Keratin (protein)

• Keratins are a large group of intermediate filaments of epithelial cells, forming the structural proteins of the skin (polymers).
• Keratins are responsible for the differentiation of the epidermis into a hard solid structure basically by accumulating protein on the intermediate filaments within the cells of the skin (keratinocytes).
• Keratins secreted outside the cell form hair and fingernails.
• Mutations in keratins can weaken skin, causing a condition known as epidermolysis bullosa.

Reference: The Cell: A Very Short Introduction. Allen T. Oxford University Press