What is epithelial turnover, and where are stem cells located in stratified epithelia?

• A. Epithelial tissue continually regenerates; stem cells reside in the basal layer (stratum basale).
• B. Epithelial tissue stops regenerating after maturity; stem cells in superficial layer
• C. Epithelial tissue regenerates only in response to injury; stem cells in basement membrane
• D. Epithelial turnover is constant only in keratinized epithelia; stem cells in the stratum corneum

Answer: (A)

Epithelial turnover is the continual renewal of surface cells: cells are lost from the surface and replaced by new cells produced deeper in the tissue. In stratified epithelia, the stem cells sit in the basal layer (stratum basale) right next to the basement membrane. These basal cells divide to generate progenitors that migrate upward through the layers, differentiating as they go, until they reach the surface where older cells are shed. This upward movement and ongoing production keep the barrier intact despite wear and abrasion. In keratinized epithelia, the outermost layer consists of dead cells in the stratum corneum, which are shed and replaced by newer cells from the basal layer. The basal layer is the true source of new cells; the superficial layers are where aging and shedding occur, not where stem cells reside.