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
Maryanne VollersWool fibers are composed of keratin proteins bundled into microfibrils. A. Goho
compare keratinocyte A. Goho keratinous
kə-ˈra-tə-nəs
adjective 
Example Sentences
Recent Examples on the Web This one contains keratin and hydrating oils that repairs and nourishes your tired, depleted strands. ELLE, 23 June 2022 What happens is that the hair follicles on your body—especially around backs of the arms, thighs, buttocks, and even face—get plugged up with too much keratin or protein, and turn into small, hard bumps. Talia Abbas, Glamour, 19 July 2021 Six of the seven species have hard shells fused to their ribs and overlaid with keratin scutes. National Geographic, 10 Oct. 2019 With a keratin lash lift, more protein is added back into the lash, Imani says. Sara Spruch-feiner, Town & Country, 7 Mar. 2020 Pangolins are insect-eating mammals that are covered in protective keratin scales. Mark Olalde, USA TODAY, 10 Apr. 2020 Pangolins are mostly covered in thick scales of keratin—the substance human fingernails are made of—that protect their bodies from predators like leopards. National Geographic, 19 Mar. 2020 The horns are made of sheaths of keratin, like true horns, but shed every year like a true antler. Matt Wyatt, ExpressNews.com, 13 Feb. 2020 Much of its strength comes from a material called keratin. Svenja Lohner, Scientific American, 28 Nov. 2019 See More
ELLE, 23 June 2022 What happens is that the hair follicles on your body—especially around backs of the arms, thighs, buttocks, and even face—get plugged up with too much keratin or protein, and turn into small, hard bumps. Talia Abbas, Glamour, 19 July 2021 Six of the seven species have hard shells fused to their ribs and overlaid with keratin scutes. National Geographic, 10 Oct. 2019 With a keratin lash lift, more protein is added back into the lash, Imani says. Sara Spruch-feiner, Town & Country, 7 Mar. 2020 Pangolins are insect-eating mammals that are covered in protective keratin scales. Mark Olalde, USA TODAY, 10 Apr. 2020 Pangolins are mostly covered in thick scales of keratin—the substance human fingernails are made of—that protect their bodies from predators like leopards. National Geographic, 19 Mar. 2020 The horns are made of sheaths of keratin, like true horns, but shed every year like a true antler. Matt Wyatt, ExpressNews.com, 13 Feb. 2020 Much of its strength comes from a material called keratin. Svenja Lohner, Scientific American, 28 Nov. 2019 See More 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
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