The structure and function of the equine hoof wall
Abstract
Strength of the equine hoof wall was investigated by examining the relationships between measured mechanical properties and the histological and ultrastructural morphology of the hoof wall material. The keratinization of the stratum medium and primary lamellae of the stratum internum, and the method by which the wall is attached to the dermis, were also described. A model of hoof wall function was formulated by integrating information of morphology and mechanical properties.
Keratinization was similar to that of hard (α) keratins, with tonofilaments being synthesized within each cell concomitant to loss of organelles and degeneration of the nucleus. Keratohyalin was not formed. Spinous cells were joined by desmosomes and gap junctions. In the intertubular horn of the stratum medium processes from cells of the stratum spinosum invaginated into or between adjacent cells. These processes frequently included extensive areas of gap junction attachment. Some areas of gap junction attachment appeared to be budded off from the plasmalemma, thus forming internalized annular gap junctions. These were subsequently destroyed by lysosome-like structures.
In cells of the stratum spinosum located near the stratum spinosum-stratum corneum junction, non-laminated membrane-coating granules (MCG) aggregated on the periphery of the cell. Intercellular material was found which apparently resulted from extrusion of the contents of the MCG. Coincidently a third type of junction, the type 3 junction, and an intercellular line were found.
In cells of the stratum corneum located near the stratum spinosum-stratum corneum junction, non-membrane bound acid phosphatase reaction product was found on the periphery of the cytoplasm. In cell layers immediately distal to this the reaction product was found in most of the intercellular space except that of the type 3 junction and intercellular line.
It was therefore proposed that the establishment of the type 3 junction and intercellular line prior to leakage of acid phosphatase into the intercellular space, and the lack of permeability of these structures to this hydrolytic enzyme, created stable areas of inter-cellular adhesion between fully keratinized cells.
Stress-strain curves were obtained by compressing square hoof wall specimens in all three orthogonal directions using an Instron Universal Testing Machine. Values of modulus of elasticity, a measure of specimen rigidity, and proportional limit were determined from these curves. The water content and type and number of tubules present in each specimen was determined.
Hoof wall specimens were nonhomogeneous, mechanically anisotropic and did not exhibit a yield point. Specimens from the more exterior locations of the stratum medium were stronger, drier and more anisotropic than specimens located near the stratum internum. The mechanical properties of specimens from black hooves were not significantly different from those of white hooves.
The compressive strength (rigidity) of specimens loaded in a direction parallel to the hoof tubules was moderately influenced by both tubule, number and by water content of the specimen. Strength of specimens loaded in the other two orthogonal directions had a high negative correlation with water content.
In the model of hoof wall function it was suggested that tubules augment the rigidity of intertubular horn by acting as struts when the wall is subjected to vertical compression. This tubule-intertubular horn arrangement is probably a compromise between the need to provide rigid strength and the need to store energy, thus protecting the tissue from failure.
During weightbearing tensile stress from the stratum internum is transmitted to the stratum medium. There is a decreasing gradient of water from the interior to the exterior parts of the stratum medium and, since the mechanical properties of the tissue stressed at right angles to the tubules are highly dependent on water content, it is likely that tensile stress is passed exteriorly through increasingly rigid wall material, thus dampening the effect of this, tensile stress.