The epidermis is the outermost of the three layers forming the skin, and the inner layers are the proper and subcutaneous skin.  The epidermal layer is a barrier to infection with environmental pathogens and regulates the amount of water released from the body into the atmosphere through transepidermal water loss. The epidermis consists of multiple layers of flattened cells that cover the base layer (the base layer) composed of column cells arranged perpendicularly.




Epidermis function

 

The epidermis function serves as a barrier that protects the body against microbial pathogens, oxidative stress (UV light) and chemical compounds, and provides mechanical resistance to minor injuries. Most of this barrier is played by the stratum corneum. Physical barrier: epidermal keratinocytes are tightly bound by cell-cell connections associated with cytoskeleton proteins, giving the epidermis its mechanical strength.

 

 

Chemical barrier: Highly organized lipids, acids, hydrolytic enzymes, and antimicrobial peptides inhibit the penetration of external chemical substances and pathogens into the body.
An immunologically active barrier: the humoral and cellular components of the immune system located in the epidermis actively fight infection.

 

The water content in the stratum corneum drops towards the surface, creating hostile conditions for the development of pathogenic microorganisms.

 

Acid pH (about 5.0) and a small amount of water make the epidermis hostile to many microbial pathogens. 
Non-pathogenic microorganisms on the surface of the epidermis help to defend against pathogens, competing for food, limiting its accessibility and producing chemical secretions that inhibit the growth of pathogenic bacterial flora.

 

Permeability

 

Psychological stress, through the growth of glucocorticoids, impairs the stratum corneum, and thus the barrier function.
Sudden and large changes in humidity change hydration of the stratum corneum in a way that can allow entry of pathogenic microorganisms.




Moisturizing the skin

 

The ability of the skin to maintain water is primarily due to the stratum corneum and is critical to maintaining healthy skin. Lipids arranged in a gradient and in an organized way between cells of the stratum corneum are a barrier to transepidermal water loss.

 

 
Skin color

 

The amount and distribution of melanin pigment in the epidermis is the main reason for skin color change in Homo sapiens. Melanin can be found in small melanosomes, particles formed in melanocytes, from where they are transferred to surrounding keratinocytes. The size, number, and distribution of melanosomes differ among racial groups, but although the number of melanocytes may vary between different areas of the body, their number remains the same in individual areas of the body in all humans. In melanosomes in white and Asian skin, they are packed in “aggregates”, but in the black skin, they are larger and arranged more evenly. The number of melanosomes in keratinocytes increases with exposure to UV radiation, while their distribution remains largely unchanged.