Fetal development of the white line ( Zona alba ) of the equine hoof

Summary The fetal development of the white line ( Zona alba ) in the equine hoof is described. Its specific structure of lamellar and interlamellar horn, which in turn is composed of cap and terminal horn, is formed in the second half of the hoof's fetal development. In equine fetuses with a crown‐rump length of less than 550 mm, the hoof capsule lacks a 'characteristic' white line since no borders between stratum medium, stratum internum and sole horn are discernible. In the hoof of an equine fetus with a crown‐rump length of 550 mm, a narrow white line has taken shape. Its shallow lamellae are arranged like arcades. Between the horn lamellae lie the polyhedral cells of the interlamellar horn. Up until birth, the height of the horn lamellae and, therefore, the width of the white line increases significantly. In the white line of the hoof of newborn foals, the terminal horn contains horn tubules with a characteristic architecture.