Ultrastructural observations of hoof horn from dairy cows: the structure of the white line.

The white line (zona alba) is the specialised region of the bovine claw capsule at the junction of the wall and the sole. It consists of two distinct components, both generated from epidermis overlying the laminar corium, and these were examined in the transmission electron microscope. One constituent of the white line is the laminar horn leaflet which was composed of fusiform squames. Within these squames the bundles of keratin fibres were orientated parallel to the laminar horn leaflets. The second components, the interdigitating horn, was composed of squames which were polyhedral in outline and contained tightly packed bundles of keratin fibres, orientated in various directions. These different ultrastructural appearances are thought to be associated with different mechanical properties for the two parts of the white line. No horn tubules were present in the interdigitating horn of the white line.