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614 Regenerative Medicine and Tissue Engineering

194-202.

José A. Andrades, Lucía Narváez-Ledesma, Luna Cerón-Torres, Anyith P. Cruz-Amaya, Daniel López-Guillén, M. Laura Mesa-Almagro and José A. Moreno-Moreno

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56389

**1. Introduction**

#### **1.1. A connective tissue**

Bone is a highly specialized form of connective tissue that is nature's provision for an internal support system in all higher vertebrates. It is a complex living tissue in which the extracellular matrix (ECM) is mineralized, conferring marked rigidity and strength to the skeleton while still maintaining some degree of elasticity. In addition to its supportive and protective organic ions, it actively participates in maintaining calcium homeostasis in the body.

Bone is composed of an organic matrix that is strengthened by deposits of calcium salts. Type I collagen constitutes approximately 95% of the organic matrix; the remaining 5% is composed of proteoglycans and numerous noncollagenous proteins. The crystalline salts deposited in the organic matrix of bone under cellular control are primarily calcium and phosphate in the form of hydroxyapatite (HA).

Morphologically there are two forms of bone: cortical (compact bone) and cancellous (spongy bone). In cortical bone, densely packed collagen fibrils form concentric lamellae, and the fibrils in adjacent lamellae run in perpendicular planes as in plywood. Cancellous bone has a loosely organized, porous matrix. The differences between cortical and cancellous bone are both structural and functional. Differences in the structural arrangements of the two types are related to their primary functions: cortical bone provides the mechanical and protective functions and cancellous bone provides the metabolic functions.

© 2013 Andrades et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1.2. Bone cell structure and function**

Bone is composed of four different cell types (Fig. 1). Osteoblasts, osteoclasts, and bone lining cells are present on bone surfaces, whereas osteocytes permeate the mineralized interior. Osteoblasts, osteocytes, and bone lining cells originate from local osteoprogenitor cells, whereas osteoclasts arise from the fusion of mononuclear precursors, which originate in the various hemopoietic tissues.

**Figure 1.** The origins and locations of bone cells. *Taken from Academic Press Inc., with permission*

Osteoblasts are the fully differentiated cells responsible for the production of the bone matrix. Portions of four osteoblasts are shown in Figure 2. An osteoblast is a typical protein-producing cell with a prominent Golgi apparatus and well-developed rough endoplasmic reticulum. It secretes the type I collagen and the noncollagenous proteins of the bone matrix.

The staggered overlap of the individual collagen molecules provides the characteristic pe‐ riodicity of type I collagen in bone matrix. Numerous noncollagenous proteins have been isolated from bone matrix (Sandberg, 1991) but to date there is no consensus for a defini‐ tive function of any of them. Osteoblasts regulate the mineralization of bone matrix, al‐ though the mechanism(s) is not completely understood. In woven bone, mineralization is initiated away from the cell surface in matrix vesicles that bud from the plasma mem‐ brane of osteoblasts. This is similar to the well-documented role of matrix vesicles in car‐ tilage mineralization (Hohling et al., 1978). In lamellar bone, the mechanism of mineralization appears to be different. Mineralization begins in the hole region between overlapped collagen molecules where there are few, in any, matrix vesicles (Landis et al., 1993) and appears to be initiated by components of the collagen molecule itself or noncol‐ lagenous proteins at this site. Whatever the mechanisms of mineralization, collagen is at least a template for its initiation and propagation and there is always a layer of unminer‐ alized bone matrix (osteoid) of the surface under osteoblasts. Matrix deposition is usually

**1.2. Bone cell structure and function**

616 Regenerative Medicine and Tissue Engineering

various hemopoietic tissues.

Bone is composed of four different cell types (Fig. 1). Osteoblasts, osteoclasts, and bone lining cells are present on bone surfaces, whereas osteocytes permeate the mineralized interior. Osteoblasts, osteocytes, and bone lining cells originate from local osteoprogenitor cells, whereas osteoclasts arise from the fusion of mononuclear precursors, which originate in the

**Figure 1.** The origins and locations of bone cells. *Taken from Academic Press Inc., with permission*

secretes the type I collagen and the noncollagenous proteins of the bone matrix.

Osteoblasts are the fully differentiated cells responsible for the production of the bone matrix. Portions of four osteoblasts are shown in Figure 2. An osteoblast is a typical protein-producing cell with a prominent Golgi apparatus and well-developed rough endoplasmic reticulum. It

The staggered overlap of the individual collagen molecules provides the characteristic pe‐ riodicity of type I collagen in bone matrix. Numerous noncollagenous proteins have been isolated from bone matrix (Sandberg, 1991) but to date there is no consensus for a defini‐ tive function of any of them. Osteoblasts regulate the mineralization of bone matrix, al‐ though the mechanism(s) is not completely understood. In woven bone, mineralization is initiated away from the cell surface in matrix vesicles that bud from the plasma mem‐ brane of osteoblasts. This is similar to the well-documented role of matrix vesicles in car‐ tilage mineralization (Hohling et al., 1978). In lamellar bone, the mechanism of mineralization appears to be different. Mineralization begins in the hole region between overlapped collagen molecules where there are few, in any, matrix vesicles (Landis et al., 1993) and appears to be initiated by components of the collagen molecule itself or noncol‐ lagenous proteins at this site. Whatever the mechanisms of mineralization, collagen is at least a template for its initiation and propagation and there is always a layer of unminer‐ alized bone matrix (osteoid) of the surface under osteoblasts. Matrix deposition is usually

**Figure 2.** Transmision electron micrograph of osteoblasts (numbered) on a bone surface in which the collagenous ma‐ trix has been deposited in two layers (A and B) at right angles to each other. The Golgi apparatus (G) and rough endo‐ plasmic reticulum (r) are prominent cytoplasmic organelles in osteoblasts. (Original magnification x2800, bar 0.1 µm). *Taken from Academic Press Inc., with permission*

polarized toward the bone surface, but periodically becomes generalized, surrounding the osteoblast and producing the next layer of osteocytes. Deposition of mineral makes the matrix impermeable and to ensure a metabolic lifeline, osteocytes establish numerous cy‐ toplasmic connections with adjacent cells before mineralization.

The osteocyte is a mature osteoblast within the bone matrix and is responsible for its mainte‐ nance (Buckwalter et al., 1995). These cells have the capacity not only to synthesize, but also to resorb matrix to a limited extent. Each osteocyte occupies a space, or lacunae, within the matrix and extends filopodial processes through canaliculi in the matrix to contact processes of adjacent cells by means of gap junctions. Because diffusion of nutrients and metabolites through the mineralized matrix is limited, filopodial connections permit communication between neighbouring osteocytes, internal and external surfaces of bone and with the blood vessels traversing the matrix. The functional capacities of osteocytes can be easily ascertained from their structure.

Bone lining cells are flat, elongated, inactive cells that cover bone surfaces that are undergoing neither bone formation nor resorption. Because these cells are inactive, they have few cyto‐ plasmic organelles. Little is known regarding the function of these cells; however, it has been speculated that bone lining cells can be precursors for osteoblasts.

Osteoclasts are large, multinucleated cells which resorb bone. When active, osteoclasts rest directly on the bone surface and have two plasma membrane specializations: a ruffled border and a clear zone. The ruffled border is the central, highly infolded area of the plasma membrane where bone resorption takes place. The clear zone is a microfilament-rich, organelle-free area of the plasma membrane that surrounds the ruffled border and serves as the point of attach‐ ment of the osteoclast to the underlying bone matrix. Active osteoclasts exhibit a characteristic polarity. Nuclei are typically located in the part of the cell most removed from the bone surface and are interconnected by cytoskeletal proteins (Watanabe et al., 1995). Osteoclasts contain multiple circumnuclear Golgi stacks, a high density of mitochondria, and abundant lysosomal vesicles that arise from the Golgi and cluster near the ruffled border.

When a fracture occurs, a set of signals is triggered. These are both local signals and systemic ones; some of these signals are mediated by neuronal impulses (Nordsletten et al., 1994), by the haematoma at the site of the fracture and by the trauma caused to the tissues surrounding the fracture (Einhorn, 1998). These signals can be divided into two interactive and inter‐ changeable categories: inflammatory signals and bone building signals. These factors mitigate the migration of phagocytotic cells to the area of the fracture, removing the necrotic tissue and propagating the in-growth of new blood vessels to the site of the fracture, thus providing nutrients and cells to the fracture site and starting the healing cascade. If at the end of the healing process osteo-integration (of the new bone together with the native bone) is not achieved, even with the best type of scaffolds, the chances of long-term success are dismal (Avila et al., 2009).
