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Feel the Burn
The person in Figure (PageIndex{1}) is no doubt feeling the burn — sunburn that is. Sunburn occurs when the outer layer of the skin is damaged by UV light from the sun or tanning lamps. Some people deliberately allow UV light to burn their skin because after the redness subsides, they are left with a tan. A tan may look healthy, but it is actually a sign of skin damage. People who experience one or more serious sunburns are significantly more likely to develop skin cancer. Natural pigment molecules in the skin help protect it from UV light damage. These pigment molecules are found in the layer of the skin called the epidermis.
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: sunburn</p>
<p>The <strong>epidermis</strong> is the outer of the two main layers of the skin, the inner layer being the dermis. 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, but structurally distinct from, the mucous membranes that line the mouth, anus, urethra, and vagina.</p>
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<h3>Cells of the Epidermis</h3>
<p>There are several different types of cells in the epidermis. All of the cells are necessary for the important functions of the epidermis.</p>
<p> The epidermis consists mainly of stacks of keratin-producing epithelial cells called keratinocytes. These cells make up at least 90 percent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells. Another 8 percent of epidermal cells are melanocytes. These cells produce the pigment melanin that protects the dermis from UV light. About 1 percent of epidermal cells are Langerhans cells. These are immune system cells that detect and fight pathogens entering the skin. Less than 1 percent of epidermal cells are Merkel cells, which respond to light touch and connect to nerve endings in the dermis.</p>
<h3>Layers of the Epidermis</h3>
<p>The 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 thicker than it is in the rest of the body. The layers of the epidermis are shown in Figure (PageIndex{2}) and described in the following text.</p>
<figure class=)
: Five layers of epidermis: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale, from top to bottom. The epidermis is connected with Dermis with the help of the basement membrane. </p>
<p>Stratum Corneum</p>
<p>The uppermost layer of the epidermis everywhere on the body is the stratum corneum. 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 the 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.</p>
<p>Protective Functions</p>
<p>The epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.</p>
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Protection from Pathogens
When 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 in Figure (PageIndex{3}). That’s why it is important to clean and cover even a minor 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 of about 5.0), low amounts of water, the presence of antimicrobial substances produced by epidermal cells, and Langerhans cells, which phagocytize bacteria or other pathogens.
Glands
Glands in the reticular layer of the dermis include sweat glands and sebaceous (oil) glands. Both are exocrine glands, which are glands that release their secretions through ducts to nearby body surfaces. The diagram below shows these glands and also several other structures in the dermis.
Sweat glands produce the fluid called sweat, which contains mainly water and salts. The glands have ducts that carry the sweat to hair follicles or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.
Eccrine sweat glands occur in the skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in temperature regulation. Apocrine sweat glands are larger than eccrine glands and occur only in the skin of the armpits and groin. The ducts of apocrine glands empty into hair follicles, and then the sweat travels along hairs to reach the surface. Apocrine glands are inactive until puberty, at which point they start producing an oily sweat that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria is the cause of body odor.
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Sebaceous glands are exocrine glands that produce a thick, fatty substance called sebum. Sebum is secreted into hair follicles and makes its way to the skin surface along with hairs. It waterproofs the hair and skin and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin. Sebaceous glands are found in every part of the skin except for the palms of the hands and soles of the feet where hair does not grow.
