Anatomy of the Hoof Capsule
The hoof capsule is an epidermal tissue. This designates that the keratinocyte, the basic building blocks of all skin structures, is the principle cell of the hoof wall, sole, frog, and bulbs. Even though the keratinocytes of soft skin and those of the hoof capsule share a common origin and physiology, it is obvious that the hoof capsule is stiffer and stronger than skin. Similarly, the hoof wall and sole is physically different from the bulbs and frog of the capsule. This presentation is focused on the macro- and microscopic anatomy of the hoof capsule and emphasizes the histologic architectures that underlie these physical differences.
The Hoof Wall
The hoof wall forms a major portion of the protective outer surface of the foot and has been described as being its primary weight bearing structure.1,2 In older literature the wall was referred to as the coffin of the foot, implying that the wall forms the majority of the protective shell that encloses the foot's sensitive tissues.3 The hoof wall is that part of the external hoof that is lined on its inner surface by laminar epithelium. This definition designates that the bars of the foot, which have a laminar epithelial architecture on their internal surface, are a part of the wall. Being the most external part of the foot, the wall is commonly used as the index, or guide, by which the shape or conformation of the foot is described. As will be noted in the lecture on conformation, wall shape frequently reflects the conformation of the internal foot, particularly the distal phalanx (coffin bone).
When the hoof wall is isolated from the other parts of the foot and hoof, its three-dimensional shape can be seen (Figure 1). Geometrically, the basic three-dimensional shape of the wall is closely related that of a cone.4 As a three-dimensional structure, it has internal and external surfaces as well as proximal (coronary) and distal (bearing) borders. Viewed from the outer, external, surface, the wall has a convex shape, whereas the inner surface is concave. This curvature is reversed on the bars of the wall; here the outer surface is concave and the inner surface convex. It is these inverse curvatures of the external wall and bars that has long invited the hypothesis that the foot acts as a spring during loading and unloading.5
The proximal, or coronary, border of the wall has two grooves or sulci (Figure 1). The outer of these is the small and relatively shallow perioplic groove, which is most easily seen in the fresh hoof. The perioplic groove is positioned above the coronary groove so that it lies between the proximal wall and the relatively soft, flexible skin of the limb. The larger groove at the proximal edge of the wall is the coronary groove. If examined closely, it will be seen that the surface of the grooves is not smooth but instead has numerous small holes. It is from the coronary surface that the bulk of the wall is produced. The depth and width of the coronary groove is not constant: It is deepest and widest at the toe and becomes progressively shallower and narrower toward the palmar/plantar surface of the foot.
Since most of the wall is produced by the tissues that lie in the coronary groove, it follows that the thickness of the wall, defined as the perpendicular distance between the external and internal surface, is determined by the width of the coronary groove (Figure 2). At the toe where the coronary groove is deepest and widest, the wall is relatively thick. As the coronary groove tapers in width and depth through the quarters and heels, wall thickness decreases. At the heels, where the wall turns back toward the toe to form the bars, the wall thickens and can equal or exceed the thickness of the dorsal wall. The thickness of the bars tapers as they travel towards the toe and the bars gradually disappear as they blend with the sole. Note that as the wall turns back to form the bars, the coronary groove does so as well.
At any region of the foot, wall thickness is fairly constant between the coronet and bearing surface (proximal to distal). The major exceptions to this are the varying thickness at the coronary edge of the wall created by perioplic and coronary grooves. The thickness of the wall at the foot's bearing surface gives the impression that the wall at the toe is much thicker than the lateral quarter and that of the lateral quarter is thicker than the medial quarter (Figure 2). Some of the differences in wall thickness are real, resulting from the varying width of the coronary groove. Some of the apparent differences in regional wall thickness is an illusion created by the relative angles at which the wall contacts the ground. At the toe, where the angle of wall to ground approaches 50 degrees, the wall is beveled, or worn, at a greater angle than the lateral or medial walls, exposing more of it. Likewise, since the wall at the lateral wall is not as steep as the medial wall in most adult horses, more of its bearing surface is exposed, giving the impression, or illusion, that the wall is thicker on the lateral side of the foot (Figure 3).
Anatomic Subdivisions of the Wall
The appearance of the wall is not strikingly different between its coronary and bearing regions but is obviously different between its outer and internal surfaces. Even without a microscope, differences in both the color and structure can be seen. Any attempt to physically bend sections of the wall adds the appreciation that the external region of the wall is stronger and stiffer than the inner wall. The changes in the physical properties of the outer-to-inner wall exists as a gradient that enhance the ability of the external wall to withstand the loads placed on it and give the internal regions sufficient flexibility to distort instead of tearing during weight bearing. This layering of wall strength and elasticity has architectural as well as metabolic and biochemical origins.
Anatomically, three major subdivisions of the wall are described; 1) the stratum externum (stratum tectorium), 2) the stratum medium, and 3) the stratum internum (lamellum) (Figure 4). Architecturally, the cells of the stratum externum and stratum medium layers are arranged so that they form tubules that extend from the coronary groove toward the bearing surface. These tubules are united by continuous layers of flattened cells that lay between and bind to adjacent tubules. This wall architecture is designated as tubular and intratubular wall.
The stratum internum is the inner most component of the wall and serves to attach the wall to the underlying soft tissue which, in turn, is attached to the distal phalanx of the foot. The cells of the stratum internum layer are arranged as in ornately folded and refolded pattern that extends from the coronary border toward the solar surface. Each fold of epidermal hoof wall cells of the stratum internum layer is separated by tissue arising from the underlying dermis. Because this region consists of both epidermal wall and underlying dermis, it is referred to as the laminar interface.
The stratum externum and stratum medium wall layers are produced by the continually replicating and differentiating cells that line the perioplic and coronary grooves that are referred to as the coronary band epithelium. Examination of the surface of the coronary sulcus reveals numerous small round indentations (Figure 5). Each indentation is the upper end of a wall tubule, and in the live horse, each contains a small coronary papilla. Due to the close association between the dermal and epidermal cells in this region its anatomy and architecture will be discussed together in subsequent section on the dermal epidermal interface. Here the primary focus is on the anatomical architecture of the wall itself.
The tubules nearest the external surface of the wall can be seem as thin, parallel lines running from the coronet to the bearing surface. Seen in cross section, wall tubules have a distinct wall (cortex) and lumen (medulla) similar to hair. The epidermal cells at the distal tip of the papillae give rise to the medullary cells, and those concentrated between the tip and base of the papillae produce the cortex of the tubule. As the medullary cells are pushed distally, they dry out, giving the center, or lumen, of the tubule a hollow appearance.
The coronary band epithelium located between the bases of coronary papillae are responsible for producing the continuous layer of flattened epithelial cells that give rise to the intratubular wall of the stratum externum and stratum medium. As they are formed these intratubular cells are joined to the cells on the outer cortex of the wall tubules and move distally with them during growth.
The appearance of
both the tubules and intratubular wall structures varies with their
location within wall. The lumens (medullae) of tubules vary from being
relatively small and oval to being large and round. Similarly, the microscopic
appearance of the tubular walls varies due to differences in the shape
and arrangement of the cells forming the cortex. Coupled with the shape
of the lumen, at least three different types of tubules have been described
with each type being located in specific regions of the capsule.6
Stratum Medium The bulk of the hoof wall is composed of the stratum medium. This thick layer of tubular and intratubular epithelium is, like the periople, produced by a papillated coronary band epithelium. Because of its bulk this layer is usually considered to be the primary weight bearing portion of the wall.
Even though the cells of the stratum medium are all hoof wall epithelial cells they are not all alike. Some are living, reactive cells where others are described as being fully cornified and are essentially dead. Some of the cells in the wall are spindle shaped where others have a flattened shape.6 Differences also exist in the arrangement of the protein (keratin) fibers within the cells. Filaments in spindle-shaped cells are oriented longitudinally and vertically; those of the flat cells are oriented horizontally and proximodistally to the axis of the wall. 9
Based upon the differences in the tubules' architecture and the metabolic activity of its epithelial cells, the stratum medium layer of the wall is subdivided into outer and inner zones. The keratinocytes located in the outer zone can be pigmented and have tubules that are distinctly different from those of the inner, unpigmented layer. In addition, the density of tubules in the outer stratum medium is much higher than that of the inner stratum medium.10 Four distinct zones of tubular density exist within the stratum medium in its inner most regions tubular density averages 11/mm. In the outer most regions of the wall the tubular density increases to greater than 22 tubules/ mm, or double that of the inner surface of the stratum medium. Between these two regions, wall tubular density increases as a two phase gradient.
The outer zone of the stratum medium extends from the middle to the outermost wall. The tubules from the outer zone are composed solely of the flat-type epithelial cells and have small and oval lumens (Figure 7). This oval shape may, in part, result from mechanical stresses placed on the tubules as they move distally down the wall. This speculation is based on the observation that the oval shape becomes more pronounced as the tubules are pushed distally where the same amount of external wall has to cover a greater area because of the increased size of the foot at the bearing surface. The cells making up the cortex of these outer wall tubules are orientated with their long axis parallel to that of the tubule. The intratubular cells that reach between tubules are inclined at 45 degrees to the tubules.
The innermost layer of the stratum medium forms the zona alba of the stratum medium.11 Cells composing this region are unpigmented and blend structurally with the epidermal cells of the stratum lamellum, which are also unpigmented. As will be discussed later, these cells are metabolically active and are more susceptible to disease than are the cells of the outer stratum medium.
The tubules of the zona alba region of the stratum medium characteristically have large round tubules and three distinct cortical zones (Figure 7).11 The innermost cortical layer is one to two cells thick and consists of the flat-type epithelial cells oriented around the marrow. This inner cortical zone is surrounded by a middle layer of composed of numerous spindle type cells oriented with their long axes parallel to the long-axes of the tubule. The third, or outer, cortical zone is composed of a few layers of spindle-shaped cells with their long axis perpendicular to the tubular axes. In the inner zone of the stratum medium the intratubular cells are arranged perpendicular to the tubules.
The changes in
the tubular density within the stratum medium, and the changes in the
tubular structure, correlate to the mechanical gradient present in the
stratum medium. Where the stratum medium is composed of densely packed,
small diameter tubules, it is relatively stiff and has a high strength.
The zona alba region of the stratum medium where the tubular density
is low and the tubules themselves have a large diameters, the wall strength
is reduced and elasticity is high. In addition, alternating orientation
of the cells within the stratum medium protects it from developing horizontal
cracks. The progression of any small fissure that originating in the
horizontally orientated cells of the intratubular wall is inhibited
when it encounters the proximodistally orientated cells of the tubules.
The physical attachments between the dermis and the epidermis at the laminar interface is described as being responsible for attaching the wall to the foot12 and/or, allowing the suspension of the distal phalanx from the inner surface of the wall.13 Functionally, the epidermal cells of the laminar interface are not only physically attached to the interdigitating dermis but are also nourished by the underlying dermal blood vessels. Because of this close anatomic and functional interdependency these two layers will be identified as the laminar interface and the anatomy will be described a subsequent section.
The Epidermal Sole
The sole of the horse's foot fills the space between the bearing portion of the wall and the bars of the foot. It is solidly attached to the wall where the fully cornified stratum lamellum of the inner wall grows out the bottom of the foot. As described, the sole is divided into body, crus, and angle regions (Figure 7). The sole's external surface is concave; in the normal, untrimmed foot, this concavity may be partially obscured by unequal wear so that the center portion of the body of the sole is thicker than that located near the wall. The sole's inner surface is convex, conforming to the solar face of the distal phalanx.
Histologically the sole, like the wall, is composed of tubular and intratubular horn14. Like the wall at the coronary sulcus, the inner surface of the sole has numerous small depressions created by the solar papillae that give rise to the solar tubules. (Figure 7) The tubules of the sole are inclined at 45 degree angle to the ground surface and radiate abaxially toward the wall. The solar tubule has an oval lumen and two cortical zones.6 There is an inner layer of flattened keratinocytes, one to two cells thick, near the lumen. The outer cortical layer is composed of multiple layers of spindle-shaped cells oriented with their axis perpendicular to the long axes of the tubule. Between tubules the intratubular sole is composed of flat cells inclined at a 45 degree angle to the tubule.
The physical properties of the sole are different from those of the hoof wall. It as been described as having a greater relative hydration, being softer, and having the tendency to come off in flakes or slab-like pieces as it ages. These, at least in part, result from differences in the microarchitecture of sole tubules. As described above, the solar keratinocytes forming both the intratubular and tubular components are orientated horizontal to the long axis of the tubule or parallel to the ground surface where in the wall, some of the keratinocytes are horizontal and some are vertical. As a result small cracks in the intratubular sole can easily progress horizontally, following the general plane of the cells across both intratubular and tubular components. The reduction in the structural resistance to crack horizontal crack progression predisposes it to flaking off in small horizontal slabs.
The frog fills the majority of the space between the bars of the foot. Its functions have been to stated to be that of allowing the foot to spread at the heels and/or to act as a protective cushion for underlying structures in the foot.15,16 The texture of the frog varies considerably with its state of relative hydration. When dried it approaches a hardness similar to the sole; when at its normal relative hydration it has an almost rubbery texture.
The frog is generally wedge shaped from both the solar or lateral perspectives with the base located at the palmar/plantar surface, and the apex toward the toe (Figure 8). The external, solar, surface has a central sulcus bounded by paired ridge-like crura. The crura of the frog join the bars deeply within the paracuneal, or collateral sulci.
The upper, internal surface of the frog is formed of three ridges and two grooves. A central ridge or spine (frog stay) is flanked on either side by two grooves that lie opposite the crura of the external surface. Lateral and medial ridges are abaxial to the grooves and are supported by the bars of the wall. The frog spine is highest at the base and abruptly tapers at the apex. The undulating internal and external surfaces of the frog gives it a "W" shape when viewed in cross-section at its base. Because the frog spine tapers, the apex of the frog is flat and greatly reduced in height.
The frog is composed of epidermal cells arranged in tubular and intratubular patterns, like the sole and wall. There are considerably fewer tubules and much of the frog is composed of intratubular horn laid down in sheets from the germinative cell layers. In addition, the frog is distinct in that it is the only region of the foot that is glandular.17 These glands are located in the dermal/subcutis tissue just above the frog, but their ducts extend into the frog's epidermis. Histologically, these glands are tubular and fairly coiled. They have a cuboidal epithelium that stains weakly with PAS. Their secretions stains strongly with PAS indicating that they contain a glycolipid or glycoprotein.6 Some have reported these as being modified sweat glands,6 others have indicated that these glands secrete lipids that are assumed to play some role in determining the texture of the normally hydrated frog. 18
The Epidermal Bulbs
The bulbs of the foot are the two rounded, hairless regions on the palmar/plantar surface of the horse's foot. (Figure 9) They are poorly described in the literature. The most proximal regions of the bulbs are covered by a layer of periople. The cross-section thickness of the bulbs reaches a maximum approximating 2.54 - 3.81 mm (0.1-0.15 inches). Physically, the bulbs are similar to a well hydrated frog. Histologically, the entire inner surface of the bulbs represents the structural counter-part of the perioplic groove. That is, it is composed of a papillated interface that gives rise to the tubular and intratubular architecture present in the bulbs. At their most distal surface the inner surface of the terminally cornifying epithelium of the bulbs are firmly attached to the hoof wall and to the base of the frog. The distal edges of the bulbs frequently peels away from the hoof wall but remains attached to the bulbs themselves.
The bulbs cover and protect the heel regions of the foot from abrasive contact with the ground. The bulbs, like the periople, are positioned between, and are attached to, the soft skin lying just above the coronet and the relatively rigid hoof wall and frog. Like the periople and soft epidermis of the pastern, and unlike the rest of the hoof capsule, the epithelium that produces the bulbs has a stratum granulosum layer. This additional epithelial cell layer alters its material, or physical properties, such that its strength and elasticity is greater than that of the skin but less than that of the wall and frog. As a result, the bulbs represent a biomechanical transition zone between two significantly different tissues that is critical to reducing the likelihood of tearing when the heel regions of the hoof are displaced during loading.
The uniqueness and anatomical complexity of the hoof capsule reflects necessary adaptations and specializations that allow it to meet the physical demands made on it. Nearly everything about it; its shape, the variations in wall thickness, the arrangement of its epidermal cells into tubular and laminar architectures, as well as the variances in tubular structure and density within the wall all represent important anatomical contributions to enhancing the strength and resiliency of the hoof capsule.