Most of this barrier role is played by the stratum corneum. CharacteristicsPhysical 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.PermeabilityPsychological 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.
In most vertebrates, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer or stratum germinativum has formed.This inner layer is a germinal epithelium that gives rise to all epidermal cells. It divides to form the outer spinous layer (stratum spinosum). The cells of these two layers, together called the Malpighian layer(s) after Marcello Malpighi, divide to form the superficial granular layer (Stratum granulosum) of the epidermis.The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally become the cornified layer (stratum corneum), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell. After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 30 - 90 milligrams of skin flakes every hour, or 0.720 - 2.16 grams per day.Epidermal development is a product of several growth factors, two of which are:
Transforming growth factor Alpha (TGFα) is an autocrine growth factor by which basal cells stimulate their own division. Keratinocyte growth factor (KGF or FGF7) is a paracrine growth 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.