Hydra's Heads and Medusa's Blood 31
the fracture and changes into a closely related type, an osteoblast, or
bone-forming cell. These new osteoblasts build a swollen ring of bone,
called a callus, around the break.
Another repair operation is going on inside the bone, in its hollow
center, the medullary cavity. In childhood the marrow in this cavity
actively produces red and white blood cells, while in adulthood most of
the marrow turns to fat. Some active marrow cells remain, however, in
the porous convolutions of the inner surface. Around the break a new
tissue forms from the marrow cells, most readily in children and young
animals. This new tissue is unspecialized, and the marrow cells seem to
form it not by increasing their rate of division, as in the callus-forming
periosteal cells, but by reverting to a primitive, neo-embryonic state.
The unspecialized former marrow cells then change into a type of primi-
tive cartilage cells, then into mature cartilage cells, and finally into new
bone cells to help heal the break from inside. Under a microscope, the
changes seen in cells from this internal healing area, especially from
children a week or two after the bone was broken, seem incredibly cha-
otic, and they look frighteningly similar to highly malignant bone-can-
cer cells. But in most cases their transformations are under control, and
the bone heals.
Dr. Marshall Urist, one of the great researchers in orthopedics, was to
conclude in the early 1960s that this second type of bone healing is an
evolutionary throwback, the only kind of regeneration that humans share
with all other vertebrates. Regeneration in this sense means the re-
growth of a complex body part, consisting of several different kinds of
cells, in a fashion resembling the original growth of the same part in the
embryo, in which the necessary cells differentiate from simpler cells or
even from seemingly unrelated types. This process, which I'll call true
regeneration, must be distinguished from two other forms of healing.
One, sometimes considered a variety of regeneration, is physiological
repair, in which small wounds and everyday wear within a single tissue
are made good by nearby cells of the same type, which merely proliferate
to close the gap. The other kind of healing occurs when a wound is too
big for single-tissue repair but the animal lacks the true regenerative
competence to restore the damaged part. In this case the injury is simply
patched over as well as possible with collagen fibers, forming a scar.
Since true regeneration is most closely related to embryonic development
and is generally Strongest in simple animals, it may be considered the
most fundamental mode of healing.
Nonunions
failed
to knit, I reasoned,
because they were
missing
something that triggered and controlled normal healing. I'd already be-