functionally the skin acts as an interface between the internal and external environment and fulfills thermoregulatory sensory and other functions as well as playing a key role as a highly effective physical barrier against infectious agents and dehydration the skin also acts as a barrier against mechanical chemical osmotic thermal and photic damage the condition of the skin is generally considered by medical practitioners and lay people alike to reflect the state of health age and other aspects of life of an individual histologically three major tissue layers are identified the uppermost layer the epidermis is a relatively thin stratified squamous epithelium which is itself composed of five strata subjacent to the epidermis is the dermis a dense fibroelastic connective tissue stroma the third layer lying beneath the dermis is the subcutaneous layer composed of areolar and fatty connective tissue there are three basic cell types in the epidermis keratinocytes which produce keratin melanocytes which are involved in pigmentation and langerhans cells which aid the immune system by intercepting foreign bodies in the skin in the epidermis a mitotic layer at the base provides keratinocytes which continuously replace those shed at the skin surface the epidermis can be divided into layers according to the stage of maturation of keratinocytes within it these layers are from deep to superficial as follows stratum basale stratum spinosum stratum granulosum stratum lucidum and stratum corneun the first three of these layers are metabolically active while the two upper layers which have attained terminal keratinization constitute the cornified zone cells of the stratum corneum eventually become detached from the epidermal surface and are replaced from below typically the time taken for a newly formed keratinocyte to pass to the surface and be shed ranges from 45.