Epithelial Fusion - Mechanisms of Morphogenesis - page 224

accumulates in the region of the DE-cadherin contacts between fusion cells. 12 This accumu-
lation of Kakapo reorganizes the actin in the cell so that fibres form to link the new DE-
cadherin-mediated contacts between tracheae with the existing DE-cadherin-containing
adherens junctions around the lumens of each trachea. 12 Once formed, these actin fibres
seem to perform the twin roles of contraction to draw the tracheal systems closer together
(presumably mediated by myosin) and marking the place for insertion of new membrane
and apical determinants.
As well as the trees on the left and right sides of one segment connecting to the trees in
adjacent segments, they connect with one another by joining their dorsal branches. In this
case, the cells critical for fusion are not at the tip but one cell back, meaning that the tip cells
remain as dorsal 'side-branches' ( Figure 19.5 a). The sequence of events in dorsal branch
fusion is similar to that of trunk fusion: 13 cells responsible for fusion (called, reasonably
enough, 'fusion cells') reach out with highly dynamic filopodia and elongate, giving the
visual impression that they are straining to make contact. Once they have made contact,
they flatten against one another, this flattening being driven apparently by elongation of
the stalks behind them. The advance of the stalk is so insistent that it pushes into the fusion
cells' space, bringing the lumen itself forwards. Only across the fusion site itself is the lumen
lined directly by the fusion cells, which hollow out to make a seamless tube ( Figure 19.5 b,c).
E PITHELIAL FUSION IN PALATE DEVELOPMEN T
The secondary palate of mammals, the bony plate that separates the nose from the mouth,
is formed by the ingrowth of two opposing shelves, one from the left and one from the right.
These palatal shelves commence development as processes that grow downwards from the
maxillary processes each side of the tongue. They then elevate to a horizontal position and
extend across the top of the tongue to meet along the midline ( Figure 19.6 ). Because it is acces-
sible, and because 'cleft palate' ( Figure 19.7 ) is a common human abnormality, the process of
FIGURE 19.6 Formation of the secondary palate. This diagram is from a point of view looking up from under
the chin, with the lower jaw removed for clarity. The oral cavity is closed off from the nasal cavity by the meeting of
three processes. The median palatal shelf gives rise to the primary palate while the secondary palate forms by the
fusion of ingrowing lateral palatal shelves.