factor produced by the underlying dermal fibroblasts in which the proliferation of basal cells is regulated. Function Barrier The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress (UV light), and chemical compounds, and provides mechanical resistance to minor injury. Most of this barrier role is played by the stratum corneum. Characteristics Physical barrier: Epidermal keratinocytes are tightly linked by cell–cell junctions associated to cytoskeletal proteins, giving the epidermis its mechanical strength. Chemical barrier: Highly organized lipids, acids, hydrolytic enzymes, and antimicrobial peptides inhibit passage of external chemicals and pathogens into the body. Immunologically active barrier: The humoral and cellular constituents of the immune system found in the epidermis actively combat infection. 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 the epidermis hostile to many microorganic pathogens. Non-pathogenic microorganisms on the surface of the epidermis help defend against pathogens by competing for food, limiting its availability, and producing chemical secretions that inhibit the growth of pathogenic microbiota. Permeability 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. Skin hydration The ability of the skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin. Skin hydration is quantified using corneometry. Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss. Skin color 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 vary 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. Clinical significance Laboratory culture of keratinocytes