Morphogenesis by Elective Cell Death - Mechanisms of Morphogenesis - page 305

FIGURE 24.1 A summary of a typical pathway that controls elective cell death by caspase-mediated apoptosis.
Pro-apoptotic influences are coloured red, anti-apoptotic ones are coloured green. Apoptosis can be triggered either
by failure of survival signals or by activation of death receptors. It is important to note that this diagram shows just
some example pathways of control d there are many others.
Alternative internal mechanisms for elective cell death have been reviewed extensively
elsewhere 8,11 e 13 and detailed discussion of the intracellular cascades involved does not
belong in a topic that is about the generation of shape rather than about biochemical
events. It is important to note, however, that the variety of possible internal mechanisms
means that popular assays for elective cell death, which detect caspase activation or the
ends of cleaved DNA, may suggest an absence of death when it is actually taking place
but by a different mechanism. All studies on the role of elective cell death in morphogen-
esis, and particularly those reporting its absence, should therefore be interpreted with
appropriate caution.
Elective cell death can be involved in morphogenesis in two main ways: as a direct means
of the formation of a new shape, or as part of a feedback system that clears up surplus and lost
cells involved in a morphogenetic event that is driven by different mechanisms. The devel-
opment of the mammalian hand and foot provide a well-studied example in which elective
cell death plays a direct role in morphogenesis. During early development, the limb forms as
a paddle-like outgrowth from the side of the trunk. Later, cells inside it undergo condensation
to form the cartilaginous predecessors of limb bones, including the phalanges that will make
up the fingers. The fingers do not, therefore, form as processes that push out from