The integument or skin is the largest organ of the body, making up to 16% of body weight, with a surface area of 1.8m2. It has several functions, the most important is to form a physical barrier to the environment, allowing or limiting the inward and outward passage of water, electrolytes and various substances so to provide protection against micro-organisms, ultraviolet radiation, toxic agents and mechanical insults.
There are three structural layers to the skin: the epidermis, the dermis and subcutis or hypodermis.
Skin is a dynamic organ in a constant state of change, as cells of the outer layers are continuously shed and replaced by inner cells moving up to the surface.
the epidermis is the most external layer and is mainly composed of layers of keratinocytes but also containing melanocytes, Langerhans cells and Merkel cells. The main cells of the epidermis are the keratinocytes, which synthesize the protein keratin. Protein bridges called desmosomes connect the keratinocytes, which are in a constant state of transition from the deeper layers to the superficial.
The four separate layers of the epidermis are formed by the differing stages of keratin maturation. The epidermis varies in thickness from 0.05 mm on the eyelids to 0.8±1.5 mm on the soles of the feet and palms of the hand.
Moving from the lower layers upwards to the surface, the four layers of the epidermis are:
• stratum basale (basal or germinativum cell layer) –contains melanin and melanocytes
• stratum spinosum
• stratum granulosum
• stratum corneum
In addition, the stratum lucidum is a thin layer of translucent cells seen in thick epidermis. It is located between the stratum granulosum and stratum corneum and is not usually seen in thin epidermis.
Numerous nerve endings are located in different layers of the epidermis and make the skin into one large sense organ, detecting temperature, light, taste and touch.
The basement membrane is a multilayer that separates the epidermis from the dermis and creates the dermo-epidermal junction. The structure is highly irregular, with dermal papillae from the papillary dermis projecting perpendicular to the skin surface. It is via diffusion at this junction that the epidermis obtains nutrients and disposes of waste. The dermo-epidermal junction flattens during ageing which accounts in part for some of the visual signs of ageing.
The dermis is a connective tissue located between the epidermis and the subcutis. It contains sweat glands, hair roots, nervous cells and fibers, blood and lymph vessels.
Two layers comprise the dermis:
• A thin papillary layer
• A thicker reticular layer.
The dermis is made up of fibroblasts, which produce collagen, elastin and structural proteoglycans, together with immune-competent mast cells and macrophages. Collagen fibers make up 70% of the dermis, giving it strength and toughness. Elastin maintains normal elasticity and flexibility while proteoglycans provide viscosity and hydration. Embedded within the fibrous tissue of the dermis are the dermal vasculature, lymphatics, nervous cells, sweat glands, hair roots and small quantities of striated muscle.
The subcutis is made up of loose connective tissue and fat, which can be up to 3 cm thick on the abdomen.
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