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FIGURE 28.2 Use of the same low-level morphogenetic mechanism for different purposes by different higher
level mechanisms of different cells. An enlargement of the stress fibre diagram at the bottom of the figure appears in
Chapter 4.
morphogenetic mechanisms described in this topic. For an example, consider the develop-
ment of the mammalian kidney, aspects of which were described briefly in Chapters 13, 19
and 20. At a morphological level, renal development involves: growth and branching of an
epithelial tube (the ureteric bud, which becomes the branched urine-collecting duct system); 9
proliferation of mesenchyme; 10 condensation of groups of mesenchymal cells; 11 differenti-
ation of mesenchymal, which condensates to become long epithelial tubes called 'nephrons'; 12
fusion of nephrons with the collecting duct system; differentiation of other mesenchymal
cells to become supporting stroma; formation of blood vessels so that every nephron ends
up with a blood supply; and invasion of the kidney by the axons of nerves. 13 Some low-level
processes are called on in more than one of these events. For example, the low-level self-
assembly of actin/myosin tension filaments is invoked in the branching epithelium, in the
condensing mesenchyme cells and in the migrating blood cell progenitors. The formation
of motile leading edges, itself a multilayer process as described above, is invoked in
condensing mesenchyme, in blood cell progenitors and in the growth cones of invading
neurones. Similar lists can be made for most of the low-level processes mentioned earlier
in this topic. The take-home message is that the different developmental programmes
followed by different cell types can invoke the same basic process many times ( Figure 28.3 ).
Higher level mechanisms, operating beyond the level of macromolecular assemblies, are
also invoked by different cell types in our example of the developing kidney. Cell prolifer-
ation is an obvious example, affecting the epithelial and mesenchymal populations. 14 Elective
cell death is another; it seems to be used to balance the rather large number of mesenchyme
cells that are usually available to form nephrons with the smaller number actually needed, 15
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