In summary, skin has five layers and four cell types. This most elaborate organ system contains its accessory structures (hair, nails, sweat and sebaceous glands), and special nerve receptors for changes in internal or external environment (touch, cold, heat, pain, and pressure). The hypodermis is where the body stores about half its fat. It's functions include protection of internal organs, temperature regulation, gatekeeping for disease-causing microorganisms, excretion via perspiration, protection against the sun's ultraviolet rays via pigmentation, and production of Vitamin D.
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Everyone's got skin. If you happen to come across someone without any, run! It is also known as the integumentary system, is the body's largest organ, and acts as a barrier from heat, sunlight, injury, and infection. It also helps to regulate body temperature and through sweating, it helps rid the body of extra water and salts. The two main layers are the outer or upper layer called the epidermis and the inner or lower layer known as the dermis.
The epidermis is the outermost layer, and thus protects the body from environmental factors. It's made up of squamous cells which are flat and appear scaly. Beneath this layer are where rounded cells called basal cells hang out.
I know you've heard the expression “thick-skinned” which is often used to describe people who don't let problems bring them down. It really does come in different thicknesses. The epidermis varies in different types of this substance, clocking in at just .05 mm thick at the eyelids, and 1.5 mm thick on the palms of the hands and the soles of the feet.
The lower part of the epidermis is where you'll find melanocytes. These cells produce the pigment called melanin, and these are also the cells in which melanoma develops. This part of the epidermis also has what's called the Langerhans cells (part of the skin's immune system), Merkel cells and sensory nerves. There are five sublayers of the epidermis that work together to continually rebuild the surface.
The Squamous Cell Layer
The thick, “spiny” squamous cell layer is atop the basal layer, and is also known as the stratum spinosum because the cells are held together with tiny prickly appendages that interlock to support the skin. The basal cells that have been pushed upward can be found here, but at this stage in the game they are now called squamous cells, or keratinocytes. They make keratin, which is a tough, protective protein that accounts for the majority of the structure of the skin, hair, fingernails and toenails. The repair process performed by Langerhans cells also takes place in this layer. The cells overpower antigens (foreign bodies) that attack areas where it is not well, and lets the immune system know about the problem.
The Basal Cell Layer
The basal layer, or stratum germinativum if you want to get technical, is the innermost layer of the epidermis and it contains small round cells called basal cells which constantly divide, and new cells continually push older ones up toward the surface where they stay until they are shed.
Melanocytes are other cells found within the basal cell layer and they produce the coloring (pigment) called melanin, giving skin its tanned coloration as well as doing their best to protect the deep skin layers from the sun's harmful rays. Too much sun is harmful - it increases your cancer risk by affecting the genetic material of cells. Skin coloring is from melanin, carotene (yellow to orange pigment), and underlying blood reflected through the skin. Variances in coloration are due to the amount as well as the distribution of melanocytes. The variation from the number of melanocytes doesn't necessarily hold true for individuals and different races, though. It's from a different kind of melanin. Albino skin does not produce any melanin at all. Patches of melanin are what cause birthmarks, freckles and age spots. Melanoma develops when a malignant transformation of melanocytes occurs.
Merkel cells, which are associated with the sense of touch, are also located in the basal layer of the epidermis.
The Stratum Granulosum & the Stratum Lucidum
The keratinocytes are pushed upwards from the squamous layer through two thin epidermal layers called the stratum granulosum, the thin middle layer which begins the keratin production process, and the stratum lucidum, a thick layer which appears only in places like the soles of your feet and palms. As these cells get closer to the surface, they become larger, flatten out and stick together. The cells fuse together into layers of tough, durable material, which keep making their way up to the surface. Eventually they become dehydrated and die off.
The Stratum Corneum
The stratum corneum is the fifth and outermost layer of the epidermis and it consists of many thin layers of continually shedding, dead keratinocyte cells which contain soft keratin to help the skin stay elastic and keep lower cells from drying out. The stratum corneum is also referred to as the “horny layer,” because its cells are toughened like the horn on an elephant, rhinoceros or other such animal. The cells on the outside will age and wear down, but not to worry, they are replaced by new layers of strong, long-wearing cells. The stratum corneum is continually shed while new cells are put in place, but this shedding process slows down as we age. Unfortunately, many things slow down as we age.
The dermis is under the epidermis and is the thickest of the three layers at 1.5 to 4mm thick. It stores most of the body's water, makes up about 90% of the skin's thickness and it's main functions are to regulate body temperature and to send nutrient-saturated blood to the epidermis. It is in this layer that most of the skin's specialized cells and structures can be found, such as:
Hair follicles, a hair-nourishing (guess you could call it “hair raising”) tube-shaped sheath surrounding the part of the hair that is under the skin.
Lymph vessels bathe the tissues of the skin with a milky substance called lymph that contains the infection-fighting immune system cells. As the lymph makes its way to the lymph nodes, the cells do their best to destroy any infections or invading organisms.
Blood vessels supply nutrients and oxygen to the skin and take away cell waste and cell products. They also bring the vitamin D produced in the skin back around to the rest of the body.
Sweat glands have two types, and the average person has about three million of these sweat suderiferous glands.
-Eccrine glands are the true sweat glands and can produce up to two liters of sweat an hour. Luckily, they secrete mostly water which is not an ideal environment for odor-producing bacteria. These glands are located all over the body and they help to regulate body temperature by bringing water from the pores to the surface, where it evaporates and reduces skin temperature.
-Apocrine glands are specialized sweat glands that can be found only in the armpits and pubic region. They become active at puberty and are larger, deeper, and produce thicker secretions than eccrine glands. The milky sweat they secrete containins pheromones and encourages the growth of the bacteria responsible for body odor. Ceruminous glands are a variation of apocrine glands found in the external ear canal lining. They secrete cerumen (earwax), a sticky substance that is thought to repel foreign material. Earwax sure repels me, that's for sure! Mammary glands in the female breasts are another form of modified apocrine glands, and are adapted to secrete milk instead of sweat.
Sebaceous (oil) glands are attached to hair follicles and can be found everywhere on the body except for the palms of the hands and the soles of the feet. These glands empty through ducts into the bases of hair follicles and secrete sebum (a mixture of fats, waxes, and hydrocarbons) that helps keep hair moist and the skin smooth and supple. The oil also helps keep skin waterproof. Oversecretion of sebum from the sebaceous glands clogs up both the gland and the hair follicle, resulting in acne.
Nerve Endings are pain and touch receptors in the dermis layer that transmit sensations of pain, itch, pressure and temperature information to the brain. If the temperature is cold, the brain will go into shivering (involuntary contraction and relaxation of muscles) which will generate body heat.
Collagen and Elastin Collagen is a tough, insoluble protein that holds the dermis together, made by fibroblasts which are cells that give the skin strength and resilience. Its found throughout the body in the connective tissue that holds muscles and organs in place. Collagen also enables the epidermis to be more durable. Elastin is a similar protein which keeps the skin flexible and allows it to be restored to it's original state when stretched.
The dermis layer is comprised of two sublayers:
The Papillary Layer
The upper (papillary) layer is found just under the epidermis and is tethered to it by papillae, some of which contain touch receptors and others which send nutrients to the epidermis and regulate temperature via a thin, extensive vascular system that operates much like other vascular systems in the body.
The Reticular Layer
The lower (reticular) layer contains sensory receptors for deep pressure, and criss-crossed collagen fibers to form a strong elastic network, strengthening the skin. Hair follicles, smooth muscle and sebaceous and sweat glands are supported by this layer. It's thicker than the papillary dermis, and its collagen fibers that are laid out parallel to the skin's surface are called Langer's lines.
The innermost layer of the skin is known as the subcutis, hypodermis or subcutaneous layer. Blood and lymph vessels as well as nerves and hair follicles cross through this layer. It's made up of a network of fat (which it stores as an energy reserve) and collagen cells and does double-duty as both an insulator by conserving body heat, and a shock absorber by protecting internal organs.
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