‘Your skin is amazing, Vaseline commercial: waterproof barrier, defense against disease, adjusts to surroundings, constantly growing and replacing itself, heals itself, all 800 million cells — do you see skin the way we do?
In zootomy and dermatology, skin is the largest organ of the integumentary system made up of multiple layers of epithelial tissues that guard underlying muscles and organs. Skin pigmentation (see: human skin color or coloring) varies among populations, and skin type can range from dry skin to oily skin.
The adjective cutaneous literally means “of the skin” (from Latin cutis, skin).
As the interface with the surroundings, skin plays the most important role in protecting (the body) against pathogens. Its other main functions are insulation and temperature regulation, sensation, and synthesis of vitamin D and the protection of vitamin B folate.
Severely damaged skin will try to heal by forming scar tissue, often giving rise to discoloration and depigmentation of the skin.
Skin Layers
Skin is composed of three primary layers: the epidermis, which provides waterproofing and serves as a barrier to infection; the dermis, which serves as a location for the appendages of skin; and the hypodermis (subcutaneous adipose layer).
Epidermis
Epidermis, “epi” coming from the Greek meaning “over” or “upon”, is the outermost layer of the skin. It forms the waterproof, protective wrap over the body’s surface and is made up of stratified squamous epithelium with an underlying basal lamina.
The outermost epidermis consists of stratified squamous epithelium with an underlying connective tissue section, or dermis, and a hypodermis, or basement membrane. The epidermis contains no blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. The main type of cells which make up the epidermis are keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, basale. Cells are formed through mitosis at the basale layer. The daughter cells, (see cell division) move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein keratin is inserted. They eventually reach the corneum and slough off (desquamation). This process is called keratinization and takes place within about 30 days. This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out, making skin a natural barrier to infection.
Epidermal Components
The epidermis contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes, melanocytes, Langerhans cells and Merkels cells.
Epidermal Layers
Epidermis is divided into several layers where cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the top layer called stratum corneum and become sloughed off, or desquamated. This process is called keratinization and takes place within weeks. The outermost layer of Epidermis consists of 25 to 30 layers of dead cells.
Epidermal Sublayers
Epidermis is divided into the following 5 sublayers or strata:
Stratum corneum
Stratum lucidum
Stratum granulosum
Stratum spinosum
Stratum germinativum (also called “stratum basale”)
Mnemonics that are good for remembering the layers of the skin (using “stratum basale” instead of “stratum germinativum”):
“Cher Likes Getting Skin Botoxed” (from superficial to deep)
“Before Signing, Get Legal Counsel” (from deep to superficial)
“Before Sex Get Latex Condoms (from deep to superficial)
Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips.
Dermis
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many nerve endings that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal to its own cells as well as the Stratum basale of the epidermis.
Dermal Structure
The dermis is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.
Papillary Region
The papillary region is composed of loose areolar connective tissue. It is named for its fingerlike projections called papillae, that extend toward the epidermis. The papillae provide the dermis with a “bumpy” surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin’s surface. These are called friction ridges, because they help the hand or foot to grasp by increasing friction. Friction ridges occur in patterns (see: fingerprint) that are genetically determined and are therefore unique to the individual, making it possible to use fingerprints or footprints as a means of identification.
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic, and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength, extensibility, and elasticity.
Also located within the reticular region are the roots of the hair, sebaceous glands, sweat glands, receptors, nails, and blood vessels. Tattoo ink is injected into the dermis. Stretch marks from pregnancy are also located in the dermis.
The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body.
Microorganisms like Staphylococcus epidermidis colonize the skin surface. These microorganisms serve as ecoorgan. The density of skin flora depends on region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings.
Components
The skin is often known as the largest organ of the human body. This applies to exterior surface, as it covers the body, appearing to have the largest surface area of all the organs. Moreover, it applies to weight, as it weighs more than any single internal organ, a
ccounting for about 15 percent of body weight. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters (8-10.8 sq.ft.), most of it is between 2-3 mm (0.10 inch) thick. The average square inch (6 cm