Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1194 Chapter 21: Development of Multicellular Organisms
Figure 21–58 Determinants of organ
size.
CELL GROWTH
CELL DIVISIONS
CELL DEATH
MATRIX DEPOSITION
The Proliferation, Death, and Size of Cells Determine
Organism Size
The nematode worm C. elegans illustrates the different ways in which size differences
can arise. This creature follows an astonishingly precise and predictable
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developmental program. Each individual of a given sex is generated by almost
exactly the same sequences of cell divisions and cell deaths, and consequently
has precisely the same number of somatic cells—959 in the adult hermaphrodite
(the sex of the majority of these animals)—although the number of germ cells is
more variable from worm to worm. The stereotyped development makes it possible
to trace somatic cell lineages in exhaustive detail. More than 1000 cell divisions
generate 1090 somatic cells during hermaphrodite development, but 131 of
these cells undergo apoptotic cell death. Thus, precise regulation of both cell division
and cell death determines the final numbers of somatic cells in the worm. In
fact, genetic screens in C. elegans identified the first genes responsible for apoptosis
and its regulation—thereby revolutionizing our molecular understanding of
this form of programmed cell death (discussed in Chapter 18).
The final number of somatic cells in the adult worm is already present at sexual
maturity (around three days after fertilization), after which no more somatic
cells are generated. Yet the worm continues to grow, doubling in size between
sexual maturity and death 2–3 weeks later. This doubling results from somatic cell
growth: although the cells no longer divide, they continue to go through rounds of
DNA synthesis; this endoreplication of the genome makes the cells polyploid. As in
all organisms, the size of a cell is proportional to its ploidy—that is, the number of
genome copies that it contains: a doubling of ploidy roughly doubles cell volume.
By artificial manipulation of somatic cell ploidy, and thereby somatic cell size, the
size of the worm as a whole can be increased or decreased. Thus the worm’s final
size is set by a combination of programmed cell divisions and cell deaths, along
with regulation of the sizes of individual cells through changes in ploidy.
In plants, as in animals, cell size increases as ploidy increases (Figure 21–59).
This effect has been exploited in the agricultural breeding of plants for large size:
most of the major fruits and vegetables that we consume are polyploid.
Animals and Organs Can Assess and Regulate Total Cell Mass
The size of an animal or organ depends on both cell number and cell size—that
is, on total cell mass. Remarkably, many animals and organs can somehow assess
their total cell mass and regulate it, providing evidence for feedback controls of
the sort highlighted earlier in our introductory account of general principles of
growth control. In contrast with C. elegans, if cell size is artificially increased or
decreased in these cases, cell numbers adjust to maintain a normal total cell mass.
This has been beautifully illustrated by experiments done long ago in salamanders,
where cell size can be manipulated by altering the animal’s ploidy. As shown in
Figure 21–59E, salamanders of different ploidies end up being the same size with
very different numbers of cells. The individual cells in a pentaploid salamander,