Cell Rearrangements from Diverse Taxa Have Some Common Features

In animals as different as frogs and flies, and in processes as distinct as gastrula- tion and germband extension, intercalary cell rearrangements show many commonalities (reviewed in Walck-Shannon and Hardin, 2014; Keller et al., 2000; Keller, 2006; Shindo, 2017; Huebner and Wallingford, 2018). The described cases of convergent extension all use two main cellular mechanisms to rearrange cells: bipolar basal cell protrusions and junctional remodeling, although they vary in the degree to which these two mechanisms are employed (reviewed in Zallen and Goldstein, 2017; Huebner and Wallingford, 2018). Basal cell protrusions make stable contacts with neighboring cells and use these contacts to exert traction. Protrusions are oriented along the mediolateral axis and the tissue elongates as the cells intercalate (Keller and Tibbetts, 1989; Wilson and Keller, 1991). Junctional remodeling requires the repositioning of the apical junctions that adhere cells to one another (Bertet et al., 2004; Blankenship et al., 2006). Repeated cycles of junctions contracting along the mediolateral axis and extending along the anteroposterior axis function to elongate the Drosophila embryo (Bertet et al., 2004; Blankenship et al., 2006). Most of the described cases of convergent extension also share common regulatory features, relying on planar cell polarity (PCP) signaling, a conserved protein network that establishes cell polarity in many different types of tissues (reviewed in Butler and Wallingford, 2017). Loss of PCP function disrupts junctional remodeling (Lienkamp et al., 2012; Nishimura et al., 2012; Shindo and Wallingford, 2014; Williams et al., 2014) and may be indirectly involved in basal protrusive activity during convergent extensions (see Huebner and Wallingford, 2018). Although PCP function in tissue polarity was first discovered in Drosophila, the PCP pathway is not required for Drosophila germband extension (Zallen and Wieschaus, 2004). Here we briefly describe the mixture of junctional remodeling and bipolar basal cell protrusions used during Drosophila germband extension, then discuss what is known from the best-studied sequentially segmenting insect, Tribolium.

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