The Role of Cell Rearrangement
The original designations of egg types of insects (Krause, 1939) focused on the length of the germ anlage relative to the anteroposterior axis of the egg and the number of presumptive segments that form from the growth zone (or from an alternative perspective, the number of segments specified in the germ anlage before the onset of gastrulation). The presumption then was that segments not yet specified originate from a short posterior growth zone, which undergoes substantial growth as additional segments are defined. As discussed earlier, this presumption fails in a number of species, and proliferative growth underlying elongation in both short and intermediate germband insects occurs throughout the length of the germband. This failure to observe the logically predicted growth in the posterior, coupled with the observation that many early germ anlage undergo substantial changes in their shape—for example, the short and wide Bombyx embryo undergoes a complete change in aspect ratio, or the bulbous posterior of a Tribolium embryo extends and narrows—led researchers to focus their attention more on the role of orchestrated cell rearrangements as an essential mechanism underlying segmentation and elongation.
The well-studied Drosophila embryo has been the arthropod model for elongation via cell rearrangement, or convergent extension. Despite the fact that the pri- mordia for all Drosophila segments are visible in the late blastoderm, the germband increases in length more than twofold (Hartenstein and Campos-Ortega, 1985). This extension post-segmentation has been the main lens through which to consider other species that elongate while adding segments. Recently, however, advances in live cell imaging have radically improved our ability to describe movements of cells during germband elongation in other arthropod species. We begin our discussion of cell rearrangements with a brief, general summary of cell rearrangement and provide an overview of convergent extension in Drosophila. We then describe what is known about cell rearrangements in sequentially segmenting arthropods, focusing on the best-studied case, Tribolium. We predict that, as more species are characterized, the cellular mechanisms underlying elongation in arthropod embryos and larvae will be diverse.