The coordination of cell–cell adhesion and actin contractility is beautifully illustrated by cellular rearrangements that occur early in the development of the fruit fly Drosophila melanogaster. Soon after gastrulation, the outer epithelium of the embryo is elongated by a process called germ-band extension, in which the cells converge inward toward the dorsal–ventral axis and extend along the anterior–posterior axis (Figure 19–15). Actin-dependent contraction along specific cell boundaries is coordinated with a loss of specific adherens junctions to allow cells to insert themselves between other cells (a process called intercalation), resulting in a longer and narrower epithelium. The mechanisms underlying the loss of adhesion along specific cell boundaries are not clear, but they depend in part on increased degradation of β-catenin, due to its phosphorylation by a protein kinase that is localized specifically at those boundaries.
Desmosomes are structurally similar to adherens junctions but contain specialized cadherins that link to intermediate filaments instead of actin filaments. Their main function is to provide mechanical strength. Desmosomes are important
Figure 19–14 The folding of an epithelial sheet to form an epithelial tube. the oriented contraction of the bundles of actin and myosin filaments running along adhesion belts causes the epithelial cells to narrow at their apex and helps the epithelial sheet to roll up into a tube. an example is the formation of the neural tube in early vertebrate development (see Figure 19–8).
Figure 19–15 Remodeling of cell–cell adhesions in embryonic Drosophila epithelium. Depicted at left is a group of cells in the outer epithelium of a Drosophila embryo. During germ-band extension, cells converge toward each other (middle) on the dorsal–ventral axis and then extend (right) along the anterior–posterior axis. the result is intercalation: cells that were originally far apart along the dorsal–ventral axis (dark green) are inserted between the cells (light green) that separated them. these rearrangements depend on the spatial regulation of actin–myosin contractile bundles, which are localized primarily at the vertical cell boundaries (red, left). Contraction of these bundles is accompanied by removal of E-cadherin (not shown) at the same cell boundaries, resulting in shrinkage and loss of adhesion along the vertical axis (middle). New cadherin-based adhesions (blue, right) then form and expand along horizontal boundaries, resulting in extension of the cells in the anterior–posterior dimension.
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