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Hyperkeratinization (American English or hyperkeratinisation in British) is a disorder of the cells lining the inside of a hair follicle.It is the normal function of these cells to detach or slough off from the skin lining at normal intervals.
Keratinization is part of the physical barrier formation (cornification), in which the keratinocytes produce more and more keratin and undergo terminal differentiation. The fully cornified keratinocytes that form the outermost layer are constantly shed off and replaced by new cells.
It does this through connections to desmosomes, cell–cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures. Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin gives skin its strength.
Keratinization begins in the stratum spinosum, [5] although the actual keratinocytes begin in the stratum basale. [4] They have large pale-staining nuclei as they are active in synthesizing fibrillar proteins, known as cytokeratin , which build up within the cells aggregating together forming tonofibrils .
Because of the differences between mucous membranes and the skin (e.g., keratinizing mucosa does not have a stratum lucidum and non keratinizing mucosa does not have this layer or normally a stratum corneum or a stratum granulosum), sometimes specialized texts give slightly different definitions of hyperkeratosis in the context of mucosae.
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.
Histologic image showing a section of epidermis. Stratum granulosum labeled near center. The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis lying above the stratum spinosum and below the stratum corneum (stratum lucidum on the soles and palms). [1]
In the process, however, keratinocytes are killed. If the keratinocytes that apoptose are in the lowest layer of the skin, this can be an issue. Typically, however, keratinocytes in higher layers are able to slough away normally. The skin peeling that occurs as sunburns heal is due to this process.