The epidermis is the thinner layer of the skin's surface, composed mainly of epithelial cells. The outermost cells contain the hard protein keratin and are known as keratinocytes.

The basal, or deepest epidermal cells, are attached to the basement membrane by adhesive molecules (or glue), specifically fibronectin. These immature cells are constantly proliferating and migrating toward the surface, to replace damaged skin cell molecules; e.g., following the event of a wound.

The same kinds of rejuvenating epidermal cells are located in hair follicles and other skin structures, which are anchored to the dermis. As the cells mature and move to the surface, they create keratin, which becomes an effective barrier to environmental dangers that cause oily skin or excessive water evaporation.

The stratum corneum is the 'outermost layer' of the epidermis, consisting of several flattened layers of devitalized keratinocytes and keratin. This layer defends against the entry of bacteria and toxins, primary causes of inflammation.

Recovery of the epidermal layer by this regenerative mechanism takes 2-3 weeks. However, regeneration depends on an ideal tissue-healing environment, which includes contact cues for cell replication and migration as well as chemical stimuli known as growth factors. These cues and stimuli at the injury surface are necessary to direct adequate orientation and mitotic reaction of the epidermal cells. Many of the cues come from skin elements, especially the matrix protein fibronectin, and matrix compounds such as hyaluronic acid.

The Dermis

DERMIS - The dermis is divided into a thin, external layer known as the 'papillary dermis', containing the anchoring epidermal rate pegs and the thicker, deeper portion known as the 'reticular dermis'. The papillary dermis is the most bioactive portion of the dermis. The primary cell type is the fibroblast, which makes the key structural extra-cellular matrix proteins, collagen and elastin, as well as matrix or ground substance. Also, these cells produce the key adhesion proteins used to attach epidermal cells to the basement membrane and are used for epidermal cell migration and proliferation.

Fibronectin is a key fibroblast derived signal protein for the orchestration of curing. The ground substance or matrix is made up of intricate polysaccharide-protein complexes known as protein, glycosaminoglycans or the GAG element, as well as hyaluronic acid. The matrix is a semi-liquid that allows cell and connective tissue arrangement, nutrient distribution to the cells, and also provides a scaffolding for cell migration.

BASEMENT MEMBRANE - The interface between the layers of the dermal-epidermal unit is the basement membrane, which secures the epidermal cells from above and the dermis from below.

THICKNESS - Normal thickness of the bilayer is 1-2 mm and is considerably thinner in infants and the elderly.

The epidermis contains the proliferating epithelial cells at the basement membrane, permitting it to move to the surface and create keratinocytes, leading to biological skin renewal. The basement membrane, rich in adherence proteins, secures the epithelial cell layer. The dermis is made of components produced by fibroblasts, including collagen (mostly type I), glycosaminoglycans, and other essential proteins.