Structure & Properties of Skin

Anatomy (Bilayer Organ)

EPIDERMIS – The outer thinner layer known as the epidermis is composed mainly of epithelial cells. The outermost cells contain the tough protein keratin and are known as keratinocytes. The basal or deepest epidermal cells are anchored to the basement membrane by adhesion molecules (or glue); namely fibronectin. These immature cells are continually dividing and migrating toward the surface, to replace lost surface cells; e.g., after an injury. The same types of regenerating epidermal cells are found in hair follicles and other skin appendages, which are anchored in the dermis. As the cells mature and migrate to the surface, they form keratin, which becomes an effective barrier to environmental hazards such as infection and excess water evaporation.

The stratum corneum is the “outermost layer” of the epidermis consisting of several flattened layers of dead keratinocytes as well as keratin. This layer protects against entry of bacteria and toxins.
Replacement of the epidermal layer by this regenerative process takes 2-3 weeks. However, regeneration depends on an optimal 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 wound surface are necessary to direct proper orientation and mitotic response of the epidermal cells. Many of the cues come from dermal elements, especially the matrix protein fibronectin, and matrix compounds such as hyaluronic acid.

DERMIS – The dermis is divided into a thin, superficial layer known as the papillary dermis, containing the anchoring epidermal rete 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 produces the key structural extra-cellular matrix proteins, collagen and elastin, as well as matrix or ground substance. In addition, these cells produce the key adhesion proteins used to attach epidermal cells to the basement membrane and are used for epidermal cell migration and replication. Fibronectin is a key fibroblast derived signal protein for orchestration of healing. The ground substance or matrix is made up of complex polysaccharide-protein complexes known as protein, glycosaminoglycans or the GAG component, as well as hyaluronic acid. The matrix is a semi-fluid that allows cell and connective tissue orientation, nutrient diffusion to the cells and also provides a scaffolding for cell migration.

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

THICKNESS – Average thickness of the bilayer is 1-2 mm and is considerably thinner in infants and the elderly, especially the dermis that is underdeveloped in infants and atrophic in the elderly.

Fig.1 Anatomy of Skin

Normal skin is shown with the key epidermal and dermal elements. The epidermis contains the dividing epithelial cells at the basement membrane migrating to the surface forming keratinocytes and maintaining the surface barrier. The basement membrane, rich in adherence proteins, anchors the epithelial cell layer. The dermis is made of components produced by fibroblasts, including collagen (mostly type I), glycosaminoglycans, and other matrix proteins.

Fig.2 Anatomy of Normal Skin

This cross section of normal skin demonstrates the key components. Keratin, a product of keratinocytes, resides on the outer most surface. Keratinocytes (epithelial cells) make up the first or outer layer of skin, i.e. the epidermis. Note that keratinocytes can also be seen in the cross sections of hair follicles found in the dermis. The dermal layer is composed of collagen seen as swirls of pink material. The lighter pink homogenous material is made up of glycosaminoglycans and other compounds produced by the fibroblasts (elongated dark cells) such as fibronectin and hyaluronic acid.

Skin Functions
Epidermis: Protection from desiccation
Protection from bacterial entry
Protection from toxins
Fluid balance: avoiding excess evaporation loss
Neurosensory
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Dermis:
Protection from trauma due to the dermal properties of elasticity and durability
Fluid balance through regulation of skin blood flow
Thermoregulation through control of skin blood flow
Growth factors and contact direction for epidermal replication and dermal repair

EPITHELIAL CELLS: These cells make up the majority of the epidermis. Immature cells are programmed to divide, migrate, and mature to keratin-producing cells called keratinocytes. The signal to activate this process comes from messenger proteins called growth factors as well as through contact direction from key dermal adhesive proteins.

FIBROBLASTS: These cells of mesenchymal origin (embryonic connective tissues) are normally present in the dermis and produce normal dermal replacement components. After injury, these cells migrate into the wound and proliferate in order to produce increased quantities of these dermal proteins and matrix.

FIBROBLAST PRODUCTS :

• Collagen (Type I predominant in skin)
• Matrix proteins
• Proteoglycans, glycosaminoglycans, hyaluronic acid, other matrix components
• Growth factors and other growth stimulants
  

ENDOTHELIAL CELLS: These cells make up the lining of micro and macro vessels and also make up the lining of new capillaries produced after injuries. These cells are attracted into the wounds by local signals.

MACROPHAGES: These cells of mesenchymal origin (embryonic connective tissues) are normally present in tissues but increase in number after injury, attracted by chemical messages released by the activation of inflammation. These long-lived cells release the protein chemical messages, growth factors, and growth stimulants which orchestrate healing in an organized fashion.

PLATELETS: These factor-rich particles release a host of growth factors and adherence proteins during the initial post-burn period.

NEUTROPHILS: These short-lived cells have been an immune function and are the first cells migrating to the wound service. Their role is to control bacteria or other toxic elements from surface penetration. Excessive inflammation, as occurs with surface dead tissue or ongoing stimulation of neutrophils sequestration, will impede healing. Surface exudate is rich in dean and dying neutrophils and in toxic protease activity.