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The epidermis is a keratinised, layered, squamous epithelium. This implies the production of a protein, keratin, and the inevitable death of the cell. The devitalized, keratinized cells (known as squames) comprise the outer layers of the epidermis (with lipids filling the intercellular spaces) and are slowly sloughed off - a mechanism termed desquamation. It has been estimated that the skin surface of the average adult is composed of approximately 2 x 10 ^ 9 squames. It takes from 2 to 4 weeks for the transfer of a cell from the basal layer to the external layer and, as a result of this mechanism, it has been calculated that the stratum corneum is renewed every 15 days. The keratin present in the cells defends the underlying tissues from extreme temperature, chemicals, and microorganisms. Melanocytes are the next most prevalent cells of the epidermis. These have long slender projections and secrete brown-black pigment melanin which is sent to keratonicytes where it absorbs UV light, thus protecting the skin from its harmful effects. The only other cells present in the epidermis are Langerhans cells (which are involved in the immune reaction to microbial intruders) and Merkel cells (which are linked with sensory neurons and are involved in the sensation of touch). The Dermis The dermis has a far more intricate composition and consists of 1) connective tissue including collagen and elastin fibers giving the skin elasticity; 2) small finger like projections (papillae) which protrude into the epidermis and contain nerve endings sensitive to touch, heat and pain; 3) hair follicles; 4) arrector pili muscles - for controlling hair movement; 5) sebaceous glands; 6) sudoriferous glands; 7) nerves; 8) adipose tissue; and 9) blood vessels and veins. On the keratin lined follicle duct, where the sebaceous glands are, Proprionibacterium acnes bacteria survives in symbiosis, feeding on and changing the sebum secreted from the sebocytes cells before it reaches the surface. This sebum is highly lipid in structure and also contains DNA, RNA, proteins, and other cellular components that result from the breakdown of sebocytes themselves. Proprionibacterium acnes are found only in sebaceous rich patches. If the nutrients in the follicular canals augment due to an active and large sebaceous system and, sebum does not flow adequately out of the canals, then colonization and high growth rates of Proprionibacterium acnes will appear. The innate bacterial flora produces naturally active molecules such as histamine, extracellular enzymes, and peptides which can initiate the inflammatory reaction that characterizes acne vulgaris. Since the follicular lining in the pilo-sebaceous unit is intact before puberty, it has been theorized that if colonization of Proprionibacterium acnes happens in sufficient numbers, they produce antigenic molecules that stimulate the initiation of inflammation. Proprionibacterium acnes can create proteinases, lipase, and hyaluronate lyase all of which can serve as the promoters or initiators of the inflammatory infiltrate which are composed of neutrophils and lymphocytes.
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