Basal cell carcinomas occur in which layer of the epidermis?

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

Answer: (A)

Basal cell carcinomas originate from the cells at the bottom of the epidermis—the basal cells in the stratum basale, which rest on the basement membrane between the epidermis and dermis. These basal cells are the proliferative, stem-like cells that constantly divide to replenish the epidermis and produce keratinocytes that migrate upward to form the other layers. Because the cancer starts in this deepest, regenerative layer, the tumor cells resemble normal basal cells and tend to grow along the dermal-epidermal junction. The other epidermal layers—spinosum, granulosum, and lucidum—represent more differentiated keratinocytes and are not the source of basal cell carcinomas (lucidum is only present in thick skin). So the correct layer is the stratum basale.