Closure of Holes - Mechanisms of Morphogenesis - page 197

production of very long filopodia, which reach out across the gap and make contact with the
other side. 9 The filopodia seem to be capable of recognizing stripes of their own sort; for
example, filopodia from cells expressing engrailed (in other words, from the anterior portion
of each para-segment) will adhere to other cells expressing engrailed but will pass right over
non-engrailed-expressing cells to find the nearest ones that do express the gene. 9 Since
Engrailed is a nuclear protein, the filopodia cannot detect it directly but presumably detect
a cell surface marker that is expressed under Engrailed control.
Once filopodia find a cell they recognize, they push against its plasma membrane and
cause it to yield around them to create a large surface area of contact between the filopodial
membrane and that of the other cell ( Figure 17.4 ). Junction molecules are recruited to the sites
of contact, and the fleeting filopodial kiss is transformed into a committed partnership as the
contacts mature to become conventional adherens junctions. 9 If formation of filopodia is
inhibited by dominant negative cdc42 (see Chapters 5 and 8), cells do not form proper adhe-
sive contacts and dorsal closure fails.
Recent work has shown that the original assumption that segment registration is perfect
and no cells are swapped is incorrect. In reality, once the hole closes there is a tightly
controlled swap of particular cell positions across segment boundaries, accompanied by
specific changes in gene expression. The unexpected behaviour of these Mixer Cells 13 does
not reduce the need for accurate meeting of the two sides of the embryo: all it means is
that the accurate meeting can then be used as the foundation for a new set of precise,
small-scale cell movements.
FIGURE 17.4 Filopodial activity at the end of dorsal closure. As the opposing sides near each other, cells
produce filopodia with which they explore one another. If they find a cell of the wrong type (represented here by
nuclear colour) they ignore it, but when they find a cell of the correct type they push into it and form adhesive
contacts. As more contacts are made, the parts of the cells betwen filopodia also advance in a lamellipodium-like
manner, meet, and adhesive contacts mature to become 'normal'.