The skin is the largest organ in the body. It protects the body against physical injuries, temperature regulation, sunlight, infection, stores water, fat, and plays a key role in metabolism including vitamin D, among other functions.

The skin consists of three main layers, epidermis (upper or outer layer), dermis (lower or inner layer) and subcutaneous tissue. Skin cancer begins in the epidermis, which is made up of three kinds of cells: keratinocytes, Melanocytes & Langerhans cells. Keratinocyte cells are found in the deepest basal layer of the stratified epithelium that comprises the epidermis. They are in charge of producing the protein keratin and making the skin waterproof and tough. Keratins form the cytoskeleton of keratinocytes and are what makes up our hair and nails. So, if defects occur in keratin expression, it would result in numerous diseases of the epidermis, hair and nails. As they mature, divide and differentiate, they lose water, flatten and move to the uppermost layer of the epidermis, the stratum corneum, which mainly accumulates dead keratinocytes, keratins and lipids. This is the tough outer layer of the skin.

 The skin completely renews itself every 3-5 weeks replacing the accumulated dead keratinocytes by other ones. Keratinocytes also play an immune system role as inmunomodulators, activating Langerhans cells in response to injury, preventing unwanted substances from penetrating the skin and secreting inhibitory cytokines in the absence of injury. Other significant cells in the epidermis are melanocytes, cells that produce melanin, the pigment responsible for skin tone and color and found in the lower part of the epidermis.

 Dermis is undoubtedly the layer most important for skin maintenance and hair follicle. It contains cells for the immune system, collagen, elastin, fibronectin which maintains it soft and stretchy and proteins that allow water absorbance for keeping the skin hydrated and with a good texture. The dermis also contains capillaries (tiny blood vessels) and lymph nodes (depots of immune cells) the blood vessels carry oxygen and nutrients and lymph nodes for protecting it from microorganisms Dermis receives blood and distributes it to the epidermis, regulates temperature, wound and removes toxins. Subcutaneous tissue is the innermost layer of the skin located under the dermis consisting of connective tissue and fat molecules.

It connects, along with the connective tissue, the dermis with the muscles and bones. 

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