The skin is composed of two main layers, the epidermis and the dermis, which lie above a layer of subcutaneous fat (William et al. 2016), the hypodermis (Timmons 2006). The relative thickness of these layers varies considerably depending on the body region, the epidermis being thickest on the palms and soles, approximately 1.5 mm, the dermis thickest on the back, approximately 30–40 times thicker than the overlying epidermis, and the subcutaneous fat most abundant on the abdomen and buttocks and scarcest on the nose and sternum (William et al. 2016).
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Figure 1. Structure of the skin (London Health Sciences Centre 2009)
1.7.1.1. Skin Structure The Epidermis
The epidermis is a stratified squamous epithelium, with several well defined layers, the stratum corneum (horny layer), stratum lucidum (clear layer), stratum granulosum (granular layer), stratum spinosum (prickle cell layer), and stratum basale (basal layer) (Timmons 2006). The principal cell type, keratinocytes, make up 95% of the epidermis and are produced by cell division in the deepest layer of the epidermis, the stratum basale (Gantwerker & Hom 2011). Melanocytes, and Langerhans cells make up the majority of the remainder of the cells (William et al. 2016). As keratinocytes replicate they move older cells toward the surface, undergoing a complex series of
morphological and biochemical changes while in transit, progressively losing their nucleus and becoming more flattened and ovoid in shape (Timmons 2006). This
process of terminal differentiation, or keratinization, produces a surface layer of tightly packed dead cells named the stratum corneum (Graham-Brown & Burns 2011). In health, the epidermis completely regenerates every 48 days maintaining a constant epidermal thickness as the rate of cell production equals that of cell loss (Gantwerker
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& Hom 2011). Epidermal kinetics are regulated by numerous growth stimulators and inhibitors (William et al. 2016). The features of this differentiation process are genetically controlled with genetic mutations accountable for a variety of diseases (William et al. 2016) . The stratum basale contains projections that interdigitate down with similar structures reaching from the dermis below forming rete ridges
(Gantwerker & Hom 2011). Newly healing wounds and skin grafts initially lack these rete ridges and are therefore susceptible to shear trauma (Gantwerker & Hom 2011). The epidermis also contains essential appendages, the eccrine sweat glands, the apocrine glands, the pilosebaceous unit, comprising of a hair follicle with an associated sebaceous gland, and the nails (Graham-Brown & Burns 2011). The pilosebaceous unit and rete ridges contain epithelial stem cells that are essential to the reepithelialisation process, as they are relatively undifferentiated, have a great proliferation potential, and have a high ability for self-renewal. Consequently, destruction of these stem cells, often seen in burn wounds, impedes the skins ability to heal normally (Gantwerker & Hom 2011). Eccrine sweat glands cover almost all the human body surface and play an important role in body temperature regulation secreting a hypotonic liquid substance, sweat, that is composed of water, electrolytes, lactate, urea and ammonia (Graham- Brown & Burns 2011). These glands consist of a secretory coil in the dermal layer and a duct that conveys the sweat to the epidermal surface (Graham-Brown & Burns 2011). The stimulation of these glands is controlled by the sympathetic nervous system, however the neurotransmitter is acetylcholine (Graham-Brown & Burns 2011).
The Dermis
Below the epidermis is a layer of connective tissue called the dermis (William et al. 2016). This layer, which forms the bulk of the skin, interdigitates with the epidermis via upward projections called the dermal papillae (Graham-Brown & Burns 2011) and delivers support and nutrients to the epidermis (Timmons 2006).The dermis comprises two layers, the papillary layer and the reticular layer (Timmons 2006). It is made up of a network of interlacing fibres, mainly collagen, and to a lesser amount, elastin, embedded in a ground substance of glycosaminoglycans (Graham-Brown & Burns 2011). These protein fibres provide the dermis great strength and elasticity (Graham- Brown & Burns 2011). The dermal layer receives the major blood supply for the skin
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and features dermal appendages such as the apocrine glands, eccrine glands, and hair follicles (Gantwerker & Hom 2011). The three main cells of the dermis are the
fibroblasts, which synthesise the dermal connective tissue matrix, the mast cells, in which granules contain substances such as histamine, prostaglandins, leukotrienes, and eosinophil and neutrophil chemotactic factors, and the macrophages, which phagocytose cellular debris and extracellular material (Graham-Brown & Burns 2011).
1.7.1.2. Skin Function
The main functions of the skin are protection, sensation, thermoregulation, excretion, metabolism, and non-verbal communication (Timmons 2006). The skin serves as the body’s main protective barrier to the penetration of external agents such as,
microorganisms, ultraviolet light (Graham-Brown & Burns 2011), and toxic substances (Timmons 2006). An antibacterial defence is provided by the components of skin secretions and the associated acidic pH of the skin surface, a physical protective layer is provided by the structure of stratum corneum with its tightly packed cells (Graham- Brown & Burns 2011), and a protective effect against ultraviolet damage is provided by the melanin pigments (Meredith & Riesz 2004). The dendritic Langerhans ’ cells of the skin provide immunological mechanisms of defence against ‘foreign’ material and are involved in the inflammatory response such as seen in allergic contact dermatitis (Graham-Brown & Burns 2011). In addition, the skin protects against the outward loss of vital bodily substances (Timmons 2006), largely due to the composition of the stratum corneum with its overlapping cells and intercellular lipid (Graham-Brown & Burns 2011), The sensation function is owed to the nerve endings present in the skin that transmit signals of pain, temperature, pressure and touch (Timmons 2006). The skin plays a vital role in thermoregulation as it responds to cold by vasoconstriction, reducing the blood flow to the skin and hence decreasing the loss of heat from the skin surface, and to heat by vasodilatation, increasing the blood flow to the skin and hence increasing the loss of heat from the skin surface (Graham-Brown & Burns 2011). Perspiration is also a key feature of thermoregulation where the production of sweat from the skin and subsequent evaporation assists in cooling of the body (Graham- Brown & Burns 2011). The skin’s excretory function involves the secretion of waste products in the form of sweat containing water, urea, and albumin, and the secretion
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of a lipid-rich product, sebum, which lubricates the skin (Timmons 2006, Graham- Brown & Burns 2011). The skin also functions in metabolism as vitamin D
(cholecalciferol) is formed by the action of ultraviolet light on dehydrocholesterol (Timmons 2006, Graham-Brown & Burns 2011). As a surface organ, the skin also plays an essential role in social interaction and sexual attraction (Graham-Brown & Burns 2011), and can convey changes in mood as well as signal inner body changes and general well-being (Timmons 2006).
As stated by Graham-Brown & Burns (2011), it is vital to have some knowledge of the normal structure and function of any organ prior to attempting to understand the abnormal. Therefore, an overview of normal structure and function of the skin has been presented. Leading on from this, the normal physiology of wound healing will be described before considering the pathophysiology of impaired wound healing.