Keratohyalin is a protein structure found in cytoplasmic granules of the

keratinocyte Keratinocytes are the primary type of cell found in the epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the basal layer (''stratum basale'') of the skin are sometimes referre ...

s in the

stratum granulosum 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).James, William; Berger, Timothy; Elston, Dirk (2005) ''A ...

of the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

. Keratohyalin granules (KHG) mainly consist of

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

, profilaggrin,

loricrin Loricrin is a protein that in humans is encoded by the ''LOR'' gene. Function Loricrin is a major protein component of the stratum corneum, cornified cell envelope found in terminally differentiated epidermis (skin), epidermal cells. Loricrin ...

and trichohyalin proteins which contribute to cornification or

ization, 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. 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

) and KHG of

Hassall's corpuscles Hassall's corpuscles (also known as thymic bodies) are structures found in the medulla of the human thymus, formed from eosinophilic type VI thymic epithelial cells arranged concentrically. These concentric corpuscles are composed of a central m ...

or type VI epithelioreticular cells of the

thymus gland The thymus (: thymuses or thymi) is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. The thymu ...

. 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

and

stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...

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 packaged with filaggrin and cover more surface. After final dehydration, the cell desquamates. Keratohyalin granules contribute significantly to the process of skin moisturization, albeit indirectly. As keratinocytes differentiate and move towards the surface of the skin, they release keratohyalin granules, which contain filaggrin. Filaggrin is then chemically modified and proteolytically processed to form natural moisturizing factor (NMF). NMF is composed of hygroscopic (water-attracting) amino acids and derivatives, which help the stratum corneum, the outermost layer of the skin, to retain moisture. Additionally, NMF serves as a UV protectant and modulates the pH of the stratum corneum, both critical functions for skin health.

References

{{reflist Keratins