Biology Reference
In-Depth Information
CHAPTER
24
Morphoge
nesis by Elective
Cell Death
As cells can be added to a system by proliferation, so they can be removed by death.
1
This
process is important for controlling the size of cell populations, for clearing up mistakes, and
also for directly driving some morphogenetic processes. The type of cell death that is a normal
feature of development is achieved by active processes, invoked by cells that have become
committed to die. Some authors use the phrase 'programmed cell death' to refer to any
process by which cells die as a normal part of embryogenesis, because cells die by running
a specific internal 'suicide programme'. Other authors restrict the phrase 'programmed
cell death' strictly to imply that the fate of an identifiable cell is reproducibly specified as
death by the general developmental programme of an embryo, in a manner similar to the
specification of differentiated states (this type of programmed death is epitomized by
the development of the nematode C. elegans).
2,3
This ambiguous usage of the phrase is
causing problems in the literature and has inspired pleas for clearer thinking.
4
Some authors
also use the term 'apoptosis'
1
to refer to active cell death in general, while others restrict the
use of that term to a specific method of self-destruction,
5,6
creating confusion around that
term as well. This is especially relevant in light of the frequency in embryonic development
of alternative mechanisms.
7
This topic will therefore use the general phrase 'elective cell
death' as a blanket term to include any process by which a cell acts to ensure its own demise,
whatever the details of the actual mechanism employed, and will use the more specific
phrases in their strict sense and only when appropriate.
Elective cell death can accomplish a variety of tasks during morphogenesis, including
clearing away temporary structures, removing lost cells, removing cells that are in the
way of another process, adjusting the size of a cell population relative to another part
of the embryo and removing damaged cells. Cells can undergo elective death using one
of a variety of possible mechanisms.
8
One is apoptosis
)
(sensu strictissimo), in which
a cascade of signalling events results in the activation of intracellular proteases of the cas-
pase family.
9
These proteases activate enzymes that cleave cellular DNA, destroy other
cellular systems and render the cell susceptible to phagocytosis (
Figure 24.1
). An alterna-
tive mechanism, one that seems to be particularly common in development,
7
is autophagy.
This also involves caspase activation and DNA cleavage
10
but, in autophagy, cells
digest themselves using lysosomes rather than relying on phagocytosis by a neighbour.
)
A footnote to the paper that introduced the term
1
makes the pronunciation clear
d
stress on the penultimate
syllable, and with the 'p' of 'ptosis' silent.
