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Of course, at tissue level, not all cells rearrange at once, and at cell level, an
individual cell does not exchange all of its adhesions at once, but loses
attachments to old neighbours and makes new ones with prospective
neighbours over some period of time.
We have postulated that the stiffness of the intercalating array of cells may
be a function of the less obvious aspect of the polarization of these cells, the
numerous small contacts along the anterior and posterior sides of these cells.
In scanning electron micrographs these contacts appear as filopodia (Figure 9
in Keller et al., 2000), but their length is probably partially due to shrinkage of
the cells, which can be to 10% or more (Keller and Schoenwolf, 1977). SEM
shows that these contacts are present in large numbers, but their number is
underestimated in the original, standard low light fluorescent movies of cells
filled with fluorescent dextrans, conditions under which few of them are visible
(Shih and Keller, 1992a). A sectional view with scanning confocal fluorescence
microscopy of cells labelled with a membrane targeted GFP shows that these
protrusions are numerous and turning over rapidly, on average about every
2min, and they appear as short, sharply tapered protrusions, most resembling
short filopodia (Keller, 2002, movie S6). These small, dynamic contacts
represent dynamic adhesions that allow the cells to shear past one another and
thus intercalate, but also may lock the intercalating array into a rigid structure
capable of bearing a compression load and exerting a pushing force (Keller
et al., 2002). These contacts could also contribute to the elongate cell shape
and the formation of the parallel array of elongated cells by sticking the long
sides of the cells together. However, we believe that it is unlikely to play a
large role in the initial polarization and elongation of the cells, and that the
larger role is played by the mediolateral, bipolar filo-lamelliform protrusive
activity. However, selective adhesion of the elongate sides to one another
could promote formation of a stable, parallel array (Elsdale and Wasoff,
A key to the mechanics of convergent extension by cell intercalation may be
regulation of the dynamics of cell-cell adhesion. These contacts are constantly
being made and broken but, at any one time, many of them attach cell to its
neighbours. The local periodic breakdown of these adhesions would allow
shearing of cells past one another and invasion of the intercalating, tractoring
protrusions between cells. For example, as the lamelliform, tractoring
protrusions advance, their way would be blocked for a time, but soon the
filiform protrusions would be retracted, allowing advance of the lamelliform
protrusion, and then new filiform protrusions would form contacts again on
the sides of the intervening cell. The tissue can be self-deforming and stiff
during cell intercalation if the adhesions are strong while they last, but have a
regulated turnover. Turnover of adhesions, of course, is not an unusual
property; it is essential for cell migration on a planar substrate. In the case of
intercalation, it is simply a matter of migration on another cell. Adhesion is
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