Researchers
suspect the root cause of bone loss in space is weightlessness.
In fact, the pull of gravity 350 km above our planet's
surface -- where the space station and the shuttle orbit
-- is 90 percent as strong as it is on the ground. That
hardly sounds weightless! But orbiting astronauts nevertheless
feel weightless because they and their spacecraft are
freely falling together toward Earth. (The space station
doesn't come crashing to the ground because it's going
forward so fast, about 28,000 km/h, that its fall matches
the curvature of the Earth. It literally "falls
around" the planet.) Just as gravity seems briefly
suspended in a downward-accelerating elevator, so does
the crew infreely-falling space station experience "zero-G."
In this mutual free-fall, bones no longer have to provide
support for locomotion or even for maintaining body
posture. As a result, little or no stress (i.e., mechanical
strain) is applied to the skeletal system. Scientists
think the lack of stress on the bones may be responsible
for the progressive bone loss seen in long-term residents
of space. (Lack of stress on bones among sedentary Earthlings,
such as those confined to beds due to illness or old
age, also contributes to osteoporosis.)
People often think of bones as rigid, unchanging calcium
pillars. But bones are actually dynamic living tissues
that constantly reshape themselves in response to the
stresses placed on them. (This is how archaeologists
can tell whether skeletal remains belonged to a laborer
or an aristocrat, for example. The incessant pull of
a laborer's muscles causes the bones to reshape themselves
slightly where the muscles were attached.)People often
think of bones as rigid, unchanging calcium pillars.
But bones are actually dynamic living tissues that constantly
reshape themselves in response to the stresses placed
on them. (This is how archaeologists can tell whether
skeletal remains belonged to a laborer or an aristocrat,
for example. The incessant pull of a laborer's muscles
causes the bones to reshape themselves slightly where
the muscles were attached.)
This reshaping is performed by two types of bone cell
that are constantly depositing and extracting calcium
phosphate minerals from the structural matrix of the
bone. The actions of these two cell types -- "osteoblasts,"
which deposit calcium phosphate, and "osteoclasts,"
which remove it -- usually balance each other out. When
the body has a calcium deficiency or during pathological
osteoporosis, the removal of the structural calcium
phosphate crystals outpaces replacement, leading to
a weakening of the bone.
In prolonged weightlessness, bone mass appears to decrease
because the lack of stress on the bones slows the formation
of osteoblast cells. Fewer bone-building cells, along
with a constant level of bone-destroying activity, translates
into a net loss of bone mass. Why weightlessness should
inhibit the development of osteoblasts is the subject
of a current study at Vanderbilt University. A key chemical
in the development of osteoblast cells from precursor
cells is an enzyme called "creatine kinase-B." Investigators
are trying to figure out which molecules in the body
regulate the activity of this enzyme and how those chemicals
are affected by low gravity, in the hope that this knowledge
will point to a way to boost osteoblast formation in
space. |
The
main weight-bearing bones of the body -- indicated with
light-purple shading in this drawing -- are also the
ones most affected by space-induced bone loss.
|
