Zebrafish lines and handling
Zebrafish (D. rerio) adults and embryos were maintained and handled according to established protocols51.
The experiments were approved and licensed by the local animal ethics committee (Landesdirektion Sachsen, Germany; licence no. DD24.1-5131/394/33) and executed in accordance with the European Communities Council Directive 2010/63/EU on the protection of animals used for scientific purposes, as well as the German Animal Welfare Act. Sample sizes were chosen to allow statistically significant experiments while minimizing the number of animals used. Adult (4 months to 2 years of age) female and male zebrafish were used to produce embryos, and the sex of the embryos was not considered. Randomization and blinding were not applicable in this study. Transgenic animals had mixed background from AB and TL strains. Zebrafish transgenic lines used in this study are listed in Supplementary Table 1.
Zebrafish sample preparation
Embryos were collected in E3 medium (5 mM NaCl, 0.17 mM KCl, 0.33 mM CaCl 2 and 0.33 mM MgSO 4 ) within 5 min after spawning and kept at 24–28 °C. Embryo clutch quality was inspected on a dissection stereomicroscope. Beads, fluorescent proteins, ferrofluid droplets or chemicals (SbTubA4P) were injected into the cytoplasm of one-cell stage transgenic zebrafish embryos according to52. Injection volumes were calibrated to 0.5 nl. Depending on the experiment, 0.5–1 nl were injected per embryo.
Embryos were mechanically dechorionated using forceps and mounted in 1% low-melting-point agarose (Sigma) in E3 medium supplemented with 25% w/v iodixanol (OptiPrep, STEMCELL Technologies 07820) for refractive index matching53 on a CELLVIEW cell culture glass bottom dish (Greiner Bio-One 627860). Embryos were brought closer to the coverslip surface by keeping the dish upside down until agarose solidified, since the embryos float in the mounting media containing OptiPrep54.
For magnetic tweezer, optical tweezer and band length experiments, dechorionated embryos were mounted in 0.5% low-melting-point agarose (Sigma) in E3 medium (without OptiPrep). Embryos were manually oriented before agarose solidification using a flamed glass capillary.
For measuring ingression velocity, embryos were mounted in 1% low-melting-point agarose (Sigma) in E3 medium (without OptiPrep) within their chorion. Embryos were manually oriented before agarose solidification using a flamed glass capillary.
Chemical treatments
Dechorionated embryos at the one-cell stage were mounted in 0.5–1% low-melting-point agarose (Sigma-Aldrich), 25% OptiPrep Density Gradient Medium (OptiPrep, STEMCELL Technologies 07820) supplemented with 10 µg ml−1 cytochalasin B from Drechslera dematioidea (Sigma-Aldrich), 200 µg ml−1 cycloheximide (239763-1GM), 10 µg ml−1 nocodazole (Thermo Scientific Chemicals 10762633) or DMSO control in a CELLVIEW cell culture glass bottom dish (Greiner Bio-One 627860).
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