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Skin is the foremost and most vital organs of the human body spanning about 20 sq. ft. in the area and contributing about 15% of body weight. It serves as an anatomical barrier of the immune system protecting the human body from external threats of infections and toxic compounds [14]. Skin is also associated with a fine network of sensory neurons, ensuring balanced sensitivity and reflexes against hot and cold stimuli. It also plays a pivotal role in thermal homeostasis of the human body [15]. Anatomy of skin comprises three layers, namely, epidermis, dermis, and subcutaneous tissue layer. The epidermis is predominantly clustered with keratin producing keratinocytes while basal dermis layer homes melanin‐producing melanocytes [16, 17]. The dermal–epidermal junction comprises the extracellular matrix, which is secreted by epidermal fibroblasts. The extracellular matrix is composed of complex polysaccharides like chondroitin sulfate and hyaluronic acid and structural proteins such as collagen, elastin, and fibrin [15]. Together, these biomolecules provide strength and elasticity to the skin. Aging of the skin is a result of abrogation in the structure or function of any of the above mentioned histological or biochemical components of the skin.

Figure 4.1 Summary of various characteristics of aging.

It is evident that skin aging is a consequence of a variety of factors encountered by an individual during his lifespan. A broad classification of various aging theories is summarized in Table 4.1.

Table 4.1 Classification of aging theories.

Classificationtheories	Genetic mutation	Wear and tear	Cellular waste accumulation
Antagonistic pleiotropy
Waste accumulation
Free radical
Insulin resistance
Advanced Glycosylation
Telomeres shortening
The error catastrophe
Autoimmunity
Circadian regulation
Evolution