The epidermis is the skin or surface layer of an animal or plant. At its most basic, it is a sheet of cells. The cells are continually replaced by cell division (mitosis), and the layer (in animals) is sensitive to stimuli such as touch. In land vertebrates the epidermis has many layers; the outer layers may be reinforced by keratin.
The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress (UV light) and chemical compounds and provides mechanical resistance. Most of that function is played by the stratum corneum.
Characteristics of the barrier
- Physical barrier through keratinocytes attached together via cell–cell junctions and associated to cytoskeletal proteins, which gives the epidermis its mechanical strength.
- Chemical barrier through the presence of highly organized lipids, acids, hydrolytic enzymes and antimicrobial peptides.
- Immunologically active barrier through humoral and cellular constituents of the immune system.
- Water content of the stratum corneum drops towards the surface, creating hostile conditions for pathogenic microorganism growth.
- An acidic pH (around 5.0) and low amounts of water make it hostile to many microorganic pathogens.
- The presence of non-pathogenic microorganism on the epidermis surface help defend against pathogenic one by limiting food availability and through chemical secretions.
Factors that alter the barrier
- Psychological stress, through an increase in glucocorticoids, compromises the stratum corneum and thus the barrier function.
- Sudden and large shifts in humidity alter stratum corneum hydration in a way that could allow entry of pathogenic microorganisms.
The ability of the skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin. Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss.
The amount and distribution of melanin pigment in the epidermis is the main reason for variation in skin color in Homo sapiens. Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melanosomes varies between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and Asian skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected.
Images for kids
Epidermis Facts for Kids. Kiddle Encyclopedia.