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The epidermal differentiation complex (EDC) is a gene complex comprising over fifty genes encoding proteins involved in the terminal differentiation and cornification of keratinocytes, the primary cell type of the epidermis. In humans, the complex is located on a 1.9 Mbp stretch within chromosome 1q21.
During this differentiation process, keratinocytes permanently withdraw from the cell cycle, initiate expression of epidermal differentiation markers, and move suprabasally as they become part of the stratum spinosum, stratum granulosum, and eventually corneocytes in the stratum corneum.
Cellular differentiation is the process in which a stem cell changes from one type to a differentiated one. [2] [3] Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types.
Keratohyalin is a protein structure found in cytoplasmic granules of the keratinocytes in the stratum granulosum of the epidermis.Keratohyalin granules (KHG) mainly consist of keratin, profilaggrin, [1] loricrin [2] and trichohyalin proteins which contribute to cornification or keratinization, the process of the formation of epidermal cornified cell envelope.
Filaggrin (filament aggregating protein) is a filament-associated protein that binds to keratin fibers in epithelial cells.Ten to twelve filaggrin units are post-translationally hydrolyzed from a large profilaggrin precursor protein during terminal differentiation of epidermal cells. [3]
This process is a normal protective mechanism of the skin to prevent pathogens from colonizing the skin, and is referred to as desquamation. In healthy skin, desquamation is an invisible process and the stratum corneum is turned over completely within 2–4 weeks, while maintaining the tissue thickness.
Once the three germ layers have been established, cellular differentiation can occur. The first major process here is neurulation, wherein the ectoderm differentiates to form the neural tube, neural crest cells and the epidermis. Each of these three components will give rise to a particular complement of cells.
Histologic image showing a section of epidermis. Stratum spinosum labeled slightly below center. The stratum spinosum (or spinous layer/prickle cell layer) [1] is a layer of the epidermis found between the stratum granulosum and stratum basale. [2] This layer is composed of polyhedral keratinocytes. [3] [4] These are joined with desmosomes. [3]