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SPECULATIONS Rules of Evidence ll: Controls something is and about concluding what something is not. A piece of sci—
entific evidence is only as good as the con—
trols it uses to tell what something is not,
and the difference between science and
non-science is usually a matter of controls.
Just because a particular compound causes
cells in the same population to differentiate
into particular cell types does not mean that
it is a morphogen. S cience is both about concluding what Negative controls In TheAdventure ofBlack Peter, Sherlock
Holmes noted, “One should always look for a
possible alternative and provide against it.”
What-other possibilities could there be be-
sides the morphogen hypothesis? A mor-
phogen is a diffusible molecule that acts
through a concentration gradient. However,
there are at least two other possibilities be-
sides gradients to explain the phenomenon
whereby cells appear to differentiate with re-
spect to their proximity-to a source of Nodal. First, there could be a cascade of induc-
tions. Nodal might only induce nearby cells
at high concentrations and do nothing at
lower concentrations. insteadr the induced
cells might put forth a second signal that
induces the next set of cells, and these cells
might put forth a third local signalr and so
on. Such a sequence of events is sometimes
called a relay cascade. A second possibility is that the cells near
the source of Nodal might respond not only
by differentiating, but by migrating away to a
distant position within the responding tissue
(or by producing daughter cells that migrate).
The stronger the signal, the farther away the
induced cells ortheir daughters migrate. Nodal mRNA + dye Ttansplanted cells
do not respond to Nodal \ i i 1‘ 25314.“5.‘6.‘7.’8.'9.‘10 1’ 2/ 314/51657I839I10 - goosecoid {head mesoderm) Transplanted cell I floating head
(notochord} FIGURE 3.21 Negative controls
confirm that Nodal is a mor-
phogen. (A) Transplanted cells that
lack the Nodal receptor do not
express the g'oosecoid, floating
head, or rip-tail genes when placed
nearthe Nodal—secreting cell. (B)
Such unresponsive cells do not
block cells farther away from
expressing the appropriate floating
head and no-tail genes, indicating
that Nodal diffusion is the most
likely means of signal transmission.
(C) A labeled responsive cell trans-
planted to a particular location
near the Nodal—secreting cell will
express the gene appropriate to
the distance from the Nodal-secret-
ing cell. (D) If a gene encoding
some other substance (in this case,
bacterial |3-galactosidase, lacZJ that
is not a signaling molecule, the
nearby cells will not express their
Nodai-responsive genes. I rip—tail (muscles) Chen and Schiei’s experiments,
described in Figure 3.20 controlied for these
alternative possibilities. First. to distinguish
whether the effects of Nodal are due to the
diffusion of Nodal or to a relay cascade, the
investigators found cells that did not
respond to Nodal. There are zebrafish with
mutations in a receptorfor Nodal, and bias-
tomeres from these fish cannot respond to Nodal protein (Figure 3.2m). (These Nodal-
deficient fish have numerous developmen—
tal anomalies, and the name of this mutant,
one—eyedpinhead, suggests it has some
major problems in forming its face.) The
mutant blastomeres can be transplanted
between the source of Nodal and the
responsive blastomeres (Figure 3.2-1 B}. If (Continued on next page) ...
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