Biology Reference
In-Depth Information
THE IDEA OF 'MECHANISM'
The word 'mechanism', as used throughout this topic, connotes a sequence of events that
take place at a molecular level and that can be explained by interactions of molecules that
follow the ordinary laws of physics and chemistry. Mechanism, in this sense, is the antithesis
of vitalism. There is no 'bio-' prefix to the words 'physics' and 'chemistry' in the first sentence
of this paragraph because there is no need for one; the biological character of morphogenetic
mechanisms lies not in the individual interactions of their molecular components but comes
from the elaborate systems of feedback that regulate where and when those interactions take
place. This emergence of biological character from simple physics is discussed further, later
in this chapter.
One of the key requirements of a molecular mechanism, as the term is used in this topic, is
that it must involve only local interactions, for molecules can be affected only by events that
take place at their surfaces and they cannot, generally, be affected by action-at-a-distance.
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This point is stressed here because traditional embryology has often used the concept of
gradients within a 'morphogenetic field' to explain particular aspects of development. The
ideas of morphogenetic gradients and fields
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e
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are very useful for providing a high-level
view of events but they cannot be a part of a molecular-level explanation because a gradient
is, by definition, non-local and cannot be sensed directly by a single molecule. At a give time,
a single receptor molecule will either be binding one of the molecules involved in the
gradient or will not, that is all. A gradient may be able to be sensed by a
system
of interacting
molecules but, to explain the mechanism of such a system, one has to focus on the purely
local influences within it. These are all that a molecule can sense. For this reason, high-level
concepts such as classical morphogenetic fields are useful for considering large-scale organ-
ization in the embryo but they do not belong in a molecular-level discussion. The more
mystical definitions of morphogenetic fields that have been offered by writers such as
Sheldrake
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are even less relevant to the mechanistic explanations being sought here.
The quest for 'mechanisms' of morphogenesis therefore aims to account for the shape
changes at the scales of cells and tissues in terms only of events that take place at the scale
of individual molecules. A simple listing of molecular interactions is not enough, though,
because an understanding of mechanism can be obtained only when the pathways of control
that regulate the behaviour of the components have also been identified and characterized.
This topic therefore uses the word 'mechanism' to refer to a combination of the molecular
interactions that directly result in shape, of other molecular interactions that control them,
and of the principles of control that are involved.
EMERGENCE
Emergence is one of the most important concepts in the study of morphogenesis, and
indeed in developmental biology as a whole. Emergence, which crops up in fields as diverse
)
Electric fields are an exception to this generalization that may be important in morphogenesis; their role in
development is discussed in Chapter 10. Gravitational and magnetic fields to which embryos are normally
subjected are very weak and have a negligible effect on the behaviour of individual molecules.
