What substance covers the outermost layer of the epidermis?

The substance that covers the outermost layer of the epidermis is keratin. Keratin is a fibrous protein that plays a crucial role in the structure and function of the skin, hair, and nails. It provides a protective layer that helps prevent water loss and shields underlying tissues from environmental damage, pathogens, and physical abrasions.

In the context of the epidermis, keratinocytes, the predominant cell type in this layer, produce keratin as they move toward the skin's surface. As these cells become more keratinized, they form a tough, protective barrier known as the stratum corneum. This barrier is essential for maintaining skin hydration and overall skin integrity.

While sebum, which is an oily substance produced by sebaceous glands, does play a role in skin health by providing moisture and creating an additional layer of protection, it is not responsible for covering the epidermis in the same way that keratin does. Water is critical for skin hydration but does not form the structural component of the epidermis itself. Collagen, on the other hand, is a protein that primarily makes up the dermis, the layer beneath the epidermis, contributing to skin strength and elasticity, rather than being part of the outer layer.