Biology Reference
In-Depth Information
FIGURE 5.11
The methods for increasing local microfilament tension, used in the study of microtubule
response to mechanical stress described in the main text, will have the effect of reducing compression in micro-
tubules if the tensegrity model is correct. The diagram depicts the method of pushing with a pipette, but the other
methods described in the text would have the same net result.
The increase in microfilaments observed in stressed parts of cells may have less to do with
mechanics and more to do with other functions of microtubules, particularly transport of
components within the cell. Given that junctional components such as cadherins and catenins
can hitch a ride to the membrane aboard microtubules, by being attached to the motor protein
kinesin,
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it may be that the main purpose of an increase in microtubules in tensed areas of
cells is to assist with transport of material needed to bolster the tension-bearing network.
The main tension and compression systems of the cytoskeleton, then, organize themselves
automatically according to the forces that act on them and the placement of the junctions that
transmit those forces. The feedback loop that ensures automatic placement of microfilaments
and microtubules is the main way in which embryos solve the problem of having to place
internal elements with an accuracy exceeding that of the tissue-level gradients that drive
pattern formation. The evidence for this feedback is strong, and, although it has been pre-
sented here in the context of the tensegrity model for global cytoskeletal function, the validity
of the feedback described does not depend on the tensegrity model standing the test of time.
FORMATION OF SPECIAL STRUCTURES: ACTIN-BASED
CELL PROTRUSIONS
'Special' morphogenesis by cells proceeds mainly by specific regulation of the generic
systems that have been described in this chapter. Cells form processes and out-pushings for
