Keratinocyte

Keratinocytes are epithelial cells defined by their keratin intermediate filaments, which they express in layer-specific pairs: K5/K14 in the proliferating basal layer and K1/K10 once they commit to differentiation in the spinous layer (K6/K16/K17 appear in wound healing and psoriasis). The filaments insert into desmosomes (desmogleins 1 and 3, desmocollins, plakoglobin, desmoplakin) that bind neighbouring cells and into hemidesmosomes that bind basal cells to the basement membrane, creating a mechanically continuous sheet.

As keratinocytes ascend they produce keratohyalin granules (profilaggrin, loricrin) and lamellar bodies (lipids and enzymes), then undergo cornification: the nucleus and organelles are digested, filaggrin aggregates keratin into dense bundles, transglutaminases cross-link a protein cornified envelope (involucrin, loricrin) and lipids are bonded to its surface. The resulting dead, flat corneocytes are eventually shed by protease-mediated digestion of corneodesmosomes.

Keratinocytes are also active immune and endocrine cells: they secrete antimicrobial peptides (β-defensins, cathelicidin), cytokines (IL-1, TNF-α, IL-33) and chemokines in response to injury and microbes, express pattern-recognition receptors, synthesise vitamin D3 under UVB, and receive melanin from melanocytes to shield their nuclei.