The stratum corneum is the outermost layer of the epidermis, and is made up of 10 to 30 thin layers of continually shedding, dead keratinocytes. The stratum corneum is also known as the "horny layer," because its cells are toughened like an animal's horn.
As the outermost cells age and wear down, they are replaced by new layers of strong, long-wearing cells. The stratum corneum is sloughed off continually as new cells take its place, but this shedding process slows down with age. Complete cell turnover occurs every 28 to 30 days in young adults, while the same process takes 45 to 50 days in elderly adults. The dermis is located beneath the epidermis and is the thickest of the three layers of the skin 1.
The main functions of the dermis are to regulate temperature and to supply the epidermis with nutrient -saturated blood. Much of the body's water supply is stored within the dermis. This layer contains most of the skins' specialized cells and structures, including:. The upper, papillary layer, contains a thin arrangement of collagen fibers.
The hypodermis also called the subcutaneous layer or superficial fascia is a layer directly below the dermis and serves to connect the skin to the underlying fascia fibrous tissue of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.
The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from trauma.
Where the fat is deposited and accumulates within the hypodermis depends on hormones testosterone, estrogen, insulin, glucagon, leptin, and others , as well as genetic factors. Fat distribution changes as our bodies mature and age.
Men tend to accumulate fat in different areas neck, arms, lower back, and abdomen than do women breasts, hips, thighs, and buttocks. The body mass index BMI is often used as a measure of fat, although this measure is, in fact, derived from a mathematical formula that compares body weight mass to height. Therefore, its accuracy as a health indicator can be called into question in individuals who are extremely physically fit.
In many animals, there is a pattern of storing excess calories as fat to be used in times when food is not readily available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption of high-calorie foods have resulted in unwanted accumulations of adipose tissue in many people. Although periodic accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now becoming chronic and considered a major health threat.
Not only is this a problem for the individuals affected, but it also has a severe impact on our healthcare system. Changes in lifestyle, specifically in diet and exercise, are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk of heart disease and diabetes.
The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome Figure 7. Figure 7. The relative coloration of the skin depends of the amount of melanin produced by melanocytes in the stratum basale and taken up by keratinocytes.
Melanin occurs in two primary forms. Eumelanin exists as black and brown, whereas pheomelanin provides a red color. Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes.
The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage and the breakdown of folic acid, a nutrient necessary for our health and well-being.
In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption. Thus, the amount of melanin present in our skin is dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin D production. It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially.
Dark-skinned individuals can also get sunburns, but are more protected than are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes; this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent. Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause sufficient DNA damage to result in skin cancer.
When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer Figure 8.
Figure 8. Moles range from benign accumulations of melanocytes to melanomas. These structures populate the landscape of our skin.
Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated. A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory organs. Although neither is fatal, it would be hard to claim that they are benign, at least to the individuals so afflicted. Albinism is a genetic disorder that affects completely or partially the coloring of skin, hair, and eyes.
The defect is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white or very pale due to the lack of melanin in their skin and hair.
Recall that melanin helps protect the skin from the harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due to the lack of pigmentation on the retinal wall.
Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes. In vitiligo , the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches Figure. Neither albinism nor vitiligo directly affects the lifespan of an individual. Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems.
Tumors of the pituitary gland can result in the secretion of large amounts of melanocyte-stimulating hormone MSH , which results in a darkening of the skin.
A sudden drop in oxygenation can affect skin color, causing the skin to initially turn ashen white. This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing because of asthma or a heart attack.
This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism? The skin is composed of two major layers: a superficial epidermis and a deeper dermis.
The epidermis consists of several layers beginning with the innermost deepest stratum basale germinatum , followed by the stratum spinosum, stratum granulosum, stratum lucidum when present , and ending with the outermost layer, the stratum corneum.
The topmost layer, the stratum corneum, consists of dead cells that shed periodically and is progressively replaced by cells formed from the basal layer.
The stratum basale also contains melanocytes, cells that produce melanin, the pigment primarily responsible for giving skin its color. Melanin is transferred to keratinocytes in the stratum spinosum to protect cells from UV rays.
The dermis connects the epidermis to the hypodermis, and provides strength and elasticity due to the presence of collagen and elastin fibers. It has only two layers: the papillary layer with papillae that extend into the epidermis and the lower, reticular layer composed of loose connective tissue.
The hypodermis, deep to the dermis of skin, is the connective tissue that connects the dermis to underlying structures; it also harbors adipose tissue for fat storage and protection. The skin consists of two layers and a closely associated layer. The epidermis provides protection, the dermis provides support and flexibility, and the hypodermis fat layer provides insulation and padding. Figure If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?
Figure These cells do not have nuclei, so you can deduce that they are dead. They appear to be sloughing off. Figure If you zoom on the cells of the stratum spinosum, what is distinctive about them? Figure These cells have desmosomes, which give the cells their spiny appearance. The papillary layer of the dermis is most closely associated with which layer of the epidermis?
What determines the color of skin, and what is the process that darkens skin when it is exposed to UV light? The pigment melanin, produced by melanocytes, is primarily responsible for skin color. Melanin comes in different shades of brown and black.
Individuals with darker skin have darker, more abundant melanin, whereas fair-skinned individuals have a lighter shade of skin and less melanin. Exposure to UV irradiation stimulates the melanocytes to produce and secrete more melanin. Cells of the epidermis derive from stem cells of the stratum basale.
Describe how the cells change as they become integrated into the different layers of the epidermis. As the cells move into the stratum spinosum, they begin the synthesis of keratin and extend cell processes, desmosomes, which link the cells. As the stratum basale continues to produce new cells, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. The cells become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin and keratohyalin.
The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that form the stratum lucidum and the stratum corneum. The keratinocytes in these layers are mostly dead and flattened. These cells are densely packed with eleiden , a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent i.
The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment see Figure 5. The increased keratinization also called cornification of the cells in this layer gives it its name.
There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum or stratum lucidum in the case of the palms and soles of feet.
The entire layer is replaced during a period of about 4 weeks. It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is mostly composed of dense irregular connective tissue that is divided to two layers: the papillary layer and reticular layer. Interwoven within these layers are numerous elastin and collagenous fibers, produced by fibroblasts Figure 5. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made.
Within the papillary layer are fibroblasts, a small number of fat cells adipocytes , and an abundance of capillary loops. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the Meissner corpuscles. Underlying the papillary layer is the much thicker reticular layer.
This layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated net-like due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis.
In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively.
0コメント