Nine
The Organ Tree
"I have yet to see any problem, however complicated, which when you
looked at it the right way did not become still more complicated," sci-
ence fiction writer Poul Anderson once observed. To a certain extent this
is true of regeneration. Intricate nature is still more than a match for our
finest-spun hypotheses. Yet we've now reached the oasis of science that
we call an interim understanding, where the data begin to shake into
place and we can sense the pattern of the rebus from the blanks we've
filled in.
Ultimately we must relate all we learn about regeneration to a general
system of communication among cells, for regrowth is only a special case
of the cooperative cohesion that's the essence of multicellular life. This
communication system includes but extends beyond the gene-protein-
enzyme subsystems that govern the specialization of cells and unite their
chemical trade routes into smoothly working tissues and organs. During
embryonic development, cells where muscle will appear must receive
instructions from their environment telling them to repress all genes
except the muscle genome, or subcode. In many tissues, perhaps in all,
chemical inductors from previously formed tissue perform this task,
leading embryonic cells step by step through the stages of differentia-
tion. However, chemical reactions and the passage of compounds from
cell to cell can't account for structure, such as the alignment of muscle
fiber bundles, the proper shape of the whole muscle, and its precise
attachment to bones. Molecullar dynamics, the simple gradients of diffu-
sion, can't explain anatomy. The control system we're seeking unites all
levels of organization, from the idiosyncratic yet regular outline of the