Fish form neural tubes by hollowing out a solid rod of cells. Within the rod, cells are elongated and occupy all positions across the rod diameter, including the midline. During tube formation, cells must move away from the midline, where the lumen will form. At the same time, the rod cells become polarized such that their future apical side is adjacent to the lumen.
Tawk et al. examined these changes by fluorescently tagging the apical marker Pard3, whose relatives help to polarize worm and fly embryonic cells. The Pard3 revealed an unusual division in early rod cells that organizes the polarity of apical proteins.
During this division, which occurred ∼15 hours after fertilization, Pard3 moved from its ubiquitous cytoplasmic distribution to the cleavage furrow. Upon division, Pard3 was partitioned with mirror symmetry, resulting in daughter cells whose apical sides faced each other.
After division, the daughter cell closer to the midline quickly moved to the opposite side. Loss of Pard3 blocked the cell migration across the midline, but the authors do not yet fully understand how crossing occurs. Until lumen formation, the daughter cells maintained a small area of cell–cell contact where Pard3 was concentrated. Clarke supposes this contact might “stop them from slipping back across one another,” thus stabilizing their relative positions.
In mutants in which the rod formed late, the mirrored division took place before rod formation. The authors thus suggest that Pard3 accumulation at the cleavage plane is triggered by a cell-intrinsic mechanism rather than by the rod's environmental influence. But they are still looking for the upstream events that trigger this sudden polarity. They speculate that other developmental cavitations, such as gut formation, might also be preceded by a mirror-symmetric division.