Furrow-associated microtubule arrays are required for the cohesion of zebrafish blastomeres following cytokinesis.

During the first few cleavages of the zebrafish embryo, daughter blastomeres are loosely associated immediately after furrow ingression, but then gradually cohere. Cohesion appears to be cadherin-dependent, as cadherin and beta-catenin are found at the membrane between cohering blastomeres, and blastomeres fail to cohere in calcium-free medium. Cadherin and beta-catenin are not initially found on the blastomere surface, but are deposited specifically at the furrow surface. An array of parallel microtubules is present on either side of the furrow tip during ingression, as seen by confocal microscopy of alpha-tubulin labelled embryos. Transient incubation of embryos in 1 microg/ml nocodazole at the start of furrowing, which causes a loss of the furrow array, inhibits the localization of beta-catenin to the furrow surface but does not prevent furrow ingression. During ingression, intracellular membranes are transported to the furrow, as shown by labelling with DiD or DiOC6(3). Concentration of these membranes near the furrow surface is microtubule-dependent. These findings suggest that microtubules are required for cohesion of blastomeres because they mediate trafficking of intracellular membranes to the furrow surface, where they are exocytosed and allow cohesion via cadherins.