Biology Reference
In-Depth Information
CHAPTER
20
E
pithelial Branchin
g
Efficient transport of resources and of waste products is a problem faced by all organisms,
particularly by large ones. Critical resources such as oxygen have to be brought close to all
cells that require them, while organs that are specialized for secretion have to pack a large
amount of glandular epithelium into a small volume and channel its secretions away.
Branched architectures provide one solution to this type of problem: they can be found at
scales varying from the finest capillaries of the microvasculature to the body plans of the
largest organisms on earth (trees and soil fungi).
1
The internal anatomy of many animals
contains branched epithelia and endothelia, and the histology of most mammalian organs
is dominated by branched tubes.
There are four basic methods of constructing a system of branched tubes. These are conflu-
ence of separate tubes, clefting of an ampulla, sprouting of new tips and intussusceptive divi-
sion of an existing tube (
Figure 20.1
).
Confluence used, for example, in the construction of pro- and meta-nephric kidneys
and also in the formation of blood vessels by vasculogenesis, is not normally considered
to be 'branching morphogenesis' and it will not therefore be described in detail here;
mechanisms of epithelial fusion, required for confluence, are discussed in Chapter 19.
Branching morphogenesis proper is usually achieved by sprouting, by clefting or by
intussusception.
BRANCHING BY SPROUTING
Sprouting is the general name given to a very common mode of branching, in which new
branch tips grow out from an existing tip or tubule. There are two basic forms: monopodial,
in which side-branches form from a principal trunk, and dipodial in which tips divide
dichotomously so that there is no dominant main stem (
Figure 20.2
). The two forms are
not exclusive and it is common for organs to use dipodial branching to set up their basic
architecture and then to use monopodial branching to add large numbers of side-branches.
The mammary gland is an example of such an organ, dipodial branching being used to create
the tree of milk ducts and monopodial branching to add alveoli (in which milk is produced)
to the branches of that tree.
2
There are also variations on the dipodial form, with trifurcations
(and so on) replacing bifurcations, and there is strong evidence for both bifurcations and
trifurcations happening in the metanephric kidney.
3
