Biology Reference
In-Depth Information
LARGER SCALE
MORPHOGENETIC
SYSTEM
LOCAL
MORPHOGENETIC
SYSTEM
SELF-ASSEMBLY
FIGURE 28.1
A general view of multilayered morphogenetic mechanisms. Simple, basic processes such as self-
assembly (for example, of cytoskeletal components) are combined with regulatory and feedback controls to form
local morphogenetic systems such as an active part of the sub-membrane actin network in a motile cell. These parts
are then further integrated into larger scale morphogenetic systems (for example, the leading edge of a cell), and so
on. Each of these levels would correspond to an integron in the vocabulary discussed in the paragraphs on
modularity.
Above the layer of self-assembly lie layers of control that dictate where and when the self-
assembly takes place. For example, at the leading edge of a migrating cell, the lowest layer is
the self-assembly of branched actin filaments nucleated by Arp2/3, but the many separate
instances of this layer are subject to a local control loop that uses proteins such as WASP
and SCAR and capping proteins so that assembly of the actin network is restricted to the
very front of the leading edge (Chapter 8). The 'make a new branch of actin' layers are there-
fore brought together, with control elements, to form a 'make a leading edge' layer of mech-
anism. The 'make a leading edge' system is in turn a component, along with a 'make
contractile actin/myosin filament' system and others, of a higher level 'make this cell
move' system. This level imposes control on the lower level systems to generate a cell that
has a leading edge at one end and contractile filaments elsewhere and, as recounted in
Chapter 8, it is capable of polarizing a cell even in the absence of external cues. Precisely
where the boundaries between layers are drawn will always be somewhat arbitrary, but
the point is that multiple layers exist, and that some lower-level systems can be used by quite
different high-level systems. For example, the low-level, Rho-mediated assembly of actin/
myosin tension fibres is invoked by the high-level process in charge of mesenchymal move-
ment (Chapter 8) and also by the high-level process in charge of closure of epithelial holes
(Chapter 17) (
Figure 28.2
).
This multilayered nesting of morphogenetic mechanisms extends over a wide variety of
scales, and the development of complex tissues and organs will include most of the
