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The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. [1] The epidermal layer provides a barrier to infection from environmental pathogens [2] and regulates the amount of water released from the body into the atmosphere through transepidermal water loss.
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.
Corneocytes are keratinocytes in their last stage of differentiation. Keratinocytes in the stratum basale of the epidermis will multiply through cell division and migrate toward the skin surface. During that migration keratinocytes will undergo multiple stages of differentiation to finally become corneocytes once they reach the stratum corneum.
The continuous division of epidermal basal cells leads to complete epidermal turnover every 40-56 days in humans and every 8-10 days in mice. [ 7 ] This process of proliferation and differentiation is regulated by multiple genetic and environmental factors including a calcium gradient, Vitamins A and D, epidermal growth factor (EGF ...
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.
Epidermal growth factor (EGF) is a protein that stimulates cell growth and differentiation by binding to its receptor, EGFR. Human EGF is 6-kDa [5] and has 53 amino acid residues and three intramolecular disulfide bonds. [6] EGF was originally described as a secreted peptide found in the submaxillary glands of mice and in human urine.