The equine hoof is very well specialised for many functions,such as shock absorption, propulsion and protection. This essay will discuss afew of the structural adaptations of the equine hoof and how they contribute toits function. The hoof wall has three divisions, the stratumexternum, the stratum medium and the stratum internum (Pollitt, 2004). The stratum externum is a thin layer covering theoutside of the hoof wall. Its functions include waterproofing (Bowker, 2003 A)as well as inhibiting dehydration (Kasapi and Gosline, 1997). The stratum medium is composed of thin, hollow tubules and is the mainsystem of load support in the foot (Bowker, 2003 A). Tubules are formed fromcoronet basal cells, which then undergo keratinisation.
The tubules arecontinual from the coronet to the ground surface (Pollitt, 2004). There are fibres that wind around the tubule andalternate direction between each layer, forming concentric lamellae (Bertramand Gosline, 1987, Kasapi and Gosline,1997). Keratinocytes are generated from theholes in the tubules, these mature to form the intertubular horn (Pollitt, 2004). The intertubular horn isformed at right angles to the tubular horn which provides “mechanically stable,fiber-reinforced composite” that has strength in all directions (Pollitt,2004). One function of the stratum medium is crack propagation. Thetubules are able to cause the pathway of the crack to be deviated, and redirectit away from sensitive structures of the foot (Kasapi and Gosline, 1998). It is argued the tubulesreinforce the hoof wall along the longitudinal axis which provides resistanceand leads the crack through a more tortuous route, while at the same timeabsorbing and dissipating fracture energy (Kasapi and Gosline, 1997).
The hollow centre of the tubules play an importantrole in crack propagation. In the middle regions of the hoof the fracture pathis redirected across hollow tubules and then along intertubular material, whichstops the cracks progressing up the hoof wall (Kasapi and Gosline, 1997). The intertubular horn isalso important in crack propagation as it has more stiffness and fracturetoughness than the tubular horn (Lancaster,Bowker and Mauer, 2013).
This function is especially important becausethe hoof wall cannot be remodelled, as only the coronet and proximal lamellaehave active cell proliferation(Pollitt, 2004), therefore will not be able to repair smallfractures. The stratum medium is required toresist compressive loads. Thedensity of the tubules changes through the stratum medium (Pollitt, 2004),producing a radial pattern of increased tubule density, leading to alower density at the dermal edge and higher density at the hoof wall.
Thisconsistent around the circumference of the hoof (Lancaster, Bowker and Mauer, 2013). The density gradient aids smoothenergy transfer, from high tubule density to low tubule density (Pollitt,2004). The intertubular horn is also able to shift within the stratum mediumwhen there are inner wall stresses and strains. This reduces the magnitude ofthe forces acting on one area of the hoof (Lancaster, Bowker and Mauer, 2013). The stratum internum is made ofkeratinized lamellae and non-keratinized secondary lamellae (Bowker, 2003 A).There is a connection between the dermis of the corium, near the distalphalanx, and the epidermis near the wall horn.
The basement membrane is astructural boundary between this connection, which is mainly composed ofextracellular matrix (Jansová et al., 2015). It is woven to create primary andsecondary epidermal lamellae that interdigitate with the dermal laminae, thisprovides a highly rigid and strong connection (Jansová et al., 2015). Thebasement membrane is important for coordinating biological processes betweenthe two tissues (Pollitt, 1994) byorganising the cytoskeleton of the epidermal cells and influences the exchangeof nutrients and molecules (Abrahamson, 1986). The lamina densa is the middlelayer of the basement membrane, this consists of extensions and anchoringfibrils which are made of collagen VII (Pollitt, 1994). The anchoring fibrilshook onto collagen I of the connective tissue fibrils in the lamellar coriumforming a vital attachment between the dermis and epidermis (Pollitt, 1994).
There is a high density of lamina densa extensions and anchoring fibrils on thetip of the secondary epidermal lamellae, this gives a larger surface area forattachments (Pollitt, 1994). Therefore, there is a strong attachment betweenthe basement membrane and the connective tissue which is especially importantin horses as they are ungulates, so all of their weight baring is on one digit.There are also electron dense plaques called hemidesmosomes which contain a seriesof proteins which can connect basal cells to the basement membrane (Pollitt,2007). It is vital that that there is a firm attachment as it important in theweight bearing hoof lamellae. The primary epidermal lamellae (PEL) can reducetension forces in the inner hoof as “there is approximately 600 PEL thatsuspend the distal phalanx and each PEL has roughly 100 secondary epidermallamellae” (Bowker, 2003 A).
Therefore, there is a large surface area forattachments, so tension forces are reduced In the horse hoof loads must be dissipated rapidly to reduce thepotential damage to bone and connective tissue (Bowker et al., 1998).This is achieved by the many internal structures of the hoof including thedigital cushion, the frog and lateral cartilages.
The digital cushion lies between the lateralcartilages but above the frog and epidermal bars (Gunkelman and Hammer, 2017).It consists of a network of collagen and elastic fibre bundles, which containproteoglycans, and small areas of adipose tissues (Bowker, 2003 B). However,above the age of 5 the internal structure begins to change. The collagenbundles and the lateral cartilage begin to form fibrocartilage, this mainlyconsists of proteoglycans which is crucial in energy dissipation (Bowker, 2003B). Internal structures of the hoof have many functions. It has beenthought that they aid in a blood pumping mechanism that encourages venous bloodreturn from the digit to the leg (Gunkelman and Hammer, 2017). There is alsoevidence to show that the internal structures have a role in absorbing anddissipating energy during locomotion and stance (Bowker et al.
, 1998).This is supported by Dyhre-Poulsen et al., 1994 who showed that thefrequency and amplitude of forces in the proximal phalanx were much lower thanthe forces in the hoof wall, therefore the laminar attachments and the distalstructures must have a role in reducing these forces.
However, the function ofthe frog during locomotion is not completely known as when it has beensurgically removed the horse’s ability to trot and canter is not diminished(Bowker et al., 1998). There are various theories about how the internal structures canabsorb such high forces. The pressure theory states that upon impact the soleand the frog compress the digital cushion. This applies pressure to the lateralcartilage on the distal phalanx, pushing the hoof wall outwards (Dyhre-Poulsen etal., 1994). Lungwitz, 1884 had conducted experiments and concluded that thehorses whose feet had no frog pressure, made “unsatisfactory angle with theground” and had “upright heels”, which therefore supported the pressure theory.
In contrast to this in 1989 Colles suggested the Lungwitz has only assumed thiswas caused by frog pressure, when there were several factors to consider.Colles also stated that frog pressure changes can result in “increasedexpansion, or contraction or may have virtually no effect”. This shows thatthere is not enough evidence to support the pressure theory The depression theory states that forces are transmitted thoughlaminar attachments in the hoof wall.
These forces are redirected as the middlephalanx in lowered, this caused the hoof walls and lateral cartilage to bepushed outwards (Bowker et al., 1998). Both theories state that blood ispumped from the foot at impact.
This is supported by experiments from Ratliff,Shindell and DeBowes which showed that there is said to be a change in venouspressure during locomotion (Ratliff, Shindell and DeBowes, 1985). On the other had neither theory explains the negative pressuresobserved within the digital cushions during stance and locomotion (Bowker, 2003B). the hydraulic mechanism involved the lateral cartilage, vasculature anddigital cushion all working within the neuromodulation of the microvasculature.At impact blood is forced through microvasculature.
This provides a resistanceto the flow of blood in microvasculature, therefore it reduced the amount ofhigh impact forced that are transferred to the bones and ligaments. Thenegative pressure change is caused by the refilling of vessels before the nextfall (Bowker, 2003 B). This system is more effective in horses with thickerlateral cartilage. This is because they have more small vessels. Afibrocartilaginous digital cushion means that more energy is dissipated asthere are more proteoglycans. Elastic tissue in the digital cushion is used asa spring to return the tissues of the foot to their original position (Bowker,2003 B). The equine hoof has a variety of structural adaptations, some of whosefunctions are still under debate.
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