The epidermis is the outer skin layer and functions as the skin's protective barrier. Whilst this barrier function is essential, it does prevent the absorption of active ingredients into the skin. The skin has not just one, but two barriers that act to prevent the absorption of active ingredients into the skin, the stratum corneum (the compressed outer portion of the epidermis) and the epidermal-dermal junction.

The dermis lies under the epidermis and these two layers are separated by the epidermal-dermal junction, a semi-permeable membrane, that allows the passage of some substances through into the dermis. Beneath the dermis lies the subcutaneous layer, which consists of fat and blood vessels. The dermis also contains hair follicles, sebaceous glands and sweat (eccrine) glands.

To successfully correct the commonest skin conditions, we need to concentrate on the epidermis and the dermis and understand their structure and function.

• Healthy skin is smooth because of undamaged and compact keratinocytes.
• Healthy skin is firm with abundant functioning collagen and elastin.
• Healthy skin is evenly coloured due to properly functioning melanocytes.
• Healthy skin is properly hydrated due to richness in glycosaminoglycans (moisture-holding cells in the dermis).
• Healthy skin has efficient and continual skin cell renewal.
• Healthy skin has a good blood supply.
• Healthy skin repairs itself quickly when scratched, scraped or injured.