Which layers of the epidermis includes the outermost layer?

The outermost layer of the epidermis is the stratum corneum. This layer plays a crucial role in the skin's barrier function, acting as the first line of defense against environmental damage, pathogens, and moisture loss. The stratum corneum is composed of dead keratinized cells that are continuously shed and replaced, which helps maintain the skin's integrity and hydration.

In contrast, the other layers mentioned serve different functions within the skin's structure. The stratum granulosum, for example, is where the process of keratinization begins, where cells start to lose their nuclei and become more flattened. The stratum lucidum is found only in certain parts of the body, like the palms and soles, providing an additional layer of protection. The stratum spinosum contains living keratinocytes and is involved in the immune response through Langerhans cells.

Overall, the stratum corneum is essential for protecting underlying layers and maintaining overall skin health, making it the distinctive outermost layer of the epidermis.