keratinocytes

Keratinocytes are the most abundant cell type in the epidermis, the outermost layer of the skin. They originate in the basal layer (stratum basale) from resident stem cells and proliferate to replenish cells shed from the surface. As they migrate outward, keratinocytes undergo a program of differentiation that progresses through the spinous (stratum spinosum) and granular (stratum granulosum) layers, culminating in the formation of the anucleate corneocytes that compose the stratum corneum.

In addition to forming a physical barrier, keratinocytes synthesize keratin intermediate filaments that reinforce cellular and tissue integrity. During differentiation, they accumulate keratohyalin granules and profilaggrin, which are processed into filaggrin to help form the cornified envelope. The lamellar bodies they release deliver lipids to the extracellular space, contributing to the lipid-rich barrier that limits water loss and protects against environmental insults.

Keratinocytes participate in innate immunity by producing antimicrobial peptides, such as defensins and cathelicidins, and by secreting cytokines and chemokines in response to injury or infection. They also contribute to vitamin D synthesis through UVB-driven conversion of 7-dehydrocholesterol to vitamin D3.

Turnover in healthy adults typically completes within about 28 to 30 days, though it can be accelerated in response to injury or certain diseases. Keratinocytes play a central role in wound healing, migrating to the wound site, proliferating, and differentiating to restore the epidermal barrier.

Dysfunction or abnormal proliferation and differentiation of keratinocytes underlie several skin conditions, including psoriasis, ichthyosis, atopic dermatitis, and skin cancers such as squamous cell carcinoma.