Self-organization of an organizer: Whole-body regeneration from reaggregated cells in cnidarians

Cnidarians like the freshwater polyp Hydra and the sea anemone Nematostella, are famous for their enormous capacity to regenerate missing head or feet upon bisection. Classical transplantation experiments have demonstrated that the hypostome, the oral tip of the freshwater polyp Hydra, acts as an axial organizer. Likewise, transplantation of the blastopore lip of an early Nematostella gastrula stage embryo to an aboral position leads to ectopic head formation. Following molecular analyses have shown that Wnt signaling is the key component of this organizer activity. Moreover, when dissociated and reaggregated head (and foot) organizer centres are re-established by self-organization. Similarly, "gastruloids", i.e. aggregates of dissociated early gastrula stage embryos, are able to self-organize. Here, we review the past and recent molecular and theoretical work in the field to explain this phenomenon. While Turing-type reaction-diffusion models involving morphogens like Wnt dominated the field for many years, recent work emphasized the importance of biophysical cues in symmetry breaking and establishment of the organizers in aggregates. The comparison with Nematostella aggregates suggests that the principles of self-organization in cnidarians is not universal.