Which epidermal layer is the sole mitotically active layer that produces cells for the superficial layers?

• A. Stratum Corneum
• B. Stratum Granulosum
• C. Stratum Spinosum
• D. Stratum Basale

Answer: (D)

The main idea is that epidermal renewal comes from the deepest living layer that contains actively dividing cells. This basal layer sits on the basement membrane and consists of a row (or few rows) of keratinocytes that are capable of mitosis. These basal cells continually divide to replace cells that are shed from the surface. Their daughter cells migrate upward through the epidermal strata, gradually differentiating as they go. In the next layer up they become more flattened and connected, then in the granulosum they accumulate keratin and keratohyalin and begin keratinization, and finally they reach the outermost corneum as dead, keratin-filled cells that form the protective barrier. Because only this deepest layer remains mitotically active, it is the source of cells for all the superficial layers. The layers above are more differentiated and cease dividing, so they cannot supply new cells.