The Ticklish Gene
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ago was a remarkable achievement. It enabled animals to advance, in
both senses of the word, quickly and efficiently. Since bone is inside the
body, it can live and grow with the animal, instead of leaving it defense-
less as an external skeleton does when cast off during a molt. It's also the
most efficient system for attaching muscles and increasing the size of
animals.
Bone is extraordinary in structure, too. It's stronger than cast iron in
resisting compression but, if killed by X rays or by cutting off its blood
supply (barely adequate to start with), it collapses into mush. The part
that's actually alive, the bone cells, comprises less than 20 percent of the
whole. The rest, the matrix, isn't just homogeneous concrete, either. It's
composed of two dissimilar materials—collagen, a long-chain, fibrous
protein that's the main structural material of the entire body, and apa-
tite, a crystalline mineral that's mainly calcium phosphate. The electron
microscope shows that the association between collagen and apatite is
highly ordered, right down to the molecular level. The collagen fibers
have raised transverse bands that divide them into regular segments. The
apatite crystals, just the right size to fit snugly between these bands, are
deposited like scales around the fibers.
This intricacy continues at higher levels of organization. The collagen
fibers lie side by side in layer upon layer wound in opposed spirals (a
double helix) around a central axis. The bone cells, or osteocytes, are
embedded in these layers, which form units a few millimeters long,
called osteones. The center of each osteone has a small canal in which
runs a blood vessel and a nerve. The osteones in turn are organized so as
to lie along the lines of maximum mechanical stress, producing a bone
of the precise shape best able to withstand the forces applied to it.
Bone has an amazing capacity to grow, which it does in three dif-
ferent ways. In childhood each long bone of the limbs has one or two
growth centers, called epiphyseal plates. Each is a body of cartilage
whose leading edge grows continuously while its trailing edge trans-
forms into bone. When the bone is the right length, the process stops,
and the remaining cartilage forms the bony knob, or epiphysis, at the
end of the bone. The "closure" of the epiphyses is an index of the body's
maturation.
Bone cannot heal. That sounds like a conundrum but it's literally
true. Fractures knit because new bone made from other tissues unites the
fracture ends. Although we sometimes speak of bone growth as part of
fracture healing, the old, preexisting bone doesn't have the capacity to
grow.
As mentioned in Chapter 1, there are two tissues that
produce
new bone at a fracture site. One is the periosteum, the bone's fibrous
covering.
It's the cells of the periosteum's innermost layer that have the