Biology Reference
In-Depth Information
CHAPTER
22
Growth, Proliferation and Death:
A Brief Overview
Growth and cell proliferation feature in the development of every multicellular
organism. They are frequently linked so that cell proliferation drives organismal growth
but they do not have to be. Early embryos can divide by cleavage without growth, tissues
can grow by cell expansion in the absence of cell proliferation, and the volume of an embryo
can increase by fluid uptake without an increase in the total volume of living cytoplasm
(
Figure 22.1
).
Because cells can alter in both volume and number, one has to know the value of both of
these variables in order to calculate the total volume of a tissue. In most cases, the volume is
determined by the product of the number and the size of the cells (exceptions to this simple
rule occur where the extracellular matrix occupies a significant fraction of the volume as it
does, for example, in cartilage). A number of classical experiments have suggested that
cell number and cell size are controlled by separate mechanisms and that either can change
without altering the other. What is more, these two variables can compensate for one another
in achieving the optimum size for a tissue.
Mutual compensation by cell size and volume is illustrated well by mutants that affect
cells in the developing wings of Drosophila melanogaster. Like other adult structures, these
develop as imaginal discs in the embryo that then evert. Cells in the wing disc can be forced
to proliferate more than usual by constitutive expression of the cell cycle regulator E2F, the
function of which will be discussed in more detail later. If E2F is over expressed, wings
have an abnormal amount of cells, but the size of the whole wing remains normal because
each cell is proportionately smaller.
1
If, on the hand, proliferation is inhibited by
temperature-sensitive inactivation of a molecule required for proliferation, the wing achieves
a normal size through cell enlargement.
2
Plant mutants, too, show compensation between
cell size and number in organs such as leaves.
3
The ability of an organ (and an organism)
to alter cell number to compensate for changes in cell volume that are beyond its control
can also be demonstrated by comparison of diploid and tetraploid salamanders. The cells
of the tetraploid animals are larger than those of diploids, yet the animals themselves are
the same size because the tissues of the tetraploid contain correspondingly fewer cells.
4
The size-regulation system therefore manipulates either or both variables (cell number, cell
size) to achieve a set product of the two (
Figure 22.2
).
