Hoof Wall Growth and Adaptability

"The incidence of problems related to the hoof wall has been estimated to be 30% or more," stated Robert M. Bowker, VMD, PhD, a professor of anatomy at Michigan State University, at the 2003 American Association of Equine Practitioners convention. "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.

"Hoof wall growth has been studied by both light and electron microscopy; however, the process is still not completely understood," he added. The mechanisms of growth and adaptation of the hoof wall to various stresses have been a big focus in recent research. Bowker presented his observations and interpretations on these phenomena.

"The morphological features of the primary epidermal laminae (PEL) and the secondary epidermal laminae (SEL) around the perimeter of the hoof wall have formed much of our understanding of the inner hoof wall, its function, and its growth," he explained. "Many practices and treatments during trimming and shoeing by the farrier and veterinarian have been based on these hypotheses. The concepts of hoof wall growth and its origins from the coronet are well accepted, although they may not be fully understood. However, these current ideas of hoof wall and sole growth do not account for the possibility that the cellular constituents can change and adapt according to the environmental stresses imposed on the foot and, thus, alter their morphological features.

"Thus, although our treatments may be formulated according to our current understanding of the growth of the hoof, these latter ideas may be flawed," he continued.

Regarding adaptation of the hoof wall to various stresses, Bowker said, "We believe that these adaptable changes occur 'normally,' but during certain diseased states, they are exaggerated and occur at a much faster rate."

From Foal to Adult

Bowker reported on work in which the feet of several near-term fetuses, newborns (up to two months of age), and yearlings were obtained for necropsy examination. He described in great detail several characteristics that were different in the feet of older horses versus younger ones, demonstrating the adaptive processes of the hoof. If there were no adaptation, then the hoof wall structure would be exactly the same at the ground end as it is at the coronet, and it would be the same in adults as in fetuses and newborns.

"In the initial findings of the fetus, foal, and yearling feet, the microanatomy of the PEL and SEL revealed definitive changes between when the fetus is born and when the foal develops into a young horse," Bowker reported. "In the newborn foal, changes in the relative density and morphology (structure) of epidermal laminae occur around the perimeter of the hoof wall, beginning within a few days of birth. In the fetal stage foal, there were equal numbers of PEL between the toe and quarter or a greater density of PEL at the quarters than at the toe. Shortly after birth, there is a significant increase in the relative number of PEL along the hoof wall length at the toe at the ground level when compared with the quarters of the same foot."

Bowker explained that the greater numbers of PEL in various areas of the foot are due to branching of PELs in response to stress.

"In the late-term fetus, only an occasional branched lamina was seen at the toe, with nearly a 10-fold increase being seen within the first two to five weeks after birth," he explained. "In the yearling, the relative branching of the PEL at the apices was significantly less."

Bowker hypothesized that structural changes in laminae, and more branching of laminae (and thus an increase in number of laminae in a given area), occurred in more stressed areas of the wall, such as the toe. This would support his theory of wall adaptation based on weight bearing.

Other Structural Changes

In addition to laminar branching, Bowker and other researchers have described an increase in the number of hoof wall tubules by "budding off" of a tubule (termed a laminar-derived tubule, or LDT) from the region between the bases of two PEL. These LDTs then move into the inner hoof wall, and might contribute up to 25% of the mass of the hoof wall in some areas, he said.

"These two processes, the bifurcation (branching) of the PEL with the secondary redistribution of the epithelial cells around the PEL and the formation of LDT at the basilar end of the PEL, suggest the notion that the PEL has at least two active zones (the base and the tip) where adaptive changes may be initiated or occur as an early response site to the applied stresses on the hoof wall," Bowker explained. The middle part of the PEL might be relatively inactive, he noted.

He described yet another structural change in the hoof, that of tertiary epidermal laminae (TEL), which branch off of a PEL closer to its base rather than splitting the tip of the lamina (the mechanism of PEL branching previously discussed). These TEL orient toward the apex of the frog, appear to only form from the bars, and are present in the foal. 

"Findings suggest that much of sole originates from the area of the bars," Bowker stated.


Branched epidermal laminae

Branching of primary epidermal laminae (arrow) appears to be a response to higher stress on certain areas of the hoof (such as the toe, as shown here).

Laminar-derived tubules

Laminar-derived tubules (LDT) add to the mass of the hoof wall; these "bud off" between the bases of two primary epidermal laminae (PEL).

Tertiary epidermal laminae

This microscopic view of the laminae shows primary (1), secondary (2), and tertiary (3) laminae. The tertiary laminae, or TEL, are only seen at the bars.


Hoof Balance and Structure

"In a general population of more than 100 feet examined from various breeds and ages, the conformation of the hoof varied considerably from a round foot to one that was flared to one side," Bowker said. "The medial side of the hoof wall was steeper in approximately 80% of the feet examined, with the lateral side being slightly flared. In the remainder of the feet examined, the lateral or outside hoof wall was steeper, with the medial side being slightly flared. In a few horses (less than 5%), the hooves were rounded and exhibited little or no asymmetries."

In feet with one side steeper than the other, the PEL distribution seems to vary with the stresses being placed on the foot, he explained. "There are fewer PEL on the steep side of the foot, as these PEL are thicker and have greater spacing between them, while on the flared side of the foot the PEL are closer together and are thinner," he explained. "The PEL are denser (more PEL in an area of hoof wall) on the flared side than on the non-flared side."

"An interesting observation is that, on the opposite side of the foot to the flare, there is also an increased density of PEL in the region that we call the 'reverse flare' and the hoof wall region that is rolled under the hoof," he added.

What's "Normal?"

"In sections from clinically sound horses, the PEL and the SEL can be seen to vary greatly from the structural conformation of the laminae that is commonly described in many textbooks," Bowker stated. Thus, it would seem to be impossible to have a very strict definition of "normal" hoof wall structure and conformation. He suggested that perhaps there is a wide range of ideal hoof structures and conformations based on the wide range of stresses different hooves experience.

"We define conformation of the foot as a 'point in time' in which the structural appearance of the foot is a product of the environmental influences on the foot at the time that it is being examined," he said. "The structural appearance of the foot is continually being modified by the interactions of the foot with the environment and the environment's influences on the foot and hoof wall. The term environmental influences includes just about everything that the horse has come into contact with since birth, including the extent of movement, ground surfaces, trimming and shoeing procedures or the lack of these practices, nutrition, etc.

"An important concept for us as hoof care professionals is to begin to appreciate the significant influence that the environment has in determining the internal structure and the composition of the hoof wall and foot, and to begin to apply these principles in the future," he concluded.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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