Cartilage tissue is structurally simple, consisting of cells of a single type— chondrocytes—embedded in a more or less uniform, highly hydrated matrix consisting of proteoglycans and type II collagen (discussed in Chapter 19). The cartilage matrix is deformable, and the tissue grows by expanding as the chondrocytes divide and secrete more matrix (Figure 22–13). Bone, by contrast, is dense and rigid; it grows by apposition—that is, by deposition of additional matrix on free surfaces. Like reinforced concrete, the bone matrix is predominantly a mixture of tough fibers (type I collagen fibrils), which resist pulling forces, and solid particles (calcium phosphate as hydroxylapatite crystals), which resist compression. The bone matrix is secreted by osteoblasts that lie at the surface of the existing matrix and deposit fresh layers of bone onto it. Some of the osteoblasts remain free at the surface, while others gradually become embedded in their own secretion. This freshly formed material (consisting chiefly of type I collagen) is rapidly converted into hard bone matrix by the deposition of calcium phosphate crystals in it.
Once imprisoned in hard matrix, the original bone-forming cell, now called an osteocyte, has no opportunity to divide, although it continues to secrete additional matrix in small quantities around itself. The osteocyte, like the chondrocyte, occupies a small cavity, or lacuna, in the matrix, but unlike the chondrocyte
Figure 22–12 Control of fibroblast differentiation by the physical properties of the extracellular matrix. on a stiff matrix, the cells form strong adhesions, spread out, and tend to turn into bone cells. on a soft matrix, where the cells are unable to form strong anchorages, they fail to spread and tend to differentiate as fat cells. these effects depend on transcription regulators (yap and taZ proteins) that move into the cell nucleus in response to tension developed in actin–myosin bundles in the cytoplasm. (based on S. dupont et al., Nature 474:179–183, 2011.)
Figure 22–13 The growth of cartilage.
the tissue expands as the chondrocytes divide and make more matrix. the freshly synthesized matrix with which each cell surrounds itself is shaded dark green. Cartilage may also grow by recruiting fibroblasts from the surrounding tissue and converting them into chondrocytes.
osteoblast osteoid (new, uncalcifed bone matrix) old, calcifed bone matrix it is not isolated from its fellows. Tiny channels, or canaliculi, radiate from each lacuna and contain cell processes from the resident osteocyte, enabling it to form gap junctions with adjacent osteocytes (Figure 22–14). Blood vessels and nerves run through the tissue, keeping the bone cells alive and reacting when the bone is damaged.
A mature bone has a complex and beautiful architecture, in which dense plates of compact bone tissue enclose spaces spanned by light frameworks of trabecular bone—a filigree of delicate shafts and flying buttresses of bone tissue, with soft marrow in the interstices (Figure 22–15). The creation, maintenance, and repair of this structure depend not only on the cells of the connective-tissue family that synthesize matrix, but also on a separate class of cells called osteoclasts that degrade it, as we explain below.
bone is Continually Remodeled by the Cells within it
For all its rigidity, bone is by no means a permanent and immutable tissue. Running through the hard extracellular matrix are channels and cavities occupied by living cells, which account for about 15% of the weight of compact bone. These cells are engaged in an unceasing process of remodeling: while osteoblasts deposit new bone matrix, osteoclasts demolish old bone matrix. This mechanism provides for continuous turnover and replacement of the matrix in the interior of the bone.