Joyce Barker, age 14, of Columbus, Ohio, for her question:

WHAT IS KERATIN?

Keratin 1s a highly fibrous and resistant protein that makes up most of the material in the cells forming the epidermis, hair, nails, scales, feathers, beaks, horns and hooves of animals.

These cells originate from permanent populations of germinal cells, and as they migrate outward they undergo specific patterns of differentiation in the process called keratinization. That is, the cells become increasingly filled with microfibrils of keratin, the cell walls harden and the nuclei and organelles of the cells are reabsorbed.

Little is understood, however, about how the cells differentiate to form such diverse and efficient structures as the elastic outer layer of mammals akin or the stiff scales of fish.

An important quality of keratin is its ability to extend and contract reversibly.