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Recent discoveries have shown that it is a mistake, however, to assume that the epithelium
is incapable of organizing itself into a reasonably spaced tree using only its own resources.
For example, when the ureteric bud of a kidney is isolated and placed in a three-dimensional
matrix, it will still form a branched tree. 90,91
One problem that is faced by most epithelial branching systems is that of spreading
branches out so that they do not tangle and collide. This is achieved partly by paracrine
action from surrounding cells, as described above, but can also be achieved by autocrine
inhibitory signalling. If two kidneys are cultured in very close proximity, their mesenchymes
fuse at once and their branching collecting duct systems grow and approach one another.
They do not, however, collide, stopping short even when this local stopping greatly distorts
the shape of each tree. The trees secrete autocrine signalling proteins such as BMPs. If BMP
signalling is blocked in this system, two effects are seen. First, collisions now take place freely.
Second, within each tree and even in the trees of kidneys cultured on their own, some
branches run almost parallel to one another. This suggests that autocrine BMP-mediated
chemo-repulsion is important in ensuring that branches normally diverge and avoid any
subsequent convergence. The mesenchyme-derived attractive signals superimpose add-
itional information on this basic system. Thus the shape of a branched system is formed
by interaction of the innate tendencies of the epithelium and signals from its environment d
'nature and nurture' in microcosm.
INTUSSUSCEPTIVE BRANCHING
Intussusception ) is a mode of branching that is seen mainly in the vascular systems of
vertebrates. The process contrasts with sprouting, also used in vascular systems, in that it
does not involve the formation of blind-ended tubes, and formation of loops does not require
tubes to find each other and to join. Instead, ramifications and the increased surface area they
bring can be added without any parts of the vessel ceasing to carry moving fluid. This feature
makes interssusception ideally suited for the modification and elaboration of blood vessels.
The process begins when a vessel wall folds inwards. The infolding may degenerate to leave
a 'pillar' that divides the stream of blood 92 ( Figure 20.13 ) or it may fuse with an infolding that
is approaching from the opposite wall of the vessel ( Figure 20.14 ).
FIGURE 20.13 Formation of a pillar by infolding followed by loss of the basal parts of the fold.
) The term interssusception means a taking within (Oxford English Dictionary, 2nd Edn, 1989); it has been used
to describe this mode of branching morphogenesis by authors such as Patan. 91
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