Which skin layer is most closely associated with the barrier to diffusion in the skin?

• A. Stratum Corneum
• B. Dermis
• C. Epidermis
• D. Hypodermis

Answer: (A)

The outermost skin layer, the stratum corneum, is the primary barrier to diffusion. It’s made of dead keratin-packed cells (corneocytes) embedded in a thick lipid matrix, arranged in a brick-and-mortar style. This lipid-rich layer forms dense lamellae that strongly resist both water loss and the passage of many chemicals. While lipophilic substances can partition into these lipids, they still must cross this tight barrier, which makes diffusion across the stratum corneum much more difficult than through the deeper skin layers. Deeper layers like the viable epidermis, dermis, and hypodermis lack this highly organized lipid barrier and are more permissive, serving roles in metabolism, blood supply, and support rather than diffusion resistance. Hence, the stratum corneum best accounts for the barrier to diffusion in the skin.