Proliferation versus Cell Rearrangement As Drivers of Segment Formation

As new segments form from the posterior, there is an addition of tissue to the segmented portion of the embryo (e.g., the segmented germband in arthropods or the somites in vertebrates). Where is this tissue coming from? Is it new tissue that results from localized cell proliferation or is it existing tissue that has been shifted into the nascent segment through cell rearrangement? Answering these questions requires developing experimental approaches that can trace individual cells over time and provide a detailed description of cell proliferation and cell movement (see Chapter 8 for some examples of these approaches).

Cell proliferation has a central role in segment formation in the cases where the segmentation process is linked to the cell cycle, as described earlier for malacostra- can crustaceans (with the exception of amphipods) and for clitellate annelids. In both these cases, we have fairly detailed data on the pattern and sequence of cell division from early development and up to the formation of distinct segmental precursors. There is no preexisting tissue that contributes to the teloblast-derived nascent segments, and all growth arises from cell proliferation within the teloblasts and their descendants.

Conversely, in a typical arthropod growth zone, there is a pool of undifferentiated cells, which are gradually recruited into the segmented germband. Throughout the process of germband extension, the growth zone becomes smaller, as the growth zone cells are recruited into nascent segments. This has been shown most dramatically in the growth zone of the geophilomorph centipede Strigamia maritima (Chipman et al., 2004), where the growth zone starts out as a very large disk, covering nearly half of the egg, and shrinks as segments are added to the germband. However, in reality the process is somewhat more complex. Even when the tissue added to the extending germband is recruited from existing cells, there is some cell proliferation in the growth zone. Auman et al. (2017) showed that in the milkweed bug Oncopeltusfasciatus, there is a constant low level cell proliferation in the posterior of the growth zone, presumably serving to replenish the pool of undifferentiated cells. There is relatively little known about cell proliferation in segmentation in other arthropods that do not have teloblasts (see Chapter 3; Williams and Nagy, 2017).

As for vertebrates and non-clitellate annelids, to my knowledge there has been no detailed work following the relative contributions of cell division and cell rearrangement to the process of segment addition.

 
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