Osteoblasts

Osteoblasts are cells derived from mesenchymal stem cells. Osteoblasts produce several substances, including osteocalcin, TGF-β, macrophage colony-stimulating factor, receptor activator of nuclear factor κβ ligand (RANKL), osteoprotegerin (OPG), and bone matrix⁶ and osteocalcin when stimulated by 1,25-dihydroxy-vitamin D. Osteoblasts are active on the outer surface of bones, where they form a single layer of cells. They initiate the formation of new bone by their synthesis of osteoid (nonmineralized bone matrix). The mechanism of osteoblast stimulation is reported to be the production of so-called coupling factors generated during the resorption process. Osteoblasts cause the formation of new bone and the orderly mineralization of bone matrix by concentrating some of the plasma proteins (growth factors) found in the bone matrix and by facilitating the deposit and exchange of calcium and other ions at the site. Osteoblasts alter levels of RANKL and OPG; the balance of these two cytokines determines overall osteoclast formation⁷ (see Figure 44-11). As new bone is formed, it is shaped and remodeled through TGF-β, as well as other plasma proteins (growth factors) found in the bone marrow (Table 43-4).

Osteocytes

An osteocyte is a transformed osteoblast that is trapped or surrounded in osteoid as it hardens from minerals that enter during ossification (see Figure 43-2, B). It is the final differentiation stage for an osteoblast. The osteocyte is within a space in the hardened bone matrix called a lacuna. Each osteocyte has a high nucleus/cytoplasm ratio with a thin layer of nonmineralized osteoid around it, similar to the egg white surrounding an egg yolk.

The cells of the osteoblastic lineage (osteoblasts and osteocytes) form a network of cells in bone that sense the shape and structure of bone (mechanoreceptors) and determine the appropriate locations for bone formation or resorption, according to Wolff ’s law (bone is shaped according to its function).

Osteocytes communicate with other bone cells and instruct both osteoblasts and osteoclasts about when and where to form and resorb bone; concentrate nutrients in the matrix; regulate bone mass and minerals (especially calcium and phosphorus); and secrete other proteins, such as sclerostin and fibroblast growth factor-23 (FGF-23), that influence mineral metabolism.3,4,8,9 Sclerostin inhibits osteoblasts, thus reducing bone formation. Additionally, osteocytes seem to function as endocrine receptors in regulating bone metabolism and are prime target cells of parathyroid hormone (PTH).^9–11 Through exchanges between these cells, hormone catalysts, and minerals, optimal levels of calcium, phosphorus, and other minerals are maintained in blood plasma. The osteocyte also aids in modifying bone matrix through release of enzymes to dissolve the mineralized walls of the lacunae to prepare the bone for remodeling. (Remodeling is described on p. 1519.)

Osteoclasts

Osteoclasts are large, multinucleated bone cells derived from hematopoietic stem cells in the bone marrow stroma and are the major resorptive cells of bone. Considered to be the major remodelers of bone, osteoclasts contain lysosomes (digestive vacuoles) filled with hydrolytic enzymes. Fine projections, or microvilli, fan out from the osteoclast cell’s surface and are known as ruffled borders; these projections result from extensive infoldings of the cell membrane adjacent to the resorptive surface.¹² Osteoclasts in regions of bone resorption lie in pits called Howship lacunae, where the infolded, ruffled borders of the osteoclasts greatly increase the surface area of the plasma membrane. The infolds end in numerous channels and vesicles in the cell cytoplasm, permitting them to resorb the bone under their ruffled, infolded borders.

Osteoclasts bind to the bone surface of cell attachment proteins called integrins.¹² In contact with bone mineral, osteoclasts use adhesive structures, called podosomes, to attach themselves to the bone surface. They then seal their cytoplasm with the underlying bone and dissolve the bone and matrix while protecting the surrounding tissue.¹³ They effect resorption of bone by secretion of hydrochloric acid (HCl) and cathepsin K (a protease enzyme), which help dissolve bone minerals and collagenase, thereby aiding the digestion of collagen along with the action of cytokines (see Table 43-4). Matrix metalloproteinases (MMPs), a group of proteolytic enzymes, help control osteoclast-matrix interactions necessary for bone resorption. Once resorption is completed, the osteoclast disappears by degeneration, either by reverting to its parent cell or by leaving the site through the process of cell mobility, wherein the osteoclast then becomes an inactive, or “resting,” osteoclast. Overall, bone homeostasis is the result of crosstalk between osteoclasts, osteoblasts, and osteocytes.¹⁴ Recent evidence has indicated that osteoclasts also may play a role in the body’s immune system.^15–17

Collagen Fibers

Collagen fibers are the major organic component of bone matrix. The fibers are approximately 90% type I collagen, are essential for bone strength, and are synthesized and secreted by osteoblasts. Once secreted, collagen molecules assemble into thin chains called α-chains, which combine in groups of 3 to form fibrils. The fibrils form a staggered pattern, overlapping nearby fibrils by approximately one fourth their length. This staggered, overlapping pattern creates regular gaps, called hole zones, into which mineral crystals are deposited.¹⁸ After mineral deposition, the fibrils link together and twist to form ropelike fibers. Collagen fibers then join to form a framework that gives bone its tensile strength and enables it to bear weight.

Collagen is the most abundant macromolecule in the body, accounting for approximately one third of all protein and providing the structural framework for nearly all tissues. Collagen is one of the extracellular components, along with proteoglycans and noncollagenous matrix proteins, of articular cartilage. To date, more than 20 types of collagen have been identified, though all their functions are not yet known. Cartilage-specific collagens include types II (the principal component), VI, IX, X, and XI. Type IX collagen is thought to be the “glue” that holds together the type II collagen scaffold of articular cartilage, helps maintain the structural integrity of cartilage, and resists tensile forces on the joint cartilage. Type XI regulates the fibril diameter of type II cartilage. Degradation of type IX collagen by proteolytic enzymes has been seen in the early stages of osteoarthritis and rheumatoid arthritis. Researchers have proposed that this degradation, or “unplugging,” may be the mechanism for the degenerative changes seen in osteoarthritic and rheumatoid cartilage. Table 43-5 gives the musculoskeletal distribution of other types of collagen.

Proteoglycans

Proteoglycans are large complexes of numerous polysaccharides attached to a common protein core. Hyaline cartilage is primarily composed of the glycosaminoglycans chondroitin sulfate and keratin sulfate. They strengthen bone by forming compression-resistant networks between the collagen fibrils. Proteoglycans also control the transport and distribution of electrically charged particles (ions), particularly calcium, through the bone matrix, thereby playing a role in bone calcium deposition and calcification.

Glycoproteins

Glycoproteins are also carbohydrate-protein complexes of bone. Glycoproteins control the collagen interactions that lead to fibril formation. They also play a role in calcification.

Some of the glycoproteins in bone matrix include bone sialoprotein, osteocalcin, osteonectin, laminin, albumin, and α-glycoprotein. Other proteins found in bone matrix are the compounds currently called bone morphogenic proteins (BMPs) (see Table 43-2). Sialoprotein (osteopontin) comprises about 8% of the noncollagenous matrix of bone and easily binds with calcium.

Osteocalcin is also a calcium-binding protein that binds preferentially to calcium that has already crystallized. The roles of osteocalcin may be to inhibit calcium phosphate precipitation and to play a part in bone resorption by recruiting osteoclasts.

Osteonectin is a bone-specific protein that binds selectively to both hydroxyapatite and collagen in the bone matrix. Laminin is an abundant bone matrix protein in humans that stabilizes basement membranes in bones and is important in neurite and axon growth.

Bone albumin is identical to serum albumin. In calcified matrix, bone albumin is permanently fixed to bone mineral crystals and remains so until the bone is resorbed. Bone albumin transports essential elements such as hormones, ions, and other metabolites to and from the bone cells and maintains the osmotic pressure of bone fluid (fluid surrounding mineral crystals and osteoblasts).