We ask an awful lot of an animal who walks on his middle toenails. Humans have recognized for centuries that the foundation of a horse’s soundness lies in his hooves–“No foot, no horse” is about as basic a principle as there is. It all comes down to the forces exerted by a 1,000- to 2,000-pound animal on four rather small and specialized bits of keratin.

And yet, there’s much we don’t understand about why some horses grow dense, tough, virtually indestructible hooves, while others produce feet with the consistency of dead cork. What do we know about the process of hoof growth, and how might we be able to influence its rate and quality?

What Makes a Hoof?

Hoof horn is a highly modified, specialized skin derivative. It has two main layers, the outer epidermis and the underlying dermis (which, when you’re talking hooves, is called the corium). The epidermis contains no blood vessels of its own; it obtains its oxygen and nutrients from the blood supply to the underlying corium of the foot.

The hoof’s hardness is derived from a fibrous protein called keratin, the same substance that makes up human fingernails. Keratin is manufactured and retained in the basal cells of the epidermis. As new keratin-containing cells are formed, they flatten, something like roof shingles. Eventually, they lose their nuclei and most of their other cell organelles and become flat, plate-like horn cells, which contain the filamentous keratin embedded in a matrix of sulfur and tyrosine proteins. The proportion of keratin to non-filamentous matrix, which can vary, is one of the factors that determines the mechanical properties of the horse’s hoof horn (in other words, how tough it is).

Keratin-containing epidermal cells arrange themselves in long tubules, with each one overlying a finger-like connective tissue papilla underneath. Between the tubules is more keratin-containing