Keratin Keratin Keratin () is one of a family of fibrous structural proteins. It is the key structural material making up hair, horns, claws, hooves, and the outer layer of human skin. Keratin is also the protein that protects epithelial cells from damage or stress. Keratin is extremely insoluble in water and organic solvents. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized epidermal appendages found in reptiles, birds, amphibians, and mammals. The only other biological matter known to approximate the toughness of keratinized tissue is chitin. Keratin derives from Greek ' from '
Outer nuclear layer Outer nuclear layer The outer nuclear layer (or layer of outer granules or external nuclear layer), is one of the layers of the vertebrate retina, the light-detecting portion of the eye. Like the inner nuclear layer, the outer nuclear layer contains several strata of oval nuclear bodies; they are of two kinds, viz.: rod and cone granules, so named on account of their being respectively connected with the rods and cones of the next layer. The spherical rod granules are much the more numerous, and are placed at different levels throughout the layer. Their nuclei present a peculiar cross-striped appearance,