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Fig. 4. Model of how an adhesion gradient could determine the direction of cell
migration during dorsal convergence in zebrafish. The dorsoventral (DV)
BMP gradient of gastrula-stage zebrafish embryos
(Hammerschmidt and Mullins,
2002) leads to the establishment of a reverse gradient of
Ca2+- and cadherin-dependent cell-cell adhesiveness
(von der Hardt et al., 2007).
Migrating lateral mesodermal cells form lamellipodia that transiently contact
neighbouring cells. At the onset of dorsal convergence, lamellipodia project
randomly in all directions. However, the BMP and adhesion gradients lead to
differences in the "functionality" of dorsal versus ventral
protrusions. In vitro measurements of adhesion forces between cells with
different cadherin levels have revealed that adhesion force is determined by
the partner with the lower levels (Krieg
et al., 2008). Accordingly, contacts of a lateral cell (in yellow)
to a ventrally located cell (in green) should be weaker than contacts to a
dorsally located cell (in red). This causes a higher likelihood of dorsal
versus ventral displacements of the lateral (yellow) cell body during
lamellipodial retraction. The model also explains why ventral-most cells
(green) do not converge at all towards the dorsal side
(Solnica-Krezel, 2006). Such
directional instructions by a tissue adhesion gradient might be particularly
important in lateral regions of the mesoderm, giving cells initial information
about medial (dorsal) versus lateral (ventral) location, and inducing a
transformation from a non- or bi-polar to a mono-polar organization of cells.
It is currently unclear (question mark) whether this transformation also leads
to a re-distribution of adhesion molecules within cells themselves, with
higher adhesiveness at the front (dorsal side), which would further enhance
the efficiency of the system. t, time.
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