The Growth and Adaptive Capabilities of the Hoof Wall and Sole: Functional Changes in Response to Stress (21-Nov-2003)

1. Introduction The hoof wall represents the outer protective structure of the foot and has long been of interest in terms of its growth, structure, and functioning, especially during disease conditions such as laminitis. The incidence of problems related to the hoof wall has been estimated to be 30% or more [1,2]. Although causal factors of some hoof wall abnormalities are obvious, such as direct trauma to the hoof wall and/or grain overload during laminitis, other problems and some forms of laminitis are not. Resolution of hoof abnormalities by corrective trimming or shoeing depends on our understanding of the basic structure and function of the hoof wall as well as its short-term and long-term growth. Many descriptions of the hoof structure and the hierarchical organization of the hoof wall have been presented previously [3,4]. The hoof wall at the toe is thicker than that of the quarters. The hoof wall becomes progressively thinner and more elastic toward the heels, and, then, it increases in thickness as the bars are formed. The structure of the hoof consists of three main layers: the stratum externum, stratum medium, and stratum internum (Fig. 1). Many studies of the hoof wall have focused on the stratum medium. It is the thickest of the three layers, and it seems to be the main load support system, whereas the outer layer serves a "waterproof" function [3-9]. The structural features of the stratum medium are the tubular and intertubular horn [4,8]. The tubular horn is composed of a central medullary cavity that extends the proximodistal length of the hoof wall. In the proximal third of the hoof wall (4-6 mm), the central medullary cavity of the horny tubule accommodates the papillae of the corium. The cortex of the tubule is surrounded by keratinized epithelial cells, which gradually merge with the keratinized cells of the intertubular horn. The epithelial cells of the intertubular horn are continuous with the epidermal cells between the papillae of the corium at the coronary band [4]. The horn tubules are generated by the germinal layer of the coronary epidermis underlying the long papillae of the corium, whereas the intertubular horn is formed from the germinal epithelium, located between the tubular projections of the coronet [4]. The relative density of the tubules within the hoof wall seems to vary across the cross section of the hoof; more tubules per unit area are located on the …