Biology Reference
In-Depth Information
The ability of growth plates to catch up is useful in the treatment of human disorders that
are characterized by abnormally low long-bone extension, such as Cushing's syndrome.
Other organs seem to have local controls as well. If foetal hearts are grafted in to adult rat
hosts, they continue to grow and develop but the adult hearts of the host do not start devel-
oping again.
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Similarly, foetal kidneys grafted into adult hosts can grow so much that they
are capable of sustaining life in the absence of host kidneys, at least over the short-term,
though there is no sign that the rest of the adult body has started to develop again.
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Even tissue-engineered kidneys
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will grow in adults to some extent.
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In both of these
cases, the foetal organs interpret the environment in which they find themselves to be condu-
cive to growth, while adult organs do not. This is true even when the adult kidneys are
present along with grafted foetal ones, which shows that the different response to the envir-
onment may be to do with either age or size but cannot just be a reflection of organ type. Some
organs, such as human liver, show an impressive facility for 'regeneration', in terms of
creating new functional biomass even if not according to precisely the original anatomy. If
a significant proportion of a rat liver is removed surgically, the remaining organ regenerates
to the original mass but not beyond that point.
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The size of the normal liver cannot there-
fore reflect a limit to cell proliferation, because the ability of cells to undergo division was
exhausted, but must be imposed by a regulatory mechanism that holds back the potential
for proliferation and releases this potential, temporarily, during regeneration.
The details of the feedback mechanisms that regulate organ and body size are still not well
understood, partly because they can only be investigated properly in the complicated context
of a complete animal rather than in tissue-culture abstractions. Nevertheless, genetic
approaches are beginning to uncover promising leads, and it may not be too long before
firm links are established between controls that operate at the scale of a whole animal, those
that operate at the scale of tissues and those that operate at the level of the proteins that
control cell cycling itself.
PLANTS SHOW A DIRECT CONNECTION BETWEEN GROWTH
AND MORPHOGENESIS
In growing animals, while cell expansion and proliferation are certainly important for the
production of shape, they are by no means solely responsible. Cell migration, neighbour
exchange and elective cell death are critically important, as is the filling of space with extra-
cellular matrix; indeed, in some systems these dominate. In plants, on the other hand, there is
very little migration or neighbour exchange and not much in the way of extracellular matrix
(cell walls are something different). Elective cell death does exist
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but it is not used as ubiq-
uitously as it is in animal development.
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The morphogenesis of plants can therefore be ana-
lysed and understood almost entirely in terms of directional cell division and expansion.
There are two main methods of understanding the connection between cell growth and
proliferation and plant morphogenesis and both are based on modelling. The general tech-
nique of discovery-by-modelling is described more fully in Section VI (Modelling Morpho-
genesis). Here the emphasis will be on the biological implications rather than on how the
models are made. There are two approaches to modelling plant growth, one analytical and
the other more exploratory. The analytical approach
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begins with a series of images taken
