Final answer:
Keratin is the insoluble protein found in the epidermis, hair, and nails, providing them with toughness and water-resistance. Keratinocytes in the epidermis produce this fibrous protein, which is key for the structural integrity of these body parts.
Step-by-step explanation:
The insoluble protein present in the epidermis and in epidermal derivatives, such as hair and nails, is keratin. Keratin is a type of structural protein that gives skin, hair, and nails their hard, water-resistant properties.
Keratinocytes are a type of epithelial cell that produces keratin, which is the most predominant type of cell found in the epidermis.
The protein keratin forms fibers that make the hair and nails tough and strong, similar to chitin, which is found in the exoskeleton of arthropods. As a fibrous protein, keratin is insoluble in water and serves structural, connective, and protective functions.