Keratinisaatioon

Keratinization, or keratinisaatio in Finnish, is the biological process by which epidermal keratinocytes terminally differentiate to form the skin’s protective outer barrier. It occurs in the stratified squamous epithelium of the epidermis and in other keratinizing epithelia. New keratinocytes arise in the basal layer (stratum basale) and migrate superficially through the spinous (stratum spinosum) and granular (stratum granulosum) layers. During this journey the cells accumulate keratin intermediate filaments, flatten, and lose their nuclei and organelles as they become corneocytes. Lipids are secreted into the extracellular space by lamellar bodies in granulosum cells, contributing to the lipid-rich barrier of the stratum corneum. At the surface, corneocytes are enveloped in a cross-linked protein envelope formed by involucrin, loricrin, and other proteins, creating the cornified envelope that helps retain water. The outermost layer, the stratum corneum, is periodically shed in a controlled desquamation process, renewing the epidermal barrier.

The process is tightly regulated by signaling pathways and by a calcium gradient across the epidermis. Notably, keratinocyte differentiation is influenced by transcription factors such as Notch and p63, and by retinoids and other external factors. The typical transit time from basal proliferation to desquamation is around 28 days in humans, though this can vary.

Impaired keratinization underlies numerous skin disorders. Hyperkeratosis and parakeratosis are hallmarks of diseases such as ichthyosis and psoriasis, which reflect disruptions in keratin expression, lipid production, or cornified envelope formation. Genetic mutations in keratin genes or in enzymes involved in lipid processing can disrupt this process, leading to abnormal barrier function and scaling.