Hoornlagen

Hoornlagen, commonly translated as horny layers, refer to the outermost part of the epidermis. In human skin they correspond to the stratum corneum, a compact layer of dead, flattened keratinocytes called corneocytes that have undergone terminal differentiation (cornification). These cells are rich in keratin filaments and are anucleate, bound together by corneodesmosomes, and embedded in an intercellular lipid matrix rich in ceramides, cholesterol, and free fatty acids.

The thickness of the hoornlagen varies with anatomical site and mechanical demand; it is thickest on the palms and soles and thinnest on the eyelids. Turnover of the epidermis from basal keratinocytes to shed corneocytes takes roughly 2-4 weeks in healthy adults. Desquamation occurs as corneodesmosomes are gradually degraded by proteases, allowing corneocytes to shed.

Functionally, the hoornlagen form a primary barrier against transepidermal water loss, invasion by pathogens, and chemical insult. The lipid matrix provides reduced permeability and contributes to skin hydration. The corneocytes also contribute to the mechanical properties and optical appearance of the skin.

Clinical relevance is linked to alterations in the hoornlagen in many dermatological conditions, such as hyperkeratosis, ichthyosis, eczema, and psoriasis. In clinical practice, barrier function is often assessed indirectly by transepidermal water loss, and therapies (emollients, moisturizers, keratolytics) aim to restore or modulate the horn layer.