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, loricrin 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.
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. 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.
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.