The special protein deposited in epidermal cells to make them waterproof is: keratin

Keratin is a fibrous structural protein that is a key component in the structure of hair, nails, and the outer layer of skin. It's known for its durability and resistance, which is why these body parts are much tougher than other tissues. Keratin's role in waterproofing comes from its ability to form a protective barrier, preventing the passage of water and other substances through the epidermis, the skin's outermost layer.

This protein is created and assembled in specialized cells known as keratinocytes, which are the most common cells in the epidermis. During their life cycle, keratinocytes migrate to the surface of the skin, continuously depositing keratin, which leads to the formation of a resilient, waterproof surface layer that protects the body from environmental stresses such as water, UV radiation, and harmful microorganisms.

Proteins, including keratin, are made up of long chains of amino acids that fold into specific three-dimensional shapes, vital for their function. The structure of a protein is organized into four levels of complexity. The primary structure is the sequence of amino acids in the chain. Secondary structures are the coils and folds along the polypeptide chain, like alpha-helices and beta-sheets—keratin predominantly forms a type of helical structure.

The tertiary structure is the overall three-dimensional shape of a single protein molecule. When protein chains bundle together as in keratin, they form a quaternary structure, providing combined functionality and stability. Keratin chains are heavily cross-linked with disulfide bonds and other chemical bridges, which give them their strength and insolubility, contributing to their protective qualities in skin cells.

The skin is the body's largest organ and consists of several layers. At the bottom is the subcutaneous tissue that attaches the skin to the underlying bones and muscles. Above this lies the dermis, which houses blood vessels, nerve endings, and hair follicles. The topmost layer is the epidermis, a multi-layered, waterproof barrier made of tightly packed cells.

The epidermis itself is composed of several sub-layers, but the outermost layer, known as the stratum corneum, is where most of the keratin in the skin is found. The cells in the stratum corneum are dead and flat, packed with keratin, which has been synthesized in living cells below and then deposited here through a process of cell differentiation and death. This keratin-rich layer is continually shed and replaced, helping to keep the skin's barrier function intact.