Читать книгу Fractures in the Horse - Группа авторов - Страница 24

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.