Biology Reference
In-Depth Information
knockout experiments are useful for identifying necessary components, but are almost
useless for identifying regulation. Finding regulators instead requires manipulations that
make small changes to the concentration or activation of a particular molecule to discover
if there is a corresponding change to the extent to which a morphogenetic event occurs.
Drugs and RNA interference d much derided for their failure to eliminate activity
completely d are actually much more useful than knockouts, as are allelic series of mutations
and overexpression techniques. Progress in gaining a high-level understanding of morpho-
genesis will be aided appreciably by a more widespread appreciation of the logic of control,
and of more careful use of words.
This topic has focused very much on basic science and has not emphasized clinical and
industrial applications of morphogenetic knowledge, but such applications undoubtedly
exist. Medically, the whole field of tissue engineering is hampered by difficulties in producing
artificial organs, whether for use ex corporo or in vivo , that have cells in the correct spatial
arrangement. Historically, the problem has been addressed by developing shaped, non-living
supports that force a particular arrangement on cells, 27 but it would be a great improvement if
cells could be programmed to do this for themselves. Industrial applications also exist; an
interesting recent example has been the development of fungi (by conventional breeding
and use of carefully controlled culture conditions, rather than by morphogenetic engineering)
that have the optimum degree of branching to act as a convincing meat-substitute for converts
to vegetarianismwho miss their favourite dishes. There are many other possible applications,
particularly in plants, where alterations in morphogenetic mechanisms may be used d for
example, to create root systems that are particularly useful in stabilizing coasts from erosion
or in modulating the speed of water flow in areas prone to flooding.
One of the most important advances in understanding is likely to come from outside the
field of development itself. The general problem of emergence crosses all of the sciences, from
physical to biological, and even enters fields outside the hard sciences such as sociology and
linguistics. It may always remain a woolly concept but, on the other hand, it may one day
gain a more rigorous definition (momentum/inertia and magnetism were, after all, woolly
concepts once). If it does, the whole of developmental biology will benefit from a rigorous
concept that will provide an even more useful framework for understanding and evaluating
different morphogenetic events.
Our understanding of morphogenesis is changing quickly. Revising the first edition of this
topic to create the second has been a startling lesson in how much the field has progressed in
less than a decade. Major mechanisms such as convergent extension and planar polarization
are understood reasonably well now d they were still very mysterious when the first edition
was being written. It will be a bitter-sweet experience to see even the second edition of this
topic soon rendered out-of-date by the pace of progress, as I am sure it will be. I hope that it
plays some role in inspiring those who will consign it to history.
Reference List
1. Le Douarin NM, Renaud D, Teillet MA, Le Douarin GH. Cholinergic differentiation of presumptive adrenergic
neuroblasts in interspecific chimeras after heterotopic transplantations. Proc Natl Acad Sci USA 1975;
:728 e 32.
2. Le Li`vre CS, Le Douarin NM. Mesenchymal derivatives of the neural crest: analysis of chimaeric quail and
chick embryos. J Embryol Exp Morphol 1975;
72
:125 e 54.
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