Synchronizing Cell Division, Cell Rearrangements, and Cell Fate
Because the available studies of elongation have revealed highly regulated temporal and spatial sequences of cell cycling and/or cell movements, an obvious question to ask is how the progressive specification of cell fates during segmentation is linked to the cell behaviors described earlier. Where and when are segmental progenitor cells restricted to their fates in different arthropod species? Are cells in the growth zone initially multipotent? Are cell fates and cell movements mechanistically linked?
Most species share the same general behaviors: some cells in the ventral epithelium that comprises the posterior growth zone move out of the epithelium and become mesodermal, while the remaining cells move through the anterior growth zone and transition to their segmental fates. We can propose a generic model for cell fate specification, based on inferences from multiple species. That the growth zone functions to maintain cells in a multipotent state has been previously proposed as a consequence of posterior Wnt signaling (McGregor et al., 2008, 2009; Chesebro et ah, 2013; Oberhofer et ah, 2014). In the emerging models of arthropod growth, cells experience a temporal sequence of transcription factors and/or signals, set up by a posterior Wnt signal, that provide a roadmap for both segmental fate, growth, and cell movement. Cells in the posterior region of high Caudal are presumed to be held in an undifferentiated state. These cells already have information as to their DV position, established in the blastoderm (as shown in Stappert et ah, 2016 for Tribolium). Dynamic waves of determinants of cell fate, e.g., pair-rule genes (El-Sherif et ah, 2012; Sarrazin et ah, 2012) or the Notch ligand Delta (Chesebro et ah, 2013), pass through this region, but do not stabilize, or presumably influence cell fate, until they are outside the region of high Caudal expression. When released from the posterior Caudal signal, cells begin their journey down a transcriptional regulatory path that allows progressive determination of fate: position within a segment, neural vs. ectodermal, tagmatic position, etc., ultimately resulting in the complete differentiation of the insect body plan. These cell fate decisions are realized as cells rearrange their positions within the embryo. Based on the temporal dynamics of gene expression, Cad (and Dichaete [Clark and Peel, 2018] or Cad and Delta [Chesebro et ah, 2013]) expressing cells would be at least multipotent (their ability to differentiate into serosa, amnion, mesoderm may have already ended).
Interestingly, some experimental data suggests posterior blastoderm cells in insects may not be multipotent, e.g., the Oncopeltus experiments described earlier
(Lawrence, 1973) or the Bombyx blastoderm irradiation experiments (Myohara, 1994). While there may be alternative explanations for these nettling results, in truth, no experimental data has verified that cells in the arthropod posterior growth zone are multipotent. In vertebrates, progenitor cells that lie upstream of somite formation use Cdx (the vertebrate caudal ortholog) expression to maintain Wnt and Fgf signaling in the growth zone. In mice, the Cdx/Wnt signaling forms a feedback circuit which, if broken, leads to premature differentiation through a failure to clear retinoic acid from the posterior (Young et al., 2009)—strong evidence that the feedback loop maintains pluripotency. Support for a role for the Cdx/Wnt feedback loop in maintaining pluripotency in the presomitic mesoderm is also confirmed in experiments that replicate somitogenesis from induced pluripotent stem cells in culture. Similar feedback loops have been demonstrated in cockroach embryos, where there is evidence for a clocklike mechanism based on Notch signaling instead of a pair-rule circuit: the Notch ligand Delta acts both up- and downstream of Wnt signaling during segmentation (Chesebro et ah, 2013). But unlike the vertebrate embryo, loss of function of the components of the feedback loop results in a failure to elongate, but do not appear to result in premature differentiation of tissue: premature appearance of segment polarity stripes of wingless in the posterior were not detected.
Ideally, we would experimentally test a cell’s determination through transplantation to different locations and score their subsequent development for segment specific markers. Unfortunately, such direct measures of cell determination are impossible in most arthropod embryos. However, in the current era of single-cell RNA-seq, comparing single-cell transcriptome profiles in anterior and posterior growth zones over time may clarify patterning signals that both limit and promote the fate of posterior cells, and propose mechanistic links between cell fate, cell division, and cell movement.