What is the only mitotic layer in the epidermis?

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

Answer: (A)

The layer that keeps epidermal renewal going is the basal layer, where keratinocytes divide to replenish the epidermis. This bottom layer sits on the basement membrane and contains stem cells that continually undergo mitosis, producing new keratinocytes. As these new cells arise, they migrate upward through the spinous, granular, and then corneous layers, differentiating along the way and ultimately becoming the flat, dead cells of the outermost stratum corneum. The spinous layer mainly shows cells connected by desmosomes and beginning differentiation, the granular layer contains keratohyalin granules and lipid processing, and the corneum is composed of anucleate, cornified cells. But mitotic activity is confined to the basal layer, making it the only mitotic layer.