Biology Reference
In-Depth Information
FIGURE 26.1
Lewis and Miller's physical model for epithelial invagination. (a) The model was based on
a series of simple brass tubes equipped with pegs. (b) Tubes were lined beside one another, and short lengths of
rubber pipe were used to connect the side pegs of neighbours. (c) Adding elastic bands to the top and bottom pegs
created a tensegrity structure. (d) Adding additional bands on one side, to simulate what we would now call 'actin-
myosin contraction', causes the tensegrity structure to bend in a manner reminiscent of an invaginating epithelium.
to keep the rods apart: in modern terms, they could be thought of as representing the micro-
tubule systems of the cell (Chapter 5). To complete the model, taut elastic bands were placed
over the remaining pegs at the end to the rods, to connect adjacent cells (
Figure 26.1
b). The
whole model formed a tensegrity structure (Chapter 5), the elastic bands exerting tension,
resisted by the brass and tubing which were in compression and, as the brass was heavy
enough to keep the model on the plane of the table, its stable state represented a row of cells
in a flat epithelium (
Figure 26.1
c). If extra elastic bands were added between pegs at the top of
the model, to simulate what would now be seen as extra actin-myosin contraction, the model
bent into a curve, representing invagination (
Figure 26.1
d). If the extra bands were added
between just some cells, distinct hinge points were seen. Simple as it was, this physical device
provided a clear demonstration that local increases in tension at one pole of some or all cells
in an epithelium were a feasible explanation for large-scale change of shape.
The idea of making a literally physical model may seem quaint in the digital age, but the
approach does carry one major advantage compared with a computer model. If the hypothesis
is that an aspect of cell or tissue behaviour can be explained in terms of compression struts and
tension elements under different strains, then modelling it physically using actual compres-
sion and tension structures involves a minimum of transformations. Modelling the same thing