Skin Cell Turnover: The 28-Day Cycle of Epidermal Renewal

The epidermis renews itself approximately every 28 days in young adults (slows to 40–60 days with age).

The Process: Epidermal Turnover

1. Basal layer (stratum basale): Stem cells and transit-amplifying cells divide (mitosis). One daughter cell remains a stem cell; the other commits to differentiation.
2. Spinous layer (stratum spinosum): Cells produce keratin filaments, desmosomes strengthen adhesion.
3. Granular layer (stratum granulosum): Keratohyalin granules, lamellar bodies (lipids) prepare for barrier formation.
4. Cornified layer (stratum corneum): Nuclei lost; keratinocytes become corneocytes—flat, anucleate, keratin-filled "bricks" embedded in lipid "mortar."
5. Desquamation: Proteolytic enzymes (kallikreins) degrade desmosomes, shedding corneocytes invisibly.

Regulation

• Growth factors (EGF, KGF), cytokines, vitamin D, retinoids.
• Barrier disruption (tape stripping, UV) accelerates turnover.
• Glucocorticoids slow turnover.

Clinical Relevance

• Psoriasis: Turnover accelerated to 3–5 days → thick scale.
• Ichthyosis: Impaired desquamation → scale retention.
• Topical retinoids: Normalize keratinization in acne, photoaging.