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Mirror symmetry in division

A division that leaves daughters with mirrored allotments of Pard3 (green) causes one cell to cross (left) the midline (yellow line). Cells fail to cross (right) in the presence of a mutant Pard3. CLARKE/MACMILLAN

By sending apical proteins to the division plane, epithelial cells create mirror image daughters, say Marcel Tawk, Jonathan Clarke (UCL, London, UK), and colleagues. This mirror symmetry helps form the zebrafish neural tube.

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.

Reference:

Tawk, M., et al. 2007. Nature. doi:10.1038/nature05722.

Nicole LeBrasseur

lebrasn{at}rockefeller.edu

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This Article Full Text (PDF, 764K) PPT slides of all figures Alert me when this article is cited Services Email this article Similar articles in this journal Alert me to new content in the JCB Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by LeBrasseur, N. Search for Related Content PubMed Articles by LeBrasseur, N. Social Bookmarking                            What's this?