How does skin keep water inside while replacing its surface?

The skin you can see keeps losing old surface cells, yet it continues to separate the body from its surroundings. Its outermost layer, the stratum corneum, slows water loss and helps resist abrasion and the entry of microbes. It is a useful barrier rather than a perfectly waterproof sheet: some water still passes through it.

The surface contains flattened cells that have lost their nuclei and become filled with protective proteins, including keratin. Lipids, fatty substances around the cells, help limit water movement through the spaces between them. In a brick-and-mortar comparison, the cells are the bricks and the surrounding lipids fill gaps. The arrangement matters as well as the materials.

New cells form deeper in the epidermis, the outer region of skin. As cells move outward, they change their structure and contribute to the protective surface. The oldest surface cells are shed. New material is therefore arriving as older material leaves, instead of the entire barrier being discarded and rebuilt at once.

This continuous renewal links two jobs that might seem incompatible: keeping a boundary and replacing the boundary's parts. The symbolic rows mark stages in that journey, not a fixed number of skin layers or a universal renewal schedule. The skin's protection depends on the organization of cells and lipids remaining useful while its material changes.