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 colonization.

The most common cell of the epidermis (comprising approximately 90% of all the cells of the skin) is the keratinocyte. New keratinocytes are constantly being created in the stratum basale. When they are moved towards the surface, they experience a process known as keratinization.

This implies the production of a unique protein called keratin, and the subsequent death of the cell. The dead keratinized cells (known as squames) populate the external layers of the epidermis with lipids that occupy the intercellular spaces and are gradually sloughed off - a process termed desquamation. It has been calculated that the skin surface of the average adult is made of approximately 2 x 10 (9) squames. It takes between 2 and 4 weeks for the passage of a cell from the basal layer to the external layer of the skin, and as a result of this process, it has been calculated that the stratum corneum is entirely renewed every 2 weeks.

The keratin contained by cells defends the underlying tissues against heat, chemicals, and microbes. Melanocytes are the next most common cells of the epidermis. These have long thin projections that secrete a dark pigment termed melanin. This pigment is then transferred to the keratonicytes where it blocks ultraviolet rays, thus defending the skin against 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 defensive response to microbial intruders) and Merkel cells (which are linked 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 including collagen and elastin fibers conferring the skin elasticity; 2) Small 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 controlling 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 lined follicle canal, where the sebaceous glands are found, Proprionibacterium bactaria exists in symbiosis, feeding on and transforming 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 rupture of sebocytes themselves. Proprionibacterium acne is found only in sebaceous rich areas.

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