Biology Reference
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FIGURE 22.10 The feedback loop that balances the rates of proliferation of tissue cells and the endothelia that
construct their blood supply (see main text for details).
too unbalanced. Similar feedback can be found between epithelial and mesenchymal
compartments of other organs. 56
The type of feedback outlined in this section can be used either to ensure that two (or
more) interdependent cell types grow proportionately during development, or to ensure
tissue homeostasis in adulthood, depending on the 'set point' of the feedback system. In
growing tissues, the default behaviour of the organ tissue would be to grow and it is held
back by lack of oxygen. In static tissues, the default behaviour would not be to grow, and
proliferation in excess of that required to replace cells lost through death would be invoked
only for special purposes such as wound healing. The 'set point' of the feedback is set up by
other, long-range signals that are integrated with the local signals that have been described
here. These are the signals that determine the size and proportions of the body as a whole.
LA RGE-SCALE CONTROL OF CELL PROLIFERATI ON
As well as being controlled on the spatial scale of individual cells and tissues, the rate of
cell proliferation is subject to controls that operate on a larger scale to organize the growth of
entire organs and of the body as a whole. Organ growth seems to be regulated by a combin-
ation of controls internal and external to the organ. Body-wide control is achieved largely by
the use of diffusible hormones. In mammalian foetal and infantile development, for example,
the hormones IGFI, IGFII and growth hormone (GH) are important regulators of cell prolif-
eration, the IGFs playing a primary role in early development and GH playing a primary role
once the pituitary gland has matured enough to make it. GH encourages the growth of
a variety of tissues and is particularly important in the growth plates of long bones, which
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