Epithelium serves as an important barrier, protecting internal organs from external threats, preserving adequate levels of fluid in the body, and controlling the permeability of substances across the epithelial barrier.[1] Epithelial cells are avascular, and nutrients pass through the barrier through absorption or diffusion. They can replicate very quickly, and as cells die, they can slough off through a process called exfoliation. In the digestive tract, the epithelium releases digestive enzymes. In the respiratory tract, epithelial cells secrete mucous that trap microorganisms and debris. Some epithelial cells have cilia, microscopic extensions of the apical membrane that help to trap particles and move the fluid. In the brain, the cilia help to circulate cerebrospinal fluid (CSF), and in the respiratory tract, the cilia help to trap pathogens in mucous. Epithelial tissues can also form exocrine glands which secrete fluids through ducts and endocrine glands that secrete hormones. Epithelial cells also form the outer layer of the skin protecting the body from friction and mechanical forces.[3]

Different anatomical connections between epithelial cells allow for varied interactions between cells: tight junctions, anchoring junctions, and gap junctions. Tight junctions split epithelial cells into apical and basal sections. Anchoring junctions are found on the basal and lateral surfaces of the epithelial cells and provide strength and flexibility in three distinct ways:

• Desmosomes hold adjacent cells together using adhesion molecules like cadherin.
• Hemidesmosomes link cells to the extracellular matrix using adhesion molecules such as integrins.
• Adherens junctions use actin along with either cadherin or integrins to influence the shape of the epithelium.
• Gap junctions are unique in that they form an intracellular connection between membranes of cells to transfer ions molecules and allow epithelium.

Skin Epithelialization

The process of epithelialization has primarily been studied in the context of skin re-epithelialization during wound healing. Skin is composed of 3 main layers: epidermis, dermis, and subcutaneous. The outer layer of the skin is created by stratified squamous epithelium called the epidermis. The epidermis can be further broken down into the outer cornified layer, granular layer, spinous layer, and basal layer.[1]

Keratinocytes are the primary component of the epidermis and play a key role in maintaining the outer skin barrier and in wound healing. Keratinocytes proliferate in the basal layer, differentiate as they rise through the granular layer, lose their nucleus and flatten to become the outer layer of skin known as the stratum corneum. Deficits in this process can result in delayed wound healing.[1]

Keratinocytes proliferate in the basal layer and are composed of keratin intermediate filaments K5 and K14. As the keratinocytes rise into the suprabasal layers, they characteristically differentiate into K1 and K10. In the granular layer, lipids and proteins are produced by the lamellar granules and fill in the crevices between the keratinocytes in the stratum corneum. High molecular weight polymers form by cornified envelope proteins (loricrin, involucrin, filaggrin) crosslinking. The cornified envelope is formed through the process of terminal differentiation, where the keratinocyte becomes dehydrated and flattens into a polyhedron referred to as terminal corneocyte. The lipid layer fills in between the corneocytes, functioning as the "mortar" and corneocytes as "bricks." This lipid layer is important for keeping water in the body. [1]

During keratinocyte differentiation, the cells change from dividing to nondividing cells as they migrate to the surface through the granular layer. This process is regulated by three major MAP kinase pathways. These pathways are activated by calcium influx, epidermal growth factor, and tumor necrosis factor. The process also uses protein kinase C isoforms. Once keratinocytes turn profilaggrin into filaggrin, they undergo changes into late terminal differentiation and are irreversibly committed to the process of differentiation. Differentiation ends when proteolytic and nucleolytic activity destroys the cellular organelles and DNA. Increased intracellular calcium forms the cornified envelope by activating transglutaminase, which then works to covalently crosslink structural proteins such as loricrin, involucrin, and filaggrin. Finally, insoluble lipids attach and the outermost layer of the epidermis is complete.[1]

The epidermal barrier is maintained by skin calmodulin-related factor (Scarf), which senses calcium and regulates the protein function in the skin barrier. Keratinocytes constantly renew to maintain this barrier. The renewing capabilities of the skin are driven by the population of epidermal stem cells (ESC). ESCs are found in three distinct niches: the bulge of hair follicles, the base of sebaceous glands, and the basal layer of the interfollicular epidermis. Each ESC niche replenishes its compartment; therefore, the depth of a wound and damage to these structures affects the skin’s ability to re-epithelialize.[1]

Reepithelialization in Wound Healing

There are 3 main phases of wound healing: the inflammatory phase, the proliferative phase, and the differentiation phase. It is important to note that there is considerable overlap of these stages. Tissue injury triggers the inflammatory phase, which is dominated by the release of inflammatory mediators, neutrophils, and macrophages traveling to the site of the wound to phagocytize bacteria and initiate the proliferative phase. The proliferative phase refers to the formation of new granulation tissue to fill the wound. This is followed by the differentiation phase, where collagen fibers in the wound become more organized and the wound strengthens.[1]

The initial stage of wound healing is the inflammatory stage, which is triggered by tissue injury. Cytokines and vasoactive substances such as histamine and serotonin are released, which cause an increase in vessel permeability. The leaky vessels allow for recruitment of lymphocytes, neutrophils, and macrophages to the wound site. Blood constituents also leak into the area and activate the clotting cascade. These blood clots serve as a matrix for cells to adhere and migrate into the wound, as well as a source of growth factors for fibroblasts and inflammatory cells. The first 48 hours are predominated by the inflammatory infiltrate with neutrophils cleaning the wound and phagocytosing bacteria.[1] Important cytokines for wound repair are also released which initiate the proliferative phase, such as IL-1, IL-6, vascular endothelial growth factor, tumor necrosis factor, and TGF-beta.[1][3]

As the inflammatory phase of wound healing ends, the proliferative phase begins and refers to the formation of granulation tissue within the wound. The onset of the proliferative phase is marked by the proliferation of fibroblasts and endothelial cells. Fibroblasts form structural proteins that make collagen and increase the strength of the wound. Angiogenesis occurs to provide oxygen and nutrients to the new tissue granulating in the wound. Once the bed of granulation tissue is laid, the process of reepithelialization can take place.[1]

Within hours of skin trauma, keratinocytes along the edges of the wound, around hair follicles, and surrounding sebaceous glands are activated by monocytes and neutrophils to migrate and proliferate to re-epithelialize the area with skin cells.[3] Basal cell keratinocytes release their attachment to the underlying dermis and migrate across the wound to cover its surface, going through rapid mitotic division as the cells move across each other in a "leapfrog" fashion, advancing in a sheet across the wound.[1]

In order to migrate across the wound, keratinocytes must undergo a structural transformation at the cellular layer. The cells lengthen, flatten, and develop actin filaments and pseudopodia, which serve as temporary protrusions for movement.[4] The keratinocytes then lose their hemidesmosome's attachment to the surrounding cells. Integrins ( proteins for attachment) release and are relocated to actin filaments which work to pull the keratinocytes into the wound. The keratinocytes migrate into and adhere to the newly formed wound matrix. These changes in the keratinocyte cytoskeleton allow for increased cell flexibility and migration of keratinocytes and are classically marked by changes in expression of keratin-6 and keratin-16.[2] Activated keratinocytes and fibroblasts in the dermis release growth factors such as keratinocyte growth factor and hepatocyte growth factor that help to regulate this process. Keratinocytes keep proliferating at the wound edges and migrating to cover the wound until they meet in the middle. Some sources report that well-approximated wounds can re-epithelialize within 48 hours, others report the process of epithelialization generally takes 2 to 3 weeks.[3] The faster this process occurs, the less scarring there is. Thick scabs over a wound can inhibit the ability of the keratinocytes to migrate across the wound and increase length of healing time.

At 2 weeks, the wound is only at 10% total wound strength.[5] The differentiation phase of wound healing occurs from 2 weeks to 1 year and refers to the reduction of macrophages and fibroblasts in the wound. Fibroblasts deposit collagen type 1 within the wound that creates a stable matrix.[1] Collagen fibers become more organized and wound strength increases. At 1 month, the wound is at 50% of total wound strength, and by 1 year the maximum scar strength of 80% is achieved.

If aspects of the wound healing process are off balance, keloid scars or delays in wound healing can result. Proteases cleave peptide bonds and are important for tissue remodeling in wound repair. If unregulated, this can lead to chronic non-healing skin ulcers.[1]