Which layer of skin is considered a barrier to water loss and microbial invasion?

The stratum corneum is the outermost layer of the epidermis and serves as the primary barrier against water loss and microbial invasion. This layer is composed of dead keratinized cells that are tightly packed together, which forms a physical barrier. The keratin in these cells provides water resistance, minimizing transepidermal water loss, while also protecting against potential pathogens and environmental hazards.

Additionally, the stratum corneum undergoes a process of keratinization, where cells produce more keratin and lose their nuclei, making them even more compact and resilient. This layer also contains lipids, which contribute to its barrier function by creating a hydrophobic environment that further prevents the entry of water and pathogens.

Other layers of the epidermis, such as the stratum lucidum, stratum spinosum, and stratum granulosum, do play important roles in skin health and structure but do not serve as the primary barrier mechanisms against water loss and microbial invasion like the stratum corneum does.