Glossary

Epidermis: The epidermis is the outer layer of the skin. It is composed of stratified squamous epithelium. It has several layers. The cells of each of these layers change as they move from the basal layer up to the surface of the skin. 1. Cornified (horny) cell layer (stratum corneum): An outer layer of overlapping flattened scale-like (squamous) remnants of cells which have lost their nuclei and which are filled with keratin, a water-insoluble protein. 2. Clear-cell layer (stratum lucidum): A thin transparent layer consisting of a substance called eleidin, a precursor of keratin. 3. Granular-cell layer (stratum granulosoma): A layer of cells containing granules of keratohyalin, an earlier precursor of kertain. 4. Prickle-cell layer (stratum spinosum) 5. Basal-cell layer (stratum basale): A layer of actively dividing columnar cells attached to a basement membrane (basal lamina) which marks the junction of the dermis and epidermis. The keratinized surface-cell remnants are continually worn away or shed while new cells are being formed by the lower layers of the epidermis. As they are formed, they are pushed outward to the surface, gradually flattened, and become filled with keratin. Keratin is a term derived from the Greek work keras meaning "horny." Melanocytes (pigment cells), located primarily in the basal layer of the epidermis, produce melanin (a dark pigment). The presence of melanin is one of the factors which determine skin color. All individuals except albinos have some melanin in their skins. It is vital for protection against the harmful effect of ultravoilet radiation. Epidermis is epithelial tissue derived from ectoderm.