keratin

noun

ker·​a·​tin ˈker-ə-tən 

: any of various sulfur-containing, fibrous, acidic or basic proteins chiefly of epithelial cells and tissues that are relatively insoluble and resistant to degradation, form filaments which assemble into bundles to provide structural support, and are the primary component of hair, nails, horns, claws, hooves, quills, scales, feathers and the epidermal layer of skin

Note: In addition to its structural properties, keratin also plays a role in cell growth and proliferation, cellular transport of substances, and intercellular communication.

Rhino horn is not actually horn but densely compacted fibers of keratin, a protein found in hair and fingernails.

—Maryanne Vollers

Wool fibers are composed of keratin proteins bundled into microfibrils.

—A. Goho

compare keratinocyte

keratinous

kə-ˈra-tə-nəs 

adjective

Examples of keratin in a Sentence

Recent Examples on the Web

On April 5, the organization announced that keratin was starting to grow on the right side of 390’s beak. —Laura Baisas, Popular Science, 10 Apr. 2025

The hero ingredient is the L-arginine, which is an amino acid found in keratin, a.k.a. —Natasha Marsh, Allure, 7 Apr. 2025

Surprisingly, the team discovered that one of the two giant bony claws has its keratin sheath—comparable to a human nail—still preserved. —Margherita Bassi, Smithsonian Magazine, 27 Mar. 2025

Though byproducts like collagen, hyaluronic acid, and keratin evolved with modern skin care, tallow was (mostly) left behind. —Boutayna Chokrane, WIRED, 14 Mar. 2025

Word History

Etymology

borrowed from German Keratin, from Greek kerat-, stem of kéras "horn" + German -in -in entry 1 — more at kerato-

Note: The term was introduced by the German physician and chemist Johann Franz Simon (1807-43) in Handbuch der angewandten medizinischen Chemie, 1. Theil, Medizinisch-analytische Chemie (Berlin, 1840), p. 49.

First Known Use

circa 1849, in the meaning defined above

Rhymes for keratin

• ferritin
• cytokeratin
• samaritan

Kids Definition

keratin

noun

ker·​a·​tin ˈker-ət-ᵊn 

: a sulfur-containing protein that makes up hair and horny tissues (as nails)

Medical Definition

keratin

noun

ker·​a·​tin ˈker-ət-ən 

: any of various sulfur-containing fibrous proteins that form the chemical basis of horny epidermal tissues (as hair and nails) and are typically not digested by enzymes of the gastrointestinal tract see pseudokeratin