Positioning and capture of cell surface-associated microtubules in epithelial tendon cells that differentiate in primary embryonic Drosophila cell cultures.

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Using primary embryonic Drosophila cell cultures, we have investigated the assembly of transcellular microtubule bundles in epidermal tendon cells. Muscles attach to the tendon cells of previously undescribed epidermal balls that form shortly after culture initiation. Basal capture of microtubule ends in cultured tendon cells is confined to discrete sites that occupy a relatively small proportion of the basal cell surface. These capturing sites are associated with hemiadherens junctions that link the ends of muscle cells to tendon cell bases. In vivo, muscle attachment and microtubule capture occur across the entire cell base. The cultured tendon cells reveal that the basal ends of their microtubules can be precisely targeted to small, pre-existing, structurally well-defined cortical capturing sites. However, a search and capture targeting procedure, such as that undertaken by kinetochore microtubules, cannot fully account for the precision of microtubule capture and positioning in tendon cells. We propose that cross-linkage of microtubules is also required to zip them into apicobasally oriented alignment, progressing from captured basal plus ends to apical minus ends. This involves repositioning of apical minus ends before they become anchored to an apical set of hemiadherens junctions. The proposal is consistent with our finding that hemiadherens junctions assemble at tendon cell bases before they do so at cell apices in both cultures and embryos. It is argued that control of microtubule positioning in the challenging spatial situations found in vitro involves the same procedures as those that operate in vivo.