The stratum corneum, the outermost layer of the epidermis, is the relatively impermeable layer of skin. When undamaged, it acts as a barrier, preventing most bacteria, viruses, and other foreign substances from entering the body. This layer's impermeability is due to its composition of tightly packed, flat, dead keratinocytes forming a waterproof keratin barrier.
While the skin is often described as waterproof, it's important to note that this is a relative term. The stratum corneum's impermeability is not absolute; it does allow for some passage of substances, and prolonged exposure to water can lead to absorption. Moisturizers and sunscreens are able to penetrate because of the nature of their molecules and formulation. The pores in skin also allow for the passage of sweat and other substances. However, the stratum corneum's primary role is to provide a significant barrier against external elements, making it the most impermeable layer.
Several sources confirm this:
- Training.seer.cancer.gov: This source describes the epidermis, including the stratum corneum, as a protective layer and highlights the role of oils in maintaining the skin's waterproof nature.
- Bio.libretexts.org: This confirms the stratum corneum is composed of "flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier."
- Toppr.com: This source explicitly identifies the stratum corneum as the keratinized and water-impermeable layer.
