossary].\r\n
Stratum Granulosum<\/h3>\r\nThe next layer above the stratum spinosum is the\u00a0[pb_glossary id=\"3547\"]stratum granulosum[\/pb_glossary]<\/strong>. In this layer, keratinocytes have become nearly filled with [pb_glossary id=\"3525\"]keratin[\/pb_glossary], giving their cytoplasm a granular appearance.\u00a0[pb_glossary id=\"5651\"]Lipids[\/pb_glossary]\u00a0are released by keratinocytes in this layer to form a\u00a0lipid\u00a0barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from\u00a0blood vessels\u00a0in the dermis to receive\u00a0nutrients. Each dying cell digests its own\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]\u00a0and\u00a0[pb_glossary id=\"5557\"]organelles[\/pb_glossary], leaving behind only a tough, keratin-filled shell.\r\n
Stratum Lucidum<\/h3>\r\nOnly on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the\u00a0[pb_glossary id=\"3548\"]stratum lucidum[\/pb_glossary]<\/strong>. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.\r\n
Stratum Corneum<\/h3>\r\nThe uppermost layer of the epidermis everywhere on the body is the\u00a0[pb_glossary id=\"3549\"]stratum corneum[\/pb_glossary]<\/strong>. This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basale to make their way to the top of the stratum corneum to replace shed cells.\r\n
\r\nFunctions of the Epidermis<\/h1>\r\n<\/div>\r\nThe epidermis has several crucial functions in the body. These functions include protection,\u00a0water\u00a0retention, and vitamin D synthesis.\r\n
Protective Functions<\/h2>\r\nThe epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.\r\n
Protection from Physical Damage<\/h3>\r\nMost of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.\r\n
Protection from Pathogens<\/h3>\r\n[caption id=\"attachment_3550\" align=\"alignright\" width=\"337\"]
Figure 10.3.3 This scrape on the knee provides an opportunity for bacteria to enter the body through the broken skin.<\/em>[\/caption]\r\n\r\nWhen pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape (like the one pictured in Figure 10.3.3). That\u2019s why it is important to clean and cover even a minor<\/em> wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH\u00a0of about 5.0), low amounts of\u00a0water, the presence of antimicrobial substances produced by epidermal cells, and\u00a0competition\u00a0with non-pathogenic microorganisms that normally live on the epidermis.\r\n\r\n \r\nProtection from UV Light<\/h3>\r\n[pb_glossary id=\"3537\"]UV light[\/pb_glossary]\u00a0that penetrates the epidermis can damage epidermal cells. In particular, it can cause\u00a0mutations\u00a0in\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]\u00a0that lead to the\u00a0development\u00a0of\u00a0skin [pb_glossary id=\"5605\"]cancer[\/pb_glossary], in which epidermal cells grow out of control. UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful\u00a0reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body\u2019s vitamin B9 level by 50 per cent.<\/em>\r\n\r\n[pb_glossary id=\"3526\"]Melanocyte[\/pb_glossary]s in the stratum basale of the epidermis contain small\u00a0organelles\u00a0called\u00a0[pb_glossary id=\"5851\"]melanosomes[\/pb_glossary]<\/strong>, which produce, store, and transport the dark brown pigment [pb_glossary id=\"5845\"]melanin[\/pb_glossary]. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to [pb_glossary id=\"3524\"]keratinocytes[\/pb_glossary]\u00a0in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin, where it can cause damage. The more melanin there is in the skin, the more UV light can be absorbed.\r\n
Water\u00a0Retention<\/h2>\r\nSkin's ability\u00a0to hold water and not lose it to the surrounding environment is due mainly to the [pb_glossary id=\"3549\"]stratum corneum[\/pb_glossary].\u00a0[pb_glossary id=\"5651\"]Lipids[\/pb_glossary]\u00a0arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.\r\n\r\nAlthough the skin is impermeable to water, it is not impermeable to\u00a0all <\/em>substances. Instead, the skin is\u00a0[pb_glossary id=\"5859\"]selectively<\/em>\u00a0permeable[\/pb_glossary], allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.\r\n
- \r\n \t
- Selective permeability allows certain medications to enter the bloodstream through the capillaries in the [pb_glossary id=\"5967\"]dermis[\/pb_glossary]. This is the basis of medications that are delivered using topical ointments, or patches (see Figure 10.3.4) that are applied to the skin. These include steroid hormones, such as [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).<\/li>\r\n \t
- Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead, as well as many pesticides.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_3554\" align=\"aligncenter\" width=\"394\"] Figure 10.3.4 This skin patch delivers small amounts of nicotine through the skin of a person in a smoking cessation program.<\/em>[\/caption]\r\n
Vitamin D Synthesis<\/h3>\r\nVitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basale and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.\r\n
\r\nWhat Gives Skin Its Colour?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5845\"]Melanin[\/pb_glossary]<\/strong> in the epidermis is the main substance that determines the colour of human skin. It explains most of the variation in skin colour in people around the world. Two other substances also contribute to skin colour, however, especially in light-skinned people: carotene and hemoglobin.\r\n
- \r\n \t
- The pigment\u00a0[pb_glossary id=\"5841\"]carotene[\/pb_glossary]<\/strong>\u00a0is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.<\/li>\r\n \t
- [pb_glossary id=\"3556\"]Hemoglobin[\/pb_glossary]<\/strong> is a red pigment found in red\u00a0blood\u00a0cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink colour is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.<\/li>\r\n<\/ul>\r\nHear what Bill Nye has to say about the subject of skin colour in the video here<\/a>.\r\n
\r\nBacteria\u00a0on Skin<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_3557\" align=\"alignleft\" width=\"251\"]
Figure 10.3.5 The bacterium Staphylococcus epidermidis is a common microorganism living on healthy human skin.<\/em>[\/caption]\r\n\r\nThe surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly one thousand bacterial species (including the one in Figure 10.3.5) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.\r\n\r\nIn general, the normal microorganisms living on the skin keep one another in check, and thereby play an important role in keeping the skin healthy.\u00a0If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain\u00a0species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas (such as\u00a0hair\u00a0follicles) and in adjacent areas of the skin.\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\nBecause of the negative health effects of excessive UV light exposure, it is important to know the facts about protecting the skin from UV light.\r\n
- [pb_glossary id=\"3556\"]Hemoglobin[\/pb_glossary]<\/strong> is a red pigment found in red\u00a0blood\u00a0cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink colour is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.<\/li>\r\n<\/ul>\r\nHear what Bill Nye has to say about the subject of skin colour in the video here<\/a>.\r\n
- The pigment\u00a0[pb_glossary id=\"5841\"]carotene[\/pb_glossary]<\/strong>\u00a0is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.<\/li>\r\n \t
- Another eight per cent of epidermal cells are [pb_glossary id=\"3526\"]melanocytes[\/pb_glossary]<\/strong>. These cells produce the pigment melanin that protects the\u00a0dermis\u00a0from UV light.<\/li>\r\n \t
![Sunburn<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Sunburn.jpg\"){kind=link}
![Blausen_0353_Epidermis<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0353_Epidermis.png\"){kind=link}
![Nicoderm<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Nicoderm.JPG\"){kind=link}