Which cells in stratified epithelia serve as mitotically active progenitors for renewal?

• A. Basal cells.
• B. Keratinocytes in superficial layer
• C. Melanocytes
• D. Langerhans cells

Answer: (A)

In stratified epithelia, renewal is driven by a mitotically active population in the basal layer that sits on the basement membrane. These basal cells include stem cells and transient amplifying progenitors. They divide to produce new keratinocytes that then move upward through the suprabasal layers, progressively differentiating as they go. The cells in the superficial layer are already mature and non-dividing, so they cannot serve as the ongoing source of new cells. Melanocytes and Langerhans cells have specialized roles (pigment production and immune surveillance, respectively) and are not the proliferative progenitors responsible for renewing the epithelial surface. Thus, the basal cells are the cells that drive renewal.