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118 M. C. H. VAN DER MEULEN AND P. J. PRENDERGAST
tissue. The third cell type, the osteoclast, unlike the other two, is pre-
sumed to arise from the fusion of blood cells. Osteoclasts are large distinc-
tive multinucleated cells that resorb bone. By sealing to a bone surface, the
osteoclast forms an acidic cavity that dissolves the underlying bone (Figure
7.2(b)).
7.2.3 Bone growth and maintenance
Bone forms through two different developmental processes: endochondral
ossification and intramembranous ossification. Endochondral ossification
involves an intermediate tissue stage, cartilage, not present in intramem-
branous formation. The long bones all form endochondrally. In these
bones, development begins with the condensation of mesenchymal cells,
which differentiate into chondrocytes (Figure 7.3), creating a cartilage pre-
pattern of the skeleton. The first bony tissue, known as the bone collar,
appears spontaneously around the midshaft. Thereafter, ossification pro-
ceeds axially towards each bone end. The same identical sequence of ossifi-
cation occurs at each location: the cartilage calcifies, blood vessels invade
the site, and the cartilage is resorbed and replaced by bone. This sequence
is regulated by genetic factors, systemic hormones, growth factors, and
mechanobiologic effects. The timing of these signals is critical to the
outcome. In the developing embryo, the first ossification of cartilage is
coincident with the first muscle contractions – if a muscle is immobilised
in the embryo, a distorted and disorganised bone forms, demonstrating the
link between mechanics and bone tissue formation.
After embryonic bone formation, the skeleton continues to grow in
length by dividing and enlarging cartilage cells, which then ossify to form
cancellous bone. Bone diameter grows by direct deposition of bone on
existing bone surfaces, accompanied by resorption of outer surfaces. As the
skeleton continues to develop, mechanical forces generate an ever-increas-
ing influence on the forming bone architectures and geometries. Cellular
proliferation increases skeletal size and needs to be exquisitely controlled
to maintain form and proportion throughout growth.
Once the skeleton is formed, continual ‘remodelling’ of bone tissue
maintains structural integrity and creates more orderly tissue structures.
Remodelling involves coupled resorption and formation on all bone sur-
faces in a well-defined sequence of events. The remodelling sequence has
been described as activation of the surface, resorption by osteoclasts, rever-
sal, formation by osteoblasts, and return to quiescence of the surface. In