Keratohyalin

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. During the keratinocyte differentiation, these granules maturate and expand in size, which leads to the conversion of keratin tonofilaments into a homogenous keratin matrix, an important step in cornification.[3]

Keratohyalin granules can be divided in three classes: globular KHG (found in quickly dividing epithelia, such as the oral mucose), stellate KHG (found in the slowly dividing normal epidermis) and KHG of Hassall's corpuscles or type VI epithelioreticular cells of the thymus gland.[4] The exact purpose of the keratinization of Hassall's corpuscles remains unknown.

During skin differentiation process, keratohyaline granules discharge their contents in the junction between stratum granulosum and stratum corneum cell layers and form the barrier. At the same time, the inner side of the cell membrane thickens forming the cornified cell envelope. After the release of the granules, nuclei, ribosomes and mitochondria disappear, the cells become densely packacked with filaggrin and cover more surface. After final dehydration, the cell desquamates.[5]

The release of keratohyalin granules plays an important role in the skin moisturization. Filaggrin undergoes chemical modification and proteolytic processing which leads to the formation of natural moisturizing factor (NMF), hygroscopic amino acids and derivatives which function as UV protectant, modulate stratum corneum pH and water retention.[6]

References
  1. Sandilands, A.; Sutherland, C.; Irvine, A. D.; McLean, W. H. I. (2009-05-01). "Filaggrin in the frontline: role in skin barrier function and disease". Journal of Cell Science. 122 (9): 1285–1294. doi:10.1242/jcs.033969. ISSN 0021-9533. PMC 2721001. PMID 19386895.
  2. Steinert, Peter M.; Marekov, Lyuben N. (1995-07-28). "The Proteins Elafin, Filaggrin, Keratin Intermediate Filaments, Loricrin, and Small Proline-rich Proteins 1 and 2 Are Isodipeptide Cross-linked Components of the Human Epidermal Cornified Cell Envelope". Journal of Biological Chemistry. 270 (30): 17702–17711. doi:10.1074/jbc.270.30.17702. ISSN 0021-9258. PMID 7543090.
  3. Freeman, S. Caleb; Sonthalia, Sidharth (2019-01-09). Histology, Keratohyalin Granules. StatPearls Publishing. PMID 30725734.
  4. Westerhof, W.; Dingemans, K. P. (1987). "The Morphology of Keratohyalin Granules in Orthokeratotic and Parakerataotic Skin and Oral Mucosa". International Journal of Dermatology. 26 (5): 308–312. doi:10.1111/j.1365-4362.1987.tb00195.x. ISSN 1365-4632. PMID 2440821. S2CID 20658832.
  5. Deo, Priya Nimish; Deshmukh, Revati (2018). "Pathophysiology of keratinization". Journal of Oral and Maxillofacial Pathology. 22 (1): 86–91. doi:10.4103/jomfp.JOMFP_195_16 (inactive 2021-01-15). ISSN 0973-029X. PMC 5917548. PMID 29731562.CS1 maint: DOI inactive as of January 2021 (link)
  6. McLean, W.H.I. (October 2016). "Filaggrin failure - from ichthyosis vulgaris to atopic eczema and beyond". British Journal of Dermatology. 175: 4–7. doi:10.1111/bjd.14997. PMC 5053269. PMID 27667308.


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