INTEGUMENTARY SYSTEM

Epidermis from the Ancient Greek language means epi: outer and derma: skin; thus, it represents the superficial outer layer of skin and plays a role in protecting other deeper layers. Epidermis consists of four layers except on palms and soles, where it becomes five layers in order to adapt with the consistent rubbing. Generally, epidermis layers differ in their cell composition. The stratum Basale is the most inner layer of the epidermis, which is in contact with the basement membrane, and as its name reveals, it is responsible for generating new epidermis cells with stem cells in addition to keratinocytes, melanocytes, and Merkel cells. Superficially located the stratum spinosum, the cells in this layer have spiny appearance due to desmosomes. Desmosomes are cellular projections that work on connecting adjacent keratinocytes. Langerhans cells are also found in this layer. The stratum Basale and the stratum spinosum form together the malpighian layer. The next superficial layer is the stratum granulosum, where keratohyalin granules are seen into keratinocytes.

Dermis is the middle and thickest layer located beneath the epidermis and above the hypodermis. It consists mainly of dense irregular connective tissue and contains all the skin appendages in addition to vessels and nerve endings. Due to its components, it is stronger and more flexible than epidermis. Dermis contact with epidermis by the basement layer which meets the upper layer of dermis: the papillary layer. The papillary layer has projections like fingers extend in the epidermis called dermal papillae. Those papillae allow nutrients and oxygen to be delivered to epidermis cells by the blood vessels. They also help with the existence of nerves in sensation of touch, pain, and temperature through the cells of the epidermis.The reticular layer is the second layer of dermis and as its name reveals, it consists of dense connective tissue with elastin and collagen fibers that form a net-like structure. This structure provides the reticular layer special characteristics as it is thicker than the upper layer, stronger and has more elasticity helping skin in moving and stretching. Blood vessels and nerve fibers also exist in this layer to support skin cells and to sense pressure and pain in the skin.

Hypodermis, subcutis, subcutaneous fat, or subcutaneous tissue, all these terms refer to the last layer of skin deep to the dermis and connects to the fascias muscles and bones. Hypodermis consists of loose areolar connective tissue and adipose tissue. Adipose tissue helps in absorbing shocks in addition to storing triglycerides in order to use them as energy resources. It also makes a barrier to reserving body heat produced by muscles. Areolar connective tissue contains elastin and collagen fibers that help skin in moving and absorbing superficial injuries.   

Hair is a keratinous filament that originates from a follicle in dermis and grows out of the epidermis. Hair follicle is an epidermal component presents in dermis. Hair shaft is the part that is visible out of the skin. The portion located under the skin is called the root. While basal cells in the follicle generate keratinocytes, the root is pushed up till the surface of skin to form the hair. Melanocytes are other cells found in follicles that pigments hair and gives it color. The medulla is located at the central core of the shaft and the root. It consists of highly pigmented cells filled with keratin. Cortex is the next layer forming most of the hair width. It also contains pigmented cells being tightly packed and having a spindle shape. These cells give the hair color. However, not all hairs contain medulla, thus, cortex will be the core layer in these hairs. The most outer layer is cuticle. It is formed by overlapping dead keratinocytes, each cell of them points away from the body.

Nails are keratinous plantlike structures composed of highly specialized epithelial cells that grow on the distal ends of fingers and toes. Nails have important protective functions for humans when doing or manipulating with accurate activities. Nails grow from dermis in a tissue called matrix which surrounds nail root, the part beneath the skin. Here, basal cells are produced and then go under maturation process to form keratinocytes, which in turn accumulate and make hard sheets of packed cells. The edge of matrix is seen as whitish crescent shape known as the lunula. As a result, to sheets accumulation, the nail grows and reaches skin surface to form the visible part of the nail, nail body. New cells are produced in the root and push continuously the old cells towards the distal part of the nail.

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Sudoriferous glands are also called sweat glands and they could be either eccrine or apocrine sweat glands. They are both located in the dermis; however, many other differences are distinguished between them. Eccrine glands secrete their products directly to skin surface, they are found all over the body. Sweat produced by them, which is formed by sodium and chloride, helps in cooling body temperature when vaporizing above skin surface. On the other hand, Apocrine glands are found in limited areas mainly in axilla, in addition to anogenital region, external ear canal, and areola. They are activated by puberty and produce a thick, oily liquid that is consumed by bacteria living on the skin. Apocrine glands open onto hair follicles and secrete their products along the hair shaft. The distinctive odor of human sweat comes from the digestion of apocrine sweat by bacteria. 

Sebaceous glands are classified as exocrine glands and they are found in skin dermis of all body regions except of palms and soles. As their name indicates, they produce sebum, which is a fatty substance released in hair follicle duct and reaches then the surface. Sebum helps in maintaining lubrication of cuticles of hairs, it defends against pathogens, and acts to waterproof and increase the elasticity of the skin. 

Ceruminous glands are modified apocrine glands located in dermis of external auditory canal. They continuously produce cerumen or what is commonly known as air wax. Cerumen is a yellowish or brown waxy substance that acts to protect skin from water, pathogens, and foreign bodies. It also lubricates the ear drum. Accumulation of newer cerumen led to pushing the older one out of the ear canal where it could be removed.   

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Several processes of skin participate in regulation of body temperature. Vasoconstriction and vasodilation of vessels result in reserve and release heat. Sweat evaporation on the surface of skin allows further heat loss. On the other hand, when hair is erected by the contraction of arrector pili muscles, it forms a layer preventing heat loss. 

Skin represents the first-line defense against different harmful substances or pathogens. Squamous cells are keratinized anucleated epithelia, which form a physical barrier against external factors. Chemical factors such as the acidic medium secreted by skin and releasing immune factors like cytokines, prevent colonization of bacteria. Lipids produced by special glands in skin help in adaptation with hot temperature by preventing water loss. The integumentary system also forms a barrier between different environments in tonicity which protects human cells from bloating. Because of all these roles, individuals with wide burn injuries face a great challenge due to losing regions of skin. 

Water, urea, and ammonia are secreted with sweat by skin. Excess vitamin B is eliminated from the body with sweat alongside the urine. Fat soluble metabolites are also secreted with sebum. 

Skin is innervated by various sensory nerve endings in addition to many mechanoreceptors; all these contribute to sensing touch, pressure, vibration, heat, and pain. When skin is exposed to ultraviolet sunlight, it is triggered to produce vitamin D that has essential role in bone health. Melatonin production is also stimulated by sunlight, which has a preventive role from adverse effects of UV rays like sunburns or skin cancer. 

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It is an autoimmune disease affecting hair follicles which usually causes hair loss in scalp hair, but it could happen in any other place of the body. It manifests grossly as bald patch(es) and histopathological characteristics reveal a “bee-swarm pattern” that involves dense lymphocytic infiltrations surrounding hair follicles. Alopecia areata classified into Alopecia totalis: affects the entire scalp; and Alopecia universalis affects the entire epidermis. The specific cause of this condition is not known. 

It is a fungal nail infection that affects the toenails or fingernails and could involve any part of the nail. Onychomycoses include five main subtypes: distal lateral subungual (DLSO), white superficial (WSO), proximal subungual (PSO), endonyx (EO), and candidal onychomycosis. Commonly, distal subungual onychomycoses affect nail bed, while proximal subungual, white superficial, candida onychomycoses affect nail plate. The most common pathogenesis is when nail bed is invaded by hyponychium from plantar skin. 

Bromhidrosis or osmidrosis is a chronic condition in which an extreme odor, usually unpleasant, is secreted by the skin. Causes include alterations in apocrine glands due to bacterial breakdown. This condition is closely linked to excessive sweating (hyperhidrosis  

Burns are classified according to the depth of injury into three degrees. The first degree is superficial burn affects only the epidermis; it causes mild pain and redness; it does not require specific medical care. The second degree is partial thickness burn that involves epidermis and part of the dermis; it is characterized by pain and blisters; it requires specialized treatment if it reaches over 1% of body surface. Third degree burn is full thickness burn that affects all layers of skin and may involves muscles and bones; it requires special health care regardless of the size or severity of injury. Another classification includes percentage of total body surface area (TBSA). These two classifications are used to estimate the severity of burn and predict the prognosis. According to WHO, there are about 180 000 deaths every year because of burn injuries; and over 486,000 people in the U.S. were treated for burn injuries in 2016, according to the American Burn Association.  

Dermatitis is any inflammation event of epidermis. It is characterized by rash, swellings, or blistering in skin. Many cases of dermatitis are triggered by several allergens, irritants or endogenous factors. Management involves avoiding triggering factors and applying topical ointments and steroids. It is estimated that Atopic dermatitis, a type of dermatitis, affects at least 28 million people, according to the American Academy of Dermatology.  

Skin cancer results from the uncontrolled proliferation of epidermal cells. It is divided into melanoma and nonmelanoma cancer. Non melanoma cancers include basal cell carcinoma and squamous cell carcinoma. Melanoma cancer has the worst prognosis. A common risk factor of non-melanoma cancer is exposure to ultraviolet waves, mainly from sunlight. Skin Cancer Foundation reported that over 3.3 million people are treated for nonmelanoma skin cancer each year; and around one in five Americans will develop skin cancer in their lifetime. 

Warts are proliferations of the skin caused by human papillomavirus (HPV). There are common anatomical sites of infections, such as hands, feet and face. Different types of warts are present with different shapes, including smooth, flat, rough, raised, dark, small and/or large. Cure is usually hard due to the high recurrence rates. Treatments include topical agents, injections, photodynamic therapy, and systemic agents. Surgical removal could also be done. 

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