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In-Depth Information
either by cell wedging or by matrix swelling. Cell multiplication is also likely to play a role,
even if only to provide for the increase in surface area, and this can be driven by local sources
of VEGF, FGFs, and so on.
99
e
101
Both the joining of folds and the formation of pillars requires
fusion of endothelial sheets. Joining of folds requires apex-apex fusion, and seems to involve
cell projections which may be similar to the filopodia seen when fusing epithelia approach
one another (Chapter 19). Formation of pillars requires some re-polarization of cells so
that the stem of the pillar can seal off in both directions as it degenerates; this is probably
analogous to the cell re-polarization and fusion that occurs when the neural tube separates
from the ectoderm (Chapter 18), except that the apicobasal polarity of the cells is reversed.
AUTOMATIC VERSUS PLANNED ARCHITECTURE
IN BRANCHING SYSTEMS
This chapter has placed strong emphasis on the ability of branching systems to grow and
to organize themselves by adaptive self-organization, according to the needs of the tissue
they serve. This type of development is used widely in developing organs, particularly for
the finer details, and is the only one available for the healing of wounded tissue. It is not,
however, generally used for the largest-diameter tubes in a branching system; the architec-
ture of the great blood vessels, of the bronchi and of the largest (earliest-formed) tubes of
the kidney, pancreas (and so on) seems to be programmed differently. A dialogue between
the developing branched system and its surroundings is still usually involved but, rather
than being shaped by signals reflecting the local balance of supply and demand, the pattern
is controlled by specific additional cues. In the zebrafish
Brachydanio rerio
, for example, the
location of largest arteries (the aortae) just below the notochord is determined by specific
cues. The angioblasts, which are precursors of endothelia, arise below and to the sides of
the somites and migrate along the medial surfaces of the somites to arrive just below the
notochord. They migrate as individual motile cells and, having a VEGF receptor called neu-
ropilin-1, they are attracted by VEGF that is produced by the medial edge of the somites. As
the dorsal migration of the angioblasts proceeds, the ventral parts of the somites produce
semaphorin-3a which repels the angioblasts and traps them into the small zone just under
the notochord.
102
The notochord itself is also a source of patterning information and in em-
bryos in which the notochord is absent, the aorta fails to form
103
(
Figure 20.17
). The main
FIGURE 20.17
Some of the signals that attract the angioblasts (purple) to the sub-notochord region in which the
aortae will form.
