Biology Reference
In-Depth Information
FIGURE 18.5
Invagination of the archenteron from the vegetal plate of a sea urchin.
invagination may be driven by the same mechanism of myosin-driven apical constriction
that has been described above. Finite element
)
computer models of this hypothesis produce
very realistic behaviour.
10
Despite this, direct tests of the role of actin filaments in this partic-
ular example of invagination fail to show any need for actin at all. Invagination begins
normally in the presence of the actin-depolymerizing drug cytochalasin D.
12
As well as
arguing against an actin-based mechanism for invagination, this observation underlines an
important warning: the fact that a hypothetical model works well on a computer does not
prove that it is the actual mechanism used in life, and the fact that a mechanism is critical
in one example of morphogenesis does not necessarily mean that it is equally critical in
a similar example in another species, or even in another place in the same species.
The most likely mechanism for archenteron invagination, based on data to hand at the
time of writing, relies more on the mechanics of extracellular matrix than of the cells them-
selves. The apical (external) surface of the epithelial cells is covered in a thick extracellular
matrix consisting of an inner 'apical lamina' and an outer, clear, hyaline layer. The matrix
seems to be important in driving invagination, which is blocked by monoclonal antibodies
raised against uncharacterized components of the apical lamina.
13
As invagination begins,
cells of the vegetal plate secrete a large chondtroitin sulphate proteoglycan into the apical
matrix but not into the hyaline layer. Chondroitin sulphates can form gels that swell as
they absorb water, so that the apical layer of the matrix expands while the overlying hyaline
layer does not. This differential expansion then forces the matrix to buckle inwards
(
Figure 18.6
). Finite-element modelling confirms that this mechanism could work in prin-
ciple, but only if the epithelium itself deforms significantly more easily than (has a
60
lower elastic modulus than) the matrix overlying it.
10
Indirect testing of the hypothesis in
living embryos show that blocking the secretion of chondtroitin sulphate blocks invagin-
ation. On the other hand, inducing the chondroitin sulphate earlier, by pharmacological
manipulation of Ca
2
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signalling, causes a corresponding advance of the timing of inva-
gination.
12
There has not yet been any direct evidence of forces generated by gel swelling
in this system, though, and the elastic moduli of the layers involved have not been measured.
The regulator RhoA, so important for cytoskeleton-mediated apical constriction in other
systems (see
Figure 18.4
), also appears to be necessary for the sea urchin's matrix-mediated
)
Finite element models consider a complex surface to be made up of a large number of small and simple
elements such a triangles, the deformations of which, and the effect of those deformations on the shape of the
entire surface, are comparatively simple to compute (see Chapter 26).
