**4. Bone physiology**

Bone is a complex, dynamic tissue that has the ability to grow ontogenically, to repair after damage, and to adaptively respond to a variety of exogenous and endogenous stimuli [23]. Composed of a mineralised organic matrix in which the cells responsible for its formation and rejuvenation are embedded, osseous tissue, through its unique physiological and biochemical properties, enables bones to perform a multitude of functions within the animal's body. The organic matrix, or osteoid [24], is formed principally of type-I collagen (around ~95% type-I [25]) which affords the bone its tensile strength, alongside trace amounts of other collagens, in addition to non-collagenous proteins whose predominant purpose is to permit the mineralisation of the matrix. The chief mineral salt found in osseous tissue is a form of hydroxyapatite [Ca10(PO4)6(OH)2] which, bound to the matrix proteins, renders the tissue resistant to compressive forces [23].

The bone is structured into either cortical or trabecular bone. Cortical, or compact, bone forms the dense outer proportions of the bone and, in the human, accounts for 80% of the total skeletal mass [23, 26]. Cortical osteons (**Figure 5**), or Haversian systems, are tubular structures consisting of a central channel (Haversian canal), through which a nerve and blood supply are provided,

#### **Figure 5.**

*Schematic of a cortical osteon. (A) Represents a Haversian canal incorporating a neurovascular bundle, (B) a lamella, (C) a lacuna containing an osteocyte, and (D) canaliculi.*

**25**

*The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof*

surrounded by coaxial lamellae of mineralised bone matrix which incorporate a number of voids, in the form of lacunae and canaliculi, inhabited by cells [23]. The remaining 20% of the osseous tissue is in the form of trabeculae (from the Latin *trabs*, meaning "beam" [27]) which provide structural support, in a buttress-like manner, to the surrounding cortex. Trabecular osteons, or packets, are similar in their lamellar architecture to those of the cortex, but are smaller in size and semi-lunar in shape [23]. This hierarchical design, a common (if not omniexistent) phenomenon in biological materials, provides bones with the physical strength they need to fulfil their roles in structural support, the protection of underlying organs, and in providing leverage to muscles and tendons, facilitating

The remodelling and general renewal of the bone is mediated by three cell types: osteoblasts, osteoclasts and osteocytes [30]. Osteoblasts are mononucleated cells formed by the differentiation of mesenchymal stem cells, and are responsible for the synthesis of osteoid and its subsequent mineralisation [25]. They reside principally in the endosteum—the vascularised cellular lining of the internal proportions of the bone, covering the walls of the Haversian canals, and the trabeculae and medullary cavity where it separates the bone matrix from the marrow [31]—and, alongside their incompletely-differentiated precursors and fibroblasts, the cambial layer of the periosteum [23]—a vascularised and innervated structure consisting of an outer fibrous layer comprising fibroblasts, collagen and elastin, and the discrete inner cambium that coats the bone's exterior [32]. Osteoclasts are multinucleated macrophagic cells derived from phagocytes in the haematopoietic bone marrow, and carry out bone resorption in localised areas of the bone surface to which they adhere upon activation [21]. Osteocytes, which inhabit the lacunae of the osteons and have multiple cytoplasmic processes which traverse the osteons' canaliculi, are the result of the terminal differentiation of osteoblasts that have become entrapped within the bone matrix that they have synthesised [25]. They act as mechanoreceptors, communicating with the osteogenic/osteolytic cells via gap junctions at the extremities of their cytoplasmic processes, and play a regulatory role in the bone

Concomitant to the physical need for the bone to be able to remodel for general maintenance, repair, and for increasing structural strength in response to stimuli, are the roles in pH balance and mineral homeostasis that the synthesis/resorption cycle affords [23]. While under normal physiological circumstances the pH of blood and extracellular fluid is maintained within narrow parameters by the removal and excretion of protons by the kidneys and lungs, a multitude of physiologically adverse conditions (e.g. kidney disease or severe exercise) can lead to acidosis [33]. The basicity of hydroxyapatite renders bone an emergency reservoir for base, buffering the acidity with the products of osteoclastic resorption. This mechanism is enabled by the osteoclast's stimulation at low pH, a peculiarity from a general cellular point of view, and the osteoblast's synergistic inhibition [33]. In a similar vein, bone acts as a reservoir for calcium and phosphorus, making them available for the maintenance of mineral homeostasis. Calcium and phosphorus are vital for a plethora of biological functions, and their homeostasis is under the endocrine regulation of the parathyroid glands, thyroid gland, and the kidney which, through the intermediary of parathyroid hormone, calcitriol (1,25-dihydroxyvitamin D, a hormone derived from vitamin D), and calcitonin respectively, affect the intestinal absorption, renal reabsorption and bone synthesis/resorption mechanisms [24, 34]. The parathyroid glands, which express Ca2+-sensing receptors, secrete PTH in response to a reduction in circulating calcium ions. Parathyroid hormone acts in the kidney to decrease phosphate and increase calcium reabsorption, and in the bone by stimulating osteocytic and osteoclastic activity [34].

*DOI: http://dx.doi.org/10.5772/intechopen.84514*

movement [23, 28, 29].

synthesis/resorption cycle [25].

#### *The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof DOI: http://dx.doi.org/10.5772/intechopen.84514*

surrounded by coaxial lamellae of mineralised bone matrix which incorporate a number of voids, in the form of lacunae and canaliculi, inhabited by cells [23]. The remaining 20% of the osseous tissue is in the form of trabeculae (from the Latin *trabs*, meaning "beam" [27]) which provide structural support, in a buttress-like manner, to the surrounding cortex. Trabecular osteons, or packets, are similar in their lamellar architecture to those of the cortex, but are smaller in size and semi-lunar in shape [23]. This hierarchical design, a common (if not omniexistent) phenomenon in biological materials, provides bones with the physical strength they need to fulfil their roles in structural support, the protection of underlying organs, and in providing leverage to muscles and tendons, facilitating movement [23, 28, 29].

The remodelling and general renewal of the bone is mediated by three cell types: osteoblasts, osteoclasts and osteocytes [30]. Osteoblasts are mononucleated cells formed by the differentiation of mesenchymal stem cells, and are responsible for the synthesis of osteoid and its subsequent mineralisation [25]. They reside principally in the endosteum—the vascularised cellular lining of the internal proportions of the bone, covering the walls of the Haversian canals, and the trabeculae and medullary cavity where it separates the bone matrix from the marrow [31]—and, alongside their incompletely-differentiated precursors and fibroblasts, the cambial layer of the periosteum [23]—a vascularised and innervated structure consisting of an outer fibrous layer comprising fibroblasts, collagen and elastin, and the discrete inner cambium that coats the bone's exterior [32]. Osteoclasts are multinucleated macrophagic cells derived from phagocytes in the haematopoietic bone marrow, and carry out bone resorption in localised areas of the bone surface to which they adhere upon activation [21]. Osteocytes, which inhabit the lacunae of the osteons and have multiple cytoplasmic processes which traverse the osteons' canaliculi, are the result of the terminal differentiation of osteoblasts that have become entrapped within the bone matrix that they have synthesised [25]. They act as mechanoreceptors, communicating with the osteogenic/osteolytic cells via gap junctions at the extremities of their cytoplasmic processes, and play a regulatory role in the bone synthesis/resorption cycle [25].

Concomitant to the physical need for the bone to be able to remodel for general maintenance, repair, and for increasing structural strength in response to stimuli, are the roles in pH balance and mineral homeostasis that the synthesis/resorption cycle affords [23]. While under normal physiological circumstances the pH of blood and extracellular fluid is maintained within narrow parameters by the removal and excretion of protons by the kidneys and lungs, a multitude of physiologically adverse conditions (e.g. kidney disease or severe exercise) can lead to acidosis [33]. The basicity of hydroxyapatite renders bone an emergency reservoir for base, buffering the acidity with the products of osteoclastic resorption. This mechanism is enabled by the osteoclast's stimulation at low pH, a peculiarity from a general cellular point of view, and the osteoblast's synergistic inhibition [33]. In a similar vein, bone acts as a reservoir for calcium and phosphorus, making them available for the maintenance of mineral homeostasis. Calcium and phosphorus are vital for a plethora of biological functions, and their homeostasis is under the endocrine regulation of the parathyroid glands, thyroid gland, and the kidney which, through the intermediary of parathyroid hormone, calcitriol (1,25-dihydroxyvitamin D, a hormone derived from vitamin D), and calcitonin respectively, affect the intestinal absorption, renal reabsorption and bone synthesis/resorption mechanisms [24, 34]. The parathyroid glands, which express Ca2+-sensing receptors, secrete PTH in response to a reduction in circulating calcium ions. Parathyroid hormone acts in the kidney to decrease phosphate and increase calcium reabsorption, and in the bone by stimulating osteocytic and osteoclastic activity [34].

*Veterinary Anatomy and Physiology*

**4. Bone physiology**

the equine foot are predisposed to local vasoconstriction and the development of ischemic disease as the arteries from the plexus have thicker walls with small lumens and are unable to auto-regulate the volume variations that are involved in contrac-

The equine hoof veins are divided into three groups depending on their location: wall dermis veins, which are separated into proximal and distal regions; coronary dermis veins; and frog and sole dermis veins [1]. The dermal lamella is drained by: the coronary vein; the independent superficial vein; the proximal branch of the caudal hoof vein; and the circumflex vein. The toe and quarters are drained via the circumflex vein [2, 20]. An additional feature of the blood circulation of the equine foot is the anastomoses of arteries and veins, which are blood vessels forming shunts [21]. Each dermal papilla in the periople, coronary band, frog, sole and terminal papillae contain a meshwork of anastomosing arteriovenous vessels located at the base of the papillae. These anastomoses are able to withdraw approximately 50% of the whole limb blood flow, and thus can be involved in ischemia due to blood flow diversion [22]. This could explain the relationship between laminitis and ischemia [3].

Bone is a complex, dynamic tissue that has the ability to grow ontogenically, to repair after damage, and to adaptively respond to a variety of exogenous and endogenous stimuli [23]. Composed of a mineralised organic matrix in which the cells responsible for its formation and rejuvenation are embedded, osseous tissue, through its unique physiological and biochemical properties, enables bones to perform a multitude of functions within the animal's body. The organic matrix, or osteoid [24], is formed principally of type-I collagen (around ~95% type-I [25]) which affords the bone its tensile strength, alongside trace amounts of other collagens, in addition to non-collagenous proteins whose predominant purpose is to permit the mineralisation of the matrix. The chief mineral salt found in osseous tissue is a form of hydroxyapatite [Ca10(PO4)6(OH)2] which, bound to the matrix

The bone is structured into either cortical or trabecular bone. Cortical, or compact, bone forms the dense outer proportions of the bone and, in the human, accounts for 80% of the total skeletal mass [23, 26]. Cortical osteons (**Figure 5**), or Haversian systems, are tubular structures consisting of a central channel (Haversian canal), through which a nerve and blood supply are provided,

tion of smooth muscle as well as encompassing arteriovenous shunts [18].

**24**

**Figure 5.**

*Schematic of a cortical osteon. (A) Represents a Haversian canal incorporating a neurovascular bundle,* 

*(B) a lamella, (C) a lacuna containing an osteocyte, and (D) canaliculi.*

proteins, renders the tissue resistant to compressive forces [23].

Bone also acts as reservoir for growth factors and cytokines, which are released during bone resorption and take effect either locally or systemically [14]. Along with the effects that these growth factors and cytokines may exert on other tissue cell types, such as the endothelial cells of the vasculature, they play important roles in bone formation and resorption, including: insulin-like growth factors, transforming growth factors, and bone morphogenic proteins as growth factors promoting osteogenesis; epidermal growth factor, granulocyte-macrophage colonystimulating factor, macrophage-colony stimulating factor, and tumour necrosis factor as growth factors stimulating osteolytic resorption; platelet-derived growth factor and fibroblastic growth factor which have contributory effects to both bone formation and resorption; prostaglandins and leukotrienes as osteolysis-stimulating cytokines; and interleukins that may directly or indirectly stimulate either bone formation or resorption depending on the interleukin family in question [35].

Strong links exist between the skeletal and vascular systems and, along with a strong vascular and nerve presence in the periosteum, numerous neurovascular bundles enter the bone through nutrient foramina, descend and ascend the canals of Haversian systems, and enter medullary cavity through Volkmann's Canals [36]. The two systems are interdependent in that the bone relies on the vasculature for the delivery of oxygen and nutrients and that, modulated by osteoblasts, haematopoiesis takes place in the bone marrow [37].

## **5. Morphological changes and pathologies in the foot**

The external morphology of the hoof capsule is indirectly associated with the function and shape of the internal segments of the hoof [38]. Dyson and colleagues [39] drew attention to the fact that, despite differences in the orientation of the distal phalanx between horses, mainly associated with changes in direction of the dished solar border, the morphology of the distal phalanx is unaffected by the external features of the hoof capsule. It is worth noting here that hoof shape can be altered when trimming and shoeing are considered [40]. The impact of trimming/shoeing on the hoof capsule shape has been explained [41] and the researchers demonstrated that the formation of the hoof wall is physically connected to the loading of the lower limb, thus protecting its optimal balance on the ground [42]. Therefore, the geometrical tendency of the foot components determines the ability of the internal structures to respond to loading through the bearing phase of the stride cycle [43].

The distal phalanx is attached within the hoof capsule through the suspensory apparatus [44], which connects the entire parietal surface of the distal phalanx to the lamellar structures of the internal hoof wall [11]. Preliminary work on equine lamellar connection found that this attachment provides the mechanism by which the weight is transferred between the distal phalanx and the epidermal laminae of the hoof wall [45]. This connection, or attachment, has a substantial role in the biomechanics of healthy foot performance, and may lead to foot lameness if damaged [41]. Indeed, the failure of the connection between the epidermal laminae and the underlying basement membrane of the dermal lamellae would weaken the suspensory apparatus of the distal phalanx [46]. Unsurprisingly, changes in the basement membrane of the suspensory tissue have been suggested to signal the first step of laminar failure [47]. While other research reported that lesions in the basement membrane appear before any clinical signs of foot lameness [48]. The dislocation of the distal phalanx, followed by its rotation, applies pressure, first on the sole at the palmar border of the distal phalanx and, secondly, on the coronet or upper area of the lamellae by the extensor process of the distal phalanx [49]. These deflections lead to impaired blood flow into the basal layers of the hoof wall [18], and can lead

**27**

**Figure 6.**

*shape of hoof capsule.*

*The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof*

heels, club foot, long-dished toe, and high-heel foot [49, 51].

straight and normal growth of the hoof capsule (**Figure 6**) [57].

or uneven feet promoting the development of foot lameness [66].

Virtually all ungulate animals can be affected by foot lameness [67–70]. However, our knowledge concerning the aetiology of the condition is often related to the economic implications of the animal in our society [58, 62]. Foot lameness is classified into acute and chronic types depending on the severity of lesions and the

*The anatomical displacement (indicated by \*) of the distal phalanx. (A, B) Longitudinal section of two laminitic feet shows lamellae wedge inside the hoof capsule indicates (\*) abnormal tissue, which changed the* 

to an inhibition of the growth rate of the hoof capsule, affecting its shape over time and induced osteolysis of bone trabeculae in chronic stages [50]. A number of hoof shapes can arise from this chronic condition, including sheared heels, crushed

One of these chronic conditions is the lamellar wedge that develops alongside laminitis and a result can be an anatomical displacement of the distal phalanx within the hoof capsule [52], is a direct consequence of the failure of the suspensory apparatus of the distal phalanx [53]. However, the molecular events involved in the lamellar wedge condition are broadly unknown [54]. In chronically laminitic horses, the lamellar wedge appears as an abnormal horny mass, that is formed between the inner hoof wall and the epidermal lamellae, and is linked to the slight rotation of the distal phalanx [55]. The separation of the distal phalanx inside the capsular wall can change the sole shape to become convex rather than be concave, due to differential growth of the proximal hoof wall portion [55]. The structural and physical appearance of this abnormal keratinized material is comparable to the white line tissue, and is therefore proposed to be an ectopic white line [56]. It was therefore thought that a large quantity of the ectopic white line could be able ultimately to prohibit the

Foot lameness is a physical impairment of a limb that has a negative effect on the freedom of movement of the animal [58, 59]. It is accompanied by clinical signs linked to a disturbance of locomotion that is related to hoof pathologies that can be caused by infection, environmental and/or genetic causes [60, 61]. Lameness can also manifest itself in pain and lesions that, in turn, lead to an abnormal gait [62, 63] with undesirable consequences on performance [64] and welfare [65]. This disruption in gait originates from involuntary and voluntary exertions to diminish the level of discomfort and/or pain that are the result of damage or injury of ligaments, muscles, nerves or integument [59, 60], or could be due to asymmetric and/

*DOI: http://dx.doi.org/10.5772/intechopen.84514*

**5.1 Lameness**

#### *The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof DOI: http://dx.doi.org/10.5772/intechopen.84514*

to an inhibition of the growth rate of the hoof capsule, affecting its shape over time and induced osteolysis of bone trabeculae in chronic stages [50]. A number of hoof shapes can arise from this chronic condition, including sheared heels, crushed heels, club foot, long-dished toe, and high-heel foot [49, 51].

One of these chronic conditions is the lamellar wedge that develops alongside laminitis and a result can be an anatomical displacement of the distal phalanx within the hoof capsule [52], is a direct consequence of the failure of the suspensory apparatus of the distal phalanx [53]. However, the molecular events involved in the lamellar wedge condition are broadly unknown [54]. In chronically laminitic horses, the lamellar wedge appears as an abnormal horny mass, that is formed between the inner hoof wall and the epidermal lamellae, and is linked to the slight rotation of the distal phalanx [55]. The separation of the distal phalanx inside the capsular wall can change the sole shape to become convex rather than be concave, due to differential growth of the proximal hoof wall portion [55]. The structural and physical appearance of this abnormal keratinized material is comparable to the white line tissue, and is therefore proposed to be an ectopic white line [56]. It was therefore thought that a large quantity of the ectopic white line could be able ultimately to prohibit the straight and normal growth of the hoof capsule (**Figure 6**) [57].
