Non-muscle myosin IIA and IIB differentially regulate cell shape changes during zebrafish brain morphogenesis.

During brain morphogenesis, the neuroepithelium must fold in specific regions to delineate functional units, and give rise to conserved embryonic brain shape. Individual cell shape changes are the basis for the morphogenetic events that occur during whole tissue shaping. We used the zebrafish to study the molecular mechanisms that regulate the first fold in the vertebrate brain, the highly conserved midbrain-hindbrain boundary (MHB). Since the contractile state of the neuroepithelium is tightly regulated by non-muscle myosin II (NMII) activity, we tested the role of NMIIA and NMIIB in regulating cell shape changes that occur during MHB morphogenesis. Using morpholino knockdown, we show that NMIIA and NMIIB are both required for normal MHB tissue angle. Quantification of cell shapes revealed that NMIIA is required for the shortening of cells specifically at the MHB constriction (MHBC), while NMIIB is required for the proper width of cells throughout the MHB region. NMIIA and NMIIB knockdown also correlated with abnormal distribution of actin within the cells of the MHBC. Thus, NMIIA and NMIIB perform distinct functions in regulating cell shape during MHB morphogenesis.