Biology Reference
In-Depth Information
FIGURE 11.1
The combination of an adhesive gradient and thermodynamic motion creates a Brownian Ratchet
mechanism that can move an object up a gradient. In this diagram, the object is a simple bead that can roll, and
kinetic energy from the thermodynamic forces impinging on the bead is shown stored temporarily in stretched
adhesive bonds. A deformable object such as a balloon or membrane vesicle can store energy temporarily by
deforming its shape into a flatter one.
The very simple form of haptotaxis described above tends not to be used in living
embryos because cells that are not actively migratory either form attachments so strong
that they are effectively immobilized (as most cells in a mature tissue tend to be) or
they tend to form essentially no attachments (for example, erythrocytes in the blood).
Actively migrating cells have labile attachments, not just because of thermal fluctuations
but also because of the 'deliberate' protrusion-attachment-tension-detachment cycle of
locomotion described in Chapter 8. These cells, therefore, are well adapted for haptotactic
steering.
The leading edge of migratory cells does not just face forwards; the edges of lamellipodia
explore the environment a little to each side of the main axis of the cell, and filopodia can
extend great distances to the side. In this way, cells can 'sample' the environment for naviga-
tional cues. An element of competition between different parts of the leading edge follows
inevitably from this lateral exploration, since the cell cannot advance simultaneously towards
the left-most parts of the leading edge and the right-most parts for long without becoming
too spread and stretched (
Figure 11.2
). Anything that tips the balance of this competition
will therefore be able to steer the cell.
The back of the leading edge is richly supplied with integrin-containing cell-substrate
adhesion complexes, against which the leading edge protrusions push as they advance,
