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Your skin is the largest organ on your body, made up of several different components, including water, protein, lipids, and different minerals and chemicals. If you're average, your skin weighs about six pounds. It's job is crucial: to protect you from infections and germs. Throughout your life, your skin will change constantly, for better or worse. In fact, your skin will regenerate itself approximately every 27 days. Proper skin care is essential to maintaining the health and vitality of this protective organ.

Over the course of your life, you should pay attention to all parts of your skin. Familiarize yourself with it, so you'll notice any changes that might occur, such as different moles or patches that might indicate skin cancer. Whenever you have a question or concern, make sure you see your doctor.

Pro Skin

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The epidermis is the thinnest layer in your skin, but it's responsible for protecting you from the harsh environment. The epidermis has five layers of its own. It also hosts different types of cells: keratinocytes produce the protein known as keratin, the main component of the epidermis. Melanocytes produce your skin pigment known as melanin. Langerhans cells prevent foreign substances from getting into your skin.

This is the layer responsible for wrinkles. The dermis is a complex combination of blood vessels, hair follicles, and sebaceous (oil) glands. Here, you'll find collagen and elastin, two proteins necessary for skin health because they offer support and elasticity. Fibroblasts are the cells you'll find in this layer, because they synthesize collagen and elastin. This layer also contains nerves that sense pain, touch, and temperature.

Reduction of tissue in this layer is what causes your skin to sag as well as wrinkle. This layer hosts sweat glands, and fat and loose connective tissue. The subcutis is responsible for conserving your body's heat and protecting your vital inner organs.

Collagen is found in the dermis and is the most abundant protein in the skin, making up 75% of this organ. This is also your fountain of youth. It's responsible for warding off wrinkles and fine lines. Over time, environmental factors and aging diminish your body's ability to produce collagen and can also break down existing collagen.

When you hear the word elastin, think elastic. This protein is found with collagen in the dermis, and is responsible for giving structure and support to your skin and organs. As with collagen, elastin is affected by time and the elements. Diminished levels of this protein cause your skin to wrinkle and sag.

Other animal coverings, such as the arthropod exoskeleton, have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis 'skin'). In mammals, the skin is an organ of the integumentary system made up of multiple layers of ectodermal tissue and guards the underlying muscles, bones, ligaments, and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds.[1] Skin (including cutaneous and subcutaneous tissues) plays crucial roles in formation, structure, and function of extraskeletal apparatus such as horns of bovids (e.g., cattle) and rhinos, cervids' antlers, giraffids' ossicones, armadillos' osteoderm, and os penis/os clitoris.[2]

All mammals have some hair on their skin, even marine mammals like whales, dolphins, and porpoises that appear to be hairless.The skin interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens[3] and excessive water loss.[4] Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar tissue. This is sometimes discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans, for example, the skin located under the eyes and around the eyelids is the thinnest skin on the body at 0.5 mm thick and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is the thickest skin on the body at 4 mm thick. The speed and quality of wound healing in skin is promoted by estrogen.[5][6][7]

Fur is dense hair.[8] Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick and can be processed to create leather. Reptiles and most fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough beta-keratins. Amphibian skin is not a strong barrier, especially regarding the passage of chemicals via skin, and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated quickly as the chemical diffuses through its skin. Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.[9]

The word skin originally only referred to dressed and tanned animal hide and the usual word for human skin was hide.Skin is a borrowing from Old Norse skinn "animal hide, fur", ultimately from the Proto-Indo-European root *sek-, meaning "to cut" (probably a reference to the fact that in those times animal hide was commonly cut off to be used as garment).[10]

The epidermis is composed of the outermost layers of the skin. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing pathogens from entering, and is a stratified squamous epithelium,[11] composed of proliferating basal and differentiated suprabasal keratinocytes.

Keratinocytes in the stratum basale proliferate through mitosis and the daughter cells move up the strata changing shape and composition as they undergo multiple stages of cell differentiation to eventually become anucleated. During that process, keratinocytes will become highly organized, forming cellular junctions (desmosomes) between each other and secreting keratin proteins and lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin.[13] Keratinocytes from the stratum corneum are eventually shed from the surface (desquamation).

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 provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils, microfibrils, and elastic fibers, embedded in hyaluronan and proteoglycans.[13] Skin proteoglycans are varied and have very specific locations.[15] For example, hyaluronan, versican and decorin are present throughout the dermis and epidermis extracellular matrix, whereas biglycan and perlecan are only found in the epidermis.

The papillary region is composed of loose areolar connective tissue. This 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.

The subcutaneous tissue (also 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 subcutaneous tissue contains 50% of body fat). Fat serves as padding and insulation for the body.

Microorganisms like Staphylococcus epidermis colonize the skin surface. 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.

The epidermis of fish and of most amphibians consists entirely of live cells, with only minimal quantities of keratin in the cells of the superficial layer.[16] It is generally permeable, and in the case of many amphibians, may actually be a major respiratory organ.[17] The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods. [16] Instead, in most species, it is largely replaced by solid, protective bony scales.[18] Apart from some particularly large dermal bones that form parts of the skull, these scales are lost in tetrapods, although many reptiles do have scales of a different kind, as do pangolins.[19] Cartilaginous fish have numerous tooth-like denticles embedded in their skin, in place of true scales.[20]

Sweat glands and sebaceous glands are both unique to mammals, but other types of skin gland are found in other vertebrates.[21] Fish typically have a numerous individual mucus-secreting skin cells that aid in insulation and protection, but may also have poison glands, photophores, or cells that produce a more watery, serous fluid. In amphibians, the mucus cells are gathered together to form sac-like glands. Most living amphibians also possess granular glands in the skin, that secrete irritating or toxic compounds.[22]

Although melanin is found in the skin of many species, in the reptiles, the amphibians, and fish, the epidermis is often relatively colorless. Instead, the color of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as chameleons and flounders may be able to change the color of their skin by adjusting the relative size of their chromatophores.[22]

Amphibians possess two types of glands, mucous and granular (serous). Both of these glands are part of the integument and thus considered cutaneous. Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. The three individual parts of the gland are the duct, the intercalary region, and lastly the alveolar gland (sac). Structurally, the duct is derived via keratinocytes and passes through to the surface of the epidermal or outer skin layer thus allowing external secretions of the body. The gland alveolus is a sac shaped structure that is found on the bottom or base region of the granular gland. The cells in this sac specialize in secretion. Between the alveolar gland and the duct is the intercalary system which can be summed up as a transitional region connecting the duct to the grand alveolar beneath the epidermal skin layer. In general, granular glands are larger in size than the mucous glands, which are greater in number.[23] ff782bc1db