Anatomy of the Integumentary System

Skin is the body’s largest organ. It consists of two layers. The outermost layer is the epidermis, which is a stratified squamous epithelium. The second layer is the dermis, comprised of both loose and dense irregular connective tissue. Skin is classified as thick or thin depending on the thickness of the epidermis. Thick skin is found on the palms of the hands and the soles of the feet, whereas skin over the rest of the body is classified as thin. The epidermis is a layered structure with continuous turnover of cells called keratinocytes. Keratinocytes form from stem cells in the basal layer of the epidermis, and then migrate to the surface. As they migrate, they accumulate keratin and eventually undergo apoptosis forming the outermost layer of the epidermis, which sheds the apoptotic cells on a daily basis. In addition to keratinocytes, the epidermis contains melanocytes. These cells produce melanin, which the melanocytes transfer to keratinocytes. Melanin determines the color of skin and provides protection from UV radiation. Skin also contains hair follicles, eccrine sweat glands, apocrine sweat glands, and sebaceous glands, all of which are derived from the epidermis. Skin is richly innervated with free nerve endings and several different types of encapsulated sensory receptors. Common benign neoplasms of the skin are melanocytic nevi (moles), and acrochordons (skin tags).