The epidermis is a keratinised, layered, squamous epithelium. Its thickness ranges 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 (constituting approximately 90% of all the cells of the skin) is the keratinocyte. New keratinocytes are constantly being produced in the stratum basale. As they are pushed towards the surface, they undergo a process called keratinization.

This implies the synthesis of a special protein known as keratin, and the subsequent death of the cell. The dead keratinized cells (called squames) comprise 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 estimated 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 estimated that the stratum corneum is completely renewed every 15 days.

The keratin contained by cells defends the underlying tissues from heat, chemicals, and microbes. Melanocytes are the next most typical cells of the epidermis. These have long thin projections that produce a dark pigment called melanin. This pigment is then moved to the keratonicytes where it absorbs ultraviolet light, thus defending the skin from its damaging effects, and preventing further actinic keratosis symptoms. The only other cells present in the epidermis are Langerhans cells (which are involved in the defensive reaction to microbial invaders) 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 a highly complex structure that consists of:

1) Connective tissue containing collagen and elastin fibers conferring the skin elasticity; 2) Small finger-like projections (papillae) which protrude into the epidermis and have 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 exists in symbiosis, feasting on and transforming the sebum secreted from the sebocytes cells before it submerges on the surface 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 increase due to an active and large sebaceous system and sebum doesn't flow properly out of the ducts, then colonization and high reproduction rates of Proprionibacterium acne will appear. The resident microbial flora creates biologically active molecules like histamine, extracellular enzymes and peptides, which may trigger the inflammatory response that characterizes acne breakouts.