Types of Cells in Segmentation Processes

Cells involved in the segmentation process are by nature pluripotent cells. Nascent segments go through a stage characterized by an undifferentiated morphological unit, which is of course composed of undifferentiated cells. These cells will go on to form precursors of multiple different organs and structures. As with other aspects of segmentation, our understanding of the specific identity of these cells is fairly limited, and there is almost no work dividing these cells into different cell types.

The exceptions to this gap in our understanding are the stem cells in malacos- tracan crustaceans and in clitellate annelids, already mentioned earlier. In these cases, there are detailed fate maps, and the individual cells are known and identified (Dohle and Scholtz, 1988; Lans et al, 1993; Gerberding and Scholtz, 1999; Kuo and Shankland, 2004). In malacostracans they include a number of ectodermal stem cells (ectoteloblasts) and a smaller number of mesodermal stem cells (mesoteloblasts). The ectoteloblasts are all roughly equivalent in behavior, and in many cases there is a fixed number of them. They are symmetrically arranged on two sides of the midline, with one cell being median and unpaired. There is no median mesoteloblast. The first two rounds of division of the daughter cells generate four rows of cells for each division of the ectoteloblasts and the mesoteloblasts. These cells remain morphologically identical, and only start to differentiate in subsequent divisions.

In clitellates, there are ten teloblasts, two of these are mesoteloblasts and eight are ectoteloblasts. Each of these cells generates a distinct lineage (with symmetrical cells being identical), with different cell fates. Thus, compared with the situation in malacostracans, the teloblasts are not as pluripotent.

Vertebrate somites are already dedicated mesodermal structures, which differentiate into a series of distinct cell fates, in a regulated regionalized manner, shortly after the somites are formed. The source of the somite cells is a series of cells known as neuromesodermal progenitors (NMPs; see Chapter 5 for a detailed discussion). These cells originate in the tailbud, and they are unusual in not being committed to a specific germ layer, even after gastrulation. As their name suggests, some of them become committed to ectodermal fate and become part of the neural tube, whereas others become committed to mesodermal fate and form the somites. The NMP cells behave like stem cells, and although they cannot be said to be totipotent, they have a range of developmental fates.

Stem cells are involved in post-traumatic segmentation in organisms that can regenerate axial structures (most notably annelids; see Chapter 10). These stem cells can come either from reserve stem cells that remain in the organisms’ tissues post- embryonically or from dedifferentiation of somatic cells. They proliferate and differentiate to regenerate all of the required adult structures (Bely et al., 2014; Zattara and Bely, 2016).

Cell lineage studies of segmental cells have mostly been performed on those species that have stereotypical stem cell division patterns (e.g., malacostracan crustaceans and leeches; Lans et al., 1993; Gerberding et al., 2002; Wolff and Scholtz, 2002; Alwes et al., 2011). There are almost no studies following the fates of cells within the undifferentiated proto-segment or the pre-segmental domains. Although it is clear that the cells in the arthropod growth zone end up in the differentiated segments (Nakamoto et al., 2015; Auman et al., 2017), there is no mapping of which cells end up where. Thus, there is currently no way to identify differing cell fates within the arthropod growth zone or nascent segment. The same is true for most other taxa.

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