Final answer:
The stratum corneum is the outermost layer of the epidermis that acts as a barrier protecting active cells beneath it from the external environment. It consists of dead keratinocytes rich in keratin, and it is responsible for preventing microbial penetration and dehydration of underlying tissues.
Step-by-step explanation:
The Stratum Corneum: Barrier of the Epidermis
It forms a barrier between the active cells of the lower epidermis and the outer dead cells; the stratum corneum is the answer to this question. The stratum corneum is the most superficial layer of the epidermis and holds a paramount role in protecting the underlying tissues. Consisting of 15 to 30 layers of dead keratinocytes that are rich in keratin, this tough and waterproof barrier prevents both microbial penetration and dehydration. These cells are periodically shed and replaced by new keratinocytes that push up from the lower layers of the epidermis. The entire process of cell replacement in the stratum corneum occurs over approximately 4 weeks, ensuring a continuous fortification of the skin's protective layer.