Keratohyalin is a protein structure found in cytoplasmic granules of the

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

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 hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

. Keratohyalin granules (KHG) mainly consist of

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

, 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 Trichohyalin is a protein that in mammals is encoded by the ''TCHH'' gene. Discovery In 1903 the name ''trichohyalin'' was assigned to the granules of the inner root sheath (IRS) of hair follicles discovered by Hans Vörner. In 1986 the name was ...

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 (or thymic corpuscles (bodies)) are structures found in the medulla of the human thymus, formed from eosinophilic type VI epithelial reticular cells arranged concentrically. These concentric corpuscles are composed of a central ...

or type VI epithelioreticular cells of the

thymus gland The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. T ...

. 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 for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compact ...

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. 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.

References

Keratins {{protein-stub