Totally tubular chitin

Flies create uniform tracheal tubes by molding them around an expanding chitin filament, according to Anna Tonning, Anne Uv, and colleagues (Göteborg University, Göteborg, Sweden).Uv's group suspected that chitin may be involved in shaping tracheal tubes after observing that a chitin synthase mutant forms tracheal branches that look like sausages rather than uniform cylinders. Branch cell fusion junctions were always constricted, with the intervening regions always grossly expanded. Indeed, fluorescent microscopy revealed that chitin normally accumulates within tube lumens during tracheal development, forms a filamentous cable, and uniformly expands to push branch cell fusion junctions outward while preventing overdilation of the remaining epithelium.

These results are consistent with two models. The chitin filament may act as a sponge. As liquid is drawn into the lumen during development, the sponge-like cable would then expand to uniformly push the epithelial cells outward. Uv, however, favors a second hypothesis in which the filament holds back lumen expansion, much like an anchor; the resultant tension would also force any constrictions to expand. This model implies the existence of a protein complex linking to the apical surface and underlying cytoskeleton.

Tube formation is a fundamental process in many organisms, so analogous mechanisms for shaping tubes may yet be found. “Different organs,” suggests Uv, “will likely use different matrix components to create uniform luminal expansion.”

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