Biology Reference
In-Depth Information
FIGURE 20.2
Dipodial and monopodial patterns of branching.
The question of whether monopodial and dipodial branching systems are minor variations
on a theme, or arise by quite distinct mechanisms, leads on to a more general debate within
the field: whether there is one conserved mechanism for sprouting a new branch or whether
we have solved the problem of making branched trees multiple times by evolving multiple,
independent mechanisms.
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The question is especially important when simple culture models
are used to study branching because, if there is not just one conserved mechanism, it is possible
that models might branch but still not properly reflect the biology of the tissue fromwhich they
are derived. This important questionwill be considered again, later in the chapter, after a concise
review of branching morphogenesis in culture models and in natural branching systems.
Much of what we think we know about branching comes from the study of simple culture
systems that involve only a single type of cell. They are valuable for their simplicity, and for
their clear implication that branching (of some kind) is an integral property of the tubule cells
themselves: it does not require any activity of surrounding mesenchyme that cannot be
substituted by simple solutions of the molecules that the mesenchyme would usually
make. This chapter will therefore proceed by considering what has been learned from these
culture models first, before focusing on examples of branching systems in real embryos and
finally considering whether they really are all variations on one theme.
The simplest systems that display multicellular branching morphogenesis are three-
dimensional cultures of epithelial cell lines that have been derived from tissues that naturally
branch. When cultured in collagen gels with normal culture media, cell lines such as MDCK
and mIMCD3s, derived from the renal collecting duct tree, form approximately spherical
FIGURE 20.3
MDCK or mIMCD-3 renal collecting duct cell lines form cysts when cultured in three-
dimensional collagen-rich gels (grey). These cysts sprout processes when treated with molecules such as hepatocyte
growth factor (HGF).
