Keratohyalinien

Keratohyalinien, commonly referred to as keratohyalin granules, are cytoplasmic inclusions found in keratinocytes of the stratum granulosum in the epidermis. They appear as basophilic granules on light microscopy and arise during the late stages of keratinocyte differentiation, serving as a reservoir of proteins required for cornification.

The granules contain several key proteins, including profilaggrin, which is processed into filaggrin, as well as loricrin, involucrin, and small proline-rich proteins. These components contribute to the condensation and organization of keratin intermediate filaments and to the formation of the cornified envelope. Filaggrin, produced from profilaggrin, aggregates keratin filaments and is subsequently degraded to amino acids that comprise the natural moisturizing factor, supporting skin hydration and barrier function. Crosslinking of enzymes like transglutaminases helps incorporate keratohyalin-derived proteins into the cornified envelope.

Functionally, keratohyalin granules play a central role in epidermal maturation and the formation of the stratum corneum, the outermost barrier of the skin. Their contents are released and reorganized during terminal differentiation, contributing to the physical strength and moisture-retaining properties of the outer epidermis.

Clinical relevance exists in disorders of keratinization. Mutations in the FLG gene, which encodes profilaggrin, reduce filaggrin production and are associated with ichthyosis vulgaris and increased risk of atopic dermatitis. Abnormal keratohyalin granule biology can reflect or contribute to altered skin barrier function in various dermatologic conditions.