Biology Reference
In-Depth Information
FIGURE 28.3
Morphogenetic modules can be used in different combinations by different cells. This figure
depicts three cell types of the developing mammalian kidney as a Venn diagram. Some mechanisms are (as far as we
know at the moment) invoked by only one of these cell types, some by two and some by all three. There are, of
course, many more processes involved and this diagram just chooses seven, of different 'levels', to illustrate the
general point.
and it clears away progenitor cells that have not been induced to do anything useful. At the
highest level, whole-embryo mechanisms seem to ensure that the size of kidney matches the
size of the developing body.
Feedback systems link all of these processes and enable the developing kidney (like any
other organ) to organize itself properly, despite variability in precisely where components
are placed and in the layout and metabolism in the rest of the embryo. Again, feedback as
a concept operates at and between many different levels. At the finest level (10
e
1000 nm
spatial resolution), it is used to ensure the adaptive self-organization of cytoskeletal com-
ponents and adhesion systems in stationary and moving cells (and to do other things). At
coarser scales (10
e
100
m spatial resolution), feedback is used to regulate the sizes of cell
condensations and the spacing between branches of the growing collecting duct tree (and
many other things). Feedback mechanisms do not operate only within a scale: the sensing
of cell density will require the function of molecular-scale machines, and the effectors of
the feedback will be molecular-scale machines (for example, for cell movement or cell death).
One important consequence of morphogenesis using systems rich in feedback is robust-
ness. Any complicated, multi-stage system that operates in the analogue domain without
feedback would be subject to a dangerous accumulation of errors. If, for example, capil-
lary-forming cells had to find developing kidney glomeruli based only on the original precise
positions of their ancestor cells with no communication or feedback between them, the chan-
ces of an accurate meeting would be minimal. This is especially true in an animal such as
a human, where around a thousand cells in a young kidney rudiment eventually produce
around a million glomeruli; for the navigation by vascular cells to work purely by dead-
reckoning from initial conditions, with no feedback of navigation performance, starting
conditions would have to be accurate to a resolution of a thousandth of a cell. Clearly, this
is not what happens. As recounted in many places in this topic, cells navigate, proliferate,
m
