In the preceding discussion, we focused our attention on the role of cell division and cell rearrangement in embryo elongation. Both types of cell behaviors figure prominently in arthropod embryo elongation, and the relative degree to which an embryo relies on one or the other is a key variable in evolution. Recent quantitative studies on cell division allow us to confirm that most arthropods do not have dedicated posterior stem cells that fuel elongation, nor has a canonical region of high posterior proliferation been discovered. Whether the initial embryonic primordia is small or large, embryo elongation is fueled by cell division throughout the embryo in all cases analyzed. Despite the fact that actual mitoses appear much less frequently than predicted in the growth zone, cell division is nonetheless present and plays a role in supplying new tissue to the embryo. One notable, and currently less explored, characteristic of cell cycling is that it is highly regulated in both space and time. The growth zone has anterior and posterior domains of cell cycling and even the speed of the cell cycle varies consistently, at least in Thamnocephalus. One exciting avenue of future research will be to document cell cycling, both M- and S-phase, more carefully in more species; uncover the regulators of cell cycling; and examine how that regulation is coordinated with segmental patterning in sequentially segmenting arthropods.
Beyond cell division, simple observations of development across arthropods make it clear that cell movements that drive convergent extension play a significant role in elongation but are currently less well characterized across arthropods (Benton et al„ 2013, Benton 2018; Sarrazin et ah, 2012; Nakamoto et ah, 2015). In Tribolium, convergent extension plays an important role in elongation, and interestingly, cell movements are temporally variable at least in extent and, possibly, in their mechanistic basis. Elements of a Drosophila model—in which pair-rule genes drive the differential expression of multiple leucine-rich receptors, which promote differential adhesion through heterophilic interactions, that in turn affect intracellular myosin localization—has some traction as a conserved mechanism to link segmental patterning to cell movement (Benton et ah, 2016). Future studies to link interactions among these receptors to the mechanical forces that move cells in more species will be important. We predict that detailed studies of embryo elongation in other species are likely to reveal additional combinations of cell division and cell movements.