Which epidermal layer is the outermost barrier in skin with dead cells at the surface?

• A. Stratum corneum
• B. Stratum granulosum
• C. Stratum spinosum
• D. Stratum basale

Answer: (A)

The outermost barrier of the epidermis is produced by a layer whose cells are dead and filled with keratin, forming a tough, waterproof shield. This is the stratum corneum, the surface layer that sits atop the skin. As epidermal cells originate in the deepest layer and move upward, they undergo cornification, losing their nuclei and organelles and becoming flattened, keratin-filled corneocytes. This stacked, keratin-rich material provides the protective barrier against water loss and pathogen entry, which is why it’s the skin’s outermost layer you see and feel. The deeper layers—where cells are still alive and dividing—include the stratum basale, spinosum, and granulosum, but they are not the surface barrier.