bones of the carpus and tarsus, the distal phalanx, navicular bone, proximal sesamoid bones and patella, share a different structural template, which consists of a thin cortical shell that encloses a network of cancellous bone throughout the medulla. The relative density of cancellous bone varies between different regions of individual bones depending on their loading history [1–3].
A soft tissue layer, the periosteum, covers the majority of the outer surface of most bones. Periosteum is absent where articular cartilage and ligamentous insertions are present. The periosteum is comprised of two layers: an outer fibrous sheath and an inner, cellular sheet frequently referred to as the cambium layer that is highly vascularized. The cambium layer is abundant in osteoprogenitor cells, which, combined with its rich blood supply, make it important in fracture healing. The inner (medullary or endosteal) surface of a bone is lined with endosteum, which is comprised of a thin membrane, only 10–40 μm thick, consisting of connective tissue and a few layers of cells. The endosteum also contains osteoprogenitor cells and has an important function in fracture healing.
The medulla of long bones is filled with haematopoietic tissue and fat. The proportion occupied by either tissue shifts towards fat in older animals. It contains osteogenic stem cells, and the fat may play an important role in bone biomechanics and absorption of impact loads [4].
Cellular Components
Healthy bone is highly cellular with four dedicated cell types responsible for different functions associated with its formation, maintenance, functional adaptation and homeostasis.
Osteoblasts synthesize the organic component of bone matrix, which they secrete as osteoid. They also play an active role in the mineralization of osteoid and moderate the extent to which it mineralizes. Osteoblasts are derived from the mesenchymal cell line. Undifferentiated mesenchymal cells are directed down the osteoprogenitor line under the influence of fibroblast growth factor, microRNAs and connexin, which stimulate the transcription of bone morphogenetic proteins (BMPs) and expression of the Wingless Wnt signalling pathway. Cells differentiate through stages during which they proliferate before developing into mature osteoblasts that express genes for various proteins, such as alkaline phosphatase (ALP), osteocalcin (OCN), bone sialoprotein (BSP) and collagen. Fully differentiated osteoblasts are relatively large cuboidal cells that form a single layer on bone surfaces. They have well‐developed rough endoplasmic reticulum and Golgi apparatus, consistent with their role in matrix synthesis. Osteoid is composed predominantly of type I collagen with traces of type II, V and other minor structural collagens, which are embedded in a ground substance of water and a wide range of non‐collagenous proteins including proteoglycans and glycosylated proteins. The majority of osteoblasts undergo apoptosis (programmed cell death) after they have made their contribution to new bone formation, but a significant proportion remain to form bone surface lining cells, covering the newly formed surfaces, or become embedded in the matrix they generate to form a dense network of residual osteocytes.
Bone surface lining cells reflect a quiescent form of osteoblasts. They form the cellular layer of periosteum and endosteum and are capable of de‐differentiating back into osteoblasts. They play an important role in ‘containing’ (forming a membrane around) cellular activity during bone remodelling and may, under certain circumstances, protect bone against osteoclastic resorption.
Osteocytes embedded in bone matrix reside within small cavities called lacunae. They are densely and evenly distributed throughout healthy lamellar bone and constitute the vast majority of the cell population. Bone can remain physically intact and serve a functional mechanical role without viable osteocytes although it is in effect necrotic. Osteocytes have numerous physiological functions, one of which is to moderate matrix mineral content: necrotic bone can become hypermineralized and thus relatively brittle. Each cell has numerous long, slender cytoplasmic projections that grow from the cell membrane during its transition from osteoblast to osteocyte. These lie within minute canals called canaliculi. Projections of adjacent osteocytes and smaller projections from bone lining cells and osteoblasts on bone surfaces contact each other and communicate via gap junctions. This effectively creates an interconnected syncytium throughout the bone. A small volume of extracellular fluid is contained within the lacunae and canaliculi, and the flow of this fluid or the small electrical current it generates may be integral to physiological mechanisms for the detection of mechanical strain. There is increasing evidence that osteocytes play the pivotal role in bone metabolism and homeostasis, through the detection of deformation and microdamage and initiation and modulation of the cellular response to these events.
Osteoclasts are large multinucleate cells that resorb bone. Osteoclasts share a haematopoietic stem‐cell precursor with cells of the monocyte/macrophage family. Stem cells are recruited from the circulation and undergo differentiation into pre‐osteoclasts and, subsequently, active osteoclasts under the influence of several factors, including macrophage colony‐stimulating factor (M‐CSF) and receptor activator of nuclear factor kappa‐B ligand (RANKL), which are secreted by osteoprogenitor cells, osteoblasts and osteocytes [5]. During the activation of bone resorption, bone lining cells first lift off the bone surface, thereby allowing osteoclasts access to the matrix. The osteoclast membrane seals to the bone surface around the margin of its contact, and the membrane within the enclosed area develops a ruffled structure. Osteoclasts secrete protons and enzymes, such as tartrate‐resistant acid phosphatase (TRAP), cathepsin K and matrix metalloproteinase‐9 (MMP‐9) into the sealed compartment to dissolve the mineral and digest the organic component. Resorption of the matrix creates a pit in the bone surface, which is referred to as a Howship'’s lacuna.
Bone Formation
Long bones of the appendicular skeleton form in the embryo as cartilage rudiments that are invaded by blood vessels and bone cells. Centres of ossification form within the anlage and progressively replace the cartilage model. Ossification usually begins at foci in the mid‐diaphysis and then the epiphyses. As a rigid tissue, bone can only grow or change shape through appositional growth, involving the addition or resorption of tissue at existing surfaces. The presence of articular cartilage at the ends of long bones prevents longitudinal growth, as new bone cannot be deposited at these surfaces. Conversely, cartilage expands by interstitial growth. Retention of a transverse section of growth cartilage, the physis, at a point where the fronts of diaphyseal and epiphyseal ossification centres meet permits the continued growth of the bone along its long axis. In addition, a layer of growth cartilage is retained between the epiphyseal centre of ossification and overlying articular cartilage to facilitate radial expansion of the epiphysis during growth. By the time of birth, functional loading necessitates that the proportion of cartilage remaining in the weight‐bearing locations of the skeleton is relatively low. In precocial animals that undergo locomotion immediately after birth, such as the horse, bones of the distal limb, e.g. the third metacarpal bone, have effectively reached their adult length by the time of parturition and retain little growth cartilage in weight‐bearing locations (Figure 2.2). Growth cartilage at the physis and around the epiphysis is eventually replaced by bone, at which stage the skeleton is considered to be mature. In the horse, this occurs relatively early in bones of the distal limb (e.g. 6 months in the third metacarpal bone) and considerably later in bones of the proximal limb (e.g. 24–36 months in the humerus). The previous location of the physis remains visible grossly and radiologically for many years as a roughening on the periosteal surface of the bone and as a transverse linear radiopacity termed the ‘physeal scar’.
Cuboidal bones of the carpus and tarsus ossify in the last two months of gestation. In normal foals, over 80% of the cartilage anlage has been replaced by bone at the time of birth [6]. The extent of ossification may be significantly less in foals born prematurely or those that are dysmature or suffering hypothyroidism. The majority of cuboidal bones ossify from a single centre and grow centrifugally. However, the third tarsal bone has two centres located in the body of the bone and dorsally. The point where the two ossifying fronts meet represents
