The epidermis is a hardened, stratified, squamous epithelium. Its density ranges from 0.5 to 3mm. depending on its location. Being the most external layer of the skin, it is certainly an important place for microbial invasion.

The most common cell of the epidermis (comprising approximately 90% of all the cells of the skin) is the keratinocyte. Fresh keratinocytes are continually being produced in the stratum basale. As they are pushed towards the surface, they experience a process known as keratinization.

This involves the production of a special protein known as keratin, and the eventual death of the cell. The devitalized keratinized cells (known as squames) comprise the external layers of the epidermis with lipids that occupy the intercellular spaces and are gradually sloughed off - a process called desquamation. It has been calculated that the skin surface of the average adult is composed of approximately 2 x 10 (9) squames. It takes between 2 and 4 weeks for the transfer of a cell from the basal layer to the outermost layer of the skin, and as a consequence of this mechanism, it has been calculated that the stratum corneum is completely renewed every 15 days.

The keratin present in cells protects the underlying tissues from heat, chemicals, and microbes. Melanocytes are the next most typical cells of the epidermis. These have long slender projections that produce a dark pigment termed melanin. This pigment is then transferred to the keratonicytes where it absorbs ultraviolet rays, thus protecting the skin from its harmful effects, stopping the development of further actinic keratosis symptoms. The only other cells observed in the epidermis are Langerhans cells (which are part of the immune response to microbial intruders) and Merkel cells (which are associated with sensory neurons and are involved in the sense of touch).

Getting to know skin anatomy

The dermis has an extremely intricate structure that consists of:

1) Connective tissue containing collagen and elastin fibers giving the skin tone; 2) Tiny finger-like projections (papillae) which reach into the epidermis and contain nerve endings sensitive to touch, heat and pain; 3) Hair follicles; 4) Arrector pili muscles for coordinating hair movement; 5) Sebaceous glands; 6) Sudoriferous glands; 7) Nerves; 8) Adipose tissue; and 9) Capillaries and veins.

Being aware of the bodily process behind acne problems is important in the search for solutions. On the keratin invested follicle canal, where the sebaceous glands reside, Proprionibacterium bactaria live in symbiosis, feeding on and changing the sebum produced by the sebocytes cells before it submerges on the exterior of the skin. The nascent sebum is largely lipid in structure and also has DNA, RNA, proteins, and other cell components that result from the breakdown of sebocytes themselves. Proprionibacterium acne is found only in sebaceous rich areas.

If the nutrients in the follicular canals augment due to an active and large sebaceous system and sebum doesn't flow adequately out of the ducts, then colonization and high growth rates of Proprionibacterium acne will form. The resident bacterial flora creates biologically active molecules like histamine, extracellular enzymes and peptides, which can trigger the inflammatory reaction that characterizes acne breakouts.