The skin is the body's primary barrier against external aggression. This organ is composed of several layers of tissue. There is the epidermis, which is the outermost part of the skin; the dermis, a connective tissue made up of fibroblasts and an extracellular matrix, which provides cohesion and nutrition to the skin; and the hypodermis, which is made up of adipocytes. 

The epidermis is made up of several cell layers of keratinocytes. Among others, we distinguish the germinal layer of the epidermis, called the basal layer, which contains skin stem cells. The spinous layer, Stralum Spinosum, made up of several layers of polygonal cells, the granular layer, Stralum granulosum, comprising one to three layers of flattened cells containing cytoplasmic inclusions, the keratohyalin granules, and finally, the horny layer, Stralum corneum, which is composed of anucleated, keratin-rich cells called coeocytes that correspond to the terminal stage of keratinocyte differentiation.    

The outermost cells of the stralum corneum are continually shed and replaced by cells from a lower layer, in a process called desquamation. Cell regeneration of the stratum corneum is based on a process of cell maturation during which cells from the basal layer of the epidermis differentiate and gradually migrate through the different layers of the epidermis until they reach the stratum corneum in the form of coeocytes.

Skin aging, whether resulting from a normal phenomenon or accentuated by an external factor such as exposure to UV rays, involves dysfunctions in cell differentiation and/or renewal leading to atrophy of all the bases of the skin.   

The skin may also experience a loss of elasticity and firmness, and a less luminous complexion. The skin may also experience other changes in its visual appearance, including stretch marks or redness.