The epidermis is a keratinised, layered, squamous epithelium. Its thickness varies from 0.5 to 3mm. depending on its location. Being the outermost layer of the skin, it is certainly an important site for microbial colonization.

The most common cell of the epidermis (comprising approximately 90% of all the cells of the skin) is the keratinocyte. Fresh keratinocytes are constantly being produced in the stratum basale. When they are pushed towards the surface, they undergo a process called keratinization.

This involves the synthesis of a unique protein known as keratin, and the subsequent death of the cell. The devitalized keratinized cells (called squames) comprise the outer 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 passage of a cell from the basal layer to the external layer of the skin, and as a consequence 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 second most common cells of the epidermis. These have long slender projections that produce a brown-black pigment termed melanin. This pigment is then moved to the keratonicytes where it absorbs ultraviolet light, thus defending the skin against its harmful effects, stopping the development of further actinic keratosis symptoms. The only other cells present in the epidermis are Langerhans cells (which are involved in the immune response to microbial invaders) and Merkel cells (which are associated with sensitive neurons and are involved in the sense of touch).

Know your skin's anatomy

The dermis has an extremely complex structure that consists of:

1) Connective tissue including collagen and elastin fibers conferring the skin tone; 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 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 are found, Proprionibacterium bactaria live in symbiosis, feeding on and transforming the sebum produced by the sebocytes cells before it reaches the exterior of the skin. The nascent sebum is largely lipid in structure and also contains DNA, RNA, proteins, and other cellular 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 canals, then colonization and high reproduction rates of Proprionibacterium acne will form. The resident microbial flora produces biologically active molecules like histamine, extracellular enzymes and peptides, which may initiate the inflammatory response that characterizes acne breakouts.