INTELLIGENT DEVELOPMENT 147THE BUILDING OF BODY STRUCTURESIn a matter of hours, the cells of the gastrula have started to diff erentiate into
hundreds of diff er ent cell types. Th e essential body structures, body axis
and segments, and organs soon begin to form under dynamical regulation.
Th e brain is, of course, the most complex organ of the body. Th e fi rst
step in its formation is the transformation of part of the ectoderm (see
fi gure 5.1) into a neuro- ectoderm. It involves interactions among a num-
ber of morphogens forming an under lying “or ga nizer.” Th is was shown
in experiments in the 1920s in which transplanting part of this or ga nizer
to another part of the ectoderm induced the development of an almost
complete second ner vous system, as well as the supporting body axis.
Embryologists seem to have had some fun showing how potential orig-
inates in morphogen interactions rather than in codes in genes. As with
the brain, teeth in mammals originate as interactions between the mes-
enchyme and ectoderm. Th e general impression is that birds (Aves) lost
their teeth when evolving from reptiles and adopting diff er ent diets. But
then mouse dental mesenchyme was experimentally graft ed onto chick
epithelium. It resulted in the development of a hen with a variety of den-
tal structures, including perfectly formed crowns! As the experimenters
explained, “Th e results suggest that the loss of teeth in Aves did not result
from a loss of ge ne tic coding... but from an alteration in the tissue
interactions requisite for odontogenesis.”^7
In a more recent review (2008), Irma Th esleff and Mark Tummers (in
Stem Book, http:// www. stembook. org / node / 551) explain how even some-
thing as seemingly simple as tooth development involves a host of mor-
phogens. Th ey are used reiteratively during advancing tooth development
at a number of hierarchical levels and are coordinated by other factors,
including a BMP. BMP is actually a family of morphogens that regulates
several developmental pro cesses, including patterning and diff erentiation
of limbs as well as teeth. Ultimately, at least two hundred genes are re-
cruited in the regulation of dental organogenesis.
In sum, changes in the cells of developing tissues appear to depend on
a seeming storm of signals from other tissues, ebbing and fl owing over
time, with criss- crossing gradients in many directions. Th e storm is more
like a symphony than a discordant racket, operating with nonlinear
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