Skin is the largest organ in the body, constituting approximately 16% of body weight. It forms an essential barrier between the organism and the external environment. A major function of skin, therefore, is protection (e.g., against desiccation, bacterial invasion, physical injury, and ultraviolet [UV] radiation). In addition, skin is important for thermoregulation, synthesis of vitamin D, and reception of sensory stimuli.
The histologic appearance of skin varies from site to site on the body, depending upon the specific function of the skin. However, skin in all regions consists of the following layers:
Epidermis. An outer epithelial layer of ectodermal origin that is in contact with the external environment. The epidermis is a continuously renewing tissue composed primarily of cells called keratinocytes. In addition, the epidermis contains melanocytes, Langerhans cells, and Merkel cells, which are specialized for production of melanin, immune protection, and sensory reception, respectively.
Dermis. A deeper layer of connective tissue of mesodermal origin that supports and nourishes the epidermis.
Hypodermis. A subcutaneous layer of loose connective tissue below the dermis that attaches the skin to underlying tissues.
Skin contains various appendages derived from epidermis, including sweat glands, hair follicles and sebaceous glands. Skin is classified as either thick or thin. Thick skin is found on the palms and the soles. Thin skin is present everywhere else on the body.
Because the skin is readily visible, it is useful in the diagnosis of genetic and systemic diseases, which can be confirmed with pathology via minimally invasive techniques.
The epidermis is a stratified squamous keratinized epithelium derived from ectoderm, and is composed primarily of a single type of cell called a keratinocyte. In addition, three other less abundant types of cells—melanocytes, Langerhans cells, and Merkel cells—are interspersed among keratinocytes.
Skin is classified as either thick (glabrous) or thin (hairy), based on the thickness of the epidermis (Figure 11-1A–C). Epidermis in thick skin, the type of skin found on the palms, flexor surfaces of the digits, and the soles of feet, is 400 to 600-μm thick. In comparison, epidermis in thin skin, which is found everywhere else on the body, is only 75 to 150-μm thick.
Organization of epidermis in thin and thick skin. A. Organization and structure of thin and thick skin. B. Thin and thick skin from the scalp and fingertip, respectively, of the same individual. HF, hair follicle. C. Comparison of the layers of keratinocytes in thin (scalp) and thick (sole of foot) skin. Note that the stratum lucidum is present only in thick skin. Images are at different magnifications. S, stratum. D. The stratum spinosum in thick skin (fingertip) shown at high magnification.