A ubiquitous biological material, keratin represents a group of insoluble, usually high-sulfur content and filament-forming proteins, constituting the bulk of epidermal appendages such as hair, nails, claws, turtle scutes, horns, beaks, and feathers. These keratinous materials are formed by cells filled with keratin and are considered ‘dead tissues’. Nevertheless, they are among the toughest biological materials, serving various interesting functions, like scales to armor the body, horns to combat aggressors, hagfish slime as a defense against predators, nails, and claws to increase prehension, hair and fur to protect against the environment.
Thinking about keratins familiar to us, it’s hard to think of how they could make an ingredient in skincare, but they can if you break them into pieces. Because they are very resistant to proteases (enzymes that break down proteins), you need to bring in the hard stuff: urea, used to denature proteins and chemicals that break down disulfide bridges. Once broken down, the keratins are reduced to minor components that are water-soluble, amino acids and peptides. The advantage of using “hydrolyzed keratin” is that animal processing frequently ends up with keratin byproducts, and it is good for the environment to find alternative uses for them. This ingredient is not suitable for vegans, but people who don’t mind using lanolin or other wool byproducts should have no problem with it.
There is a more important side to keratin. They constitute about 30–80% of skin keratinocytes’ total protein, forming the major intermediate filament cytoskeleton. Keratins are coded by more than 50 unique human genes and are expressed depending on the cell’s type, differentiation, and development stage. When the epidermis differentiates into the protective stratum corneum, the keratins become part of the “cement” that holds the dead cells together, creating the skin barrier that is so important to skin health.
What about before the cells die? The primary function of the keratin intermediate filament cytoskeleton is to provide cells with structural resilience against mechanical trauma. This is especially important for epidermal cells because, as the outermost barrier tissue, the epidermis has to be able to resist some of the most severe physical stress experienced by any human tissue.
Most disease-causing mutations occur in the well-conserved structural domains of keratins, disrupting secondary structure formation. In addition to structural and mechanical support, cell-specific keratin expression modulates growth, adhesion, migration, and invasion of epithelial cells. Dysfunction or mutations of keratin proteins are associated with various skin disorders, such as skin blistering, inflammatory disorders, and skin tumors.
Bin Wang, Wen Yang, Joanna McKittrick, Marc André Meyers (2016) Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration,
Progress in Materials Science, 76: 229-318, https://doi.org/10.1016/j.pmatsci.2015.06.001.
Bragulla HH, Homberger DG. Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia. J Anat. 2009;214(4):516-559.. doi:10.1111/j.1469-7580.2009.01066.x