The significance of Ethel Browne's research on Hydra for the organizer concept

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.

Organizer formation, organizer maintenance and epithelial cell plasticity in Hydra: Role of the Wnt3/β-catenin/TCF/Sp5/Zic4 gene network

The experimental and conceptual knowledge in 1909 led to the discovery of the Hydra head organizer through transplantation experiments between pigmented and non-pigmented animals; a discovery followed by numerous transplantations demonstrating cross-regulation between activating and inhibiting components distributed along the body axis. This experimental work inspired mathematicians, engineers, physicists and computer scientists to develop theoretical models predicting the principles of developmental mechanisms. Today, we know that the Wnt/β-catenin/Sp5/Zic4 gene regulatory network (GRN) links organizer activity, morphogenesis and cellular identity in Hydra, with variable conformations depending on the region or epithelial layer, and varied phenotypes depending on which GRN element is misregulated. In intact animals, Wnt/β-catenin signaling acts as the head activator at the tip of the hypostome, restricted by Sp5 in the other regions of the animal. Moreover, in the tentacle ring, Sp5 and Zic4 act epistatically to support tentacle differentiation and prevent basal disc differentiation. Along the body column, Sp5 is self-repressed in the epidermis and acts as a head inhibitor along the gastrodermis. Other players modulate these activities, such as TSP and Margin/RAX apically, Notch signaling in the tentacle zone, Dkk1/2/4 and HAS-7 in the body column. In the developmental context of regeneration, cells below the amputation zone switch from repressed to locally de novo activated head organizer status, a transition driven by immediate symmetrical and asymmetrical metabolic changes that lead to gene expression regulations involving components and modulators of Wnt/β-catenin signaling, early-pulse and early-late transient both often symmetrical, together with sustained ones, specific to head regeneration.

Self-organization from organs to embryoids by activin in early amphibian development

Embryonic development is a complex self-organizing process orchestrated by a series of regulatory events at the molecular and cellular levels, resulting in the formation of a fully functional organism. This review focuses on activin protein as a mesoderm-inducing factor and the self-organizing properties it confers. Activin has been detected in both unfertilized eggs and embryos, suggesting its involvement in early developmental processes. To explore its effects, animal cap cells-pluripotent cells from the animal pole of amphibian blastula-stage embryos-were treated with varying concentrations of activin. The results showed that activin induced mesodermal tissues, including blood, muscle, and notochord, in a dose-dependent manner. Co-treatment with activin and retinoic acid further promoted the development of kidney and pancreatic tissues, while activin alone stimulated the formation of beating cardiac tissue. In subsequent experiments, high concentrations of activin conferred an organizer-like activity on animal cap cells. The pretreatment duration affected outcomes: longer exposure induced anterior structures, such as eyes, while shorter exposure resulted in posterior structures, like tails. These findings reflect moderate self-assembly, where cells become increasingly organized. In another experiment, activin was used to create an artificial gradient. Explants cultured on this gradient developed into embryoids with well-defined anteroposterior, dorsoventral, and left-right axes, exemplifying higher-order self-organization. These results demonstrate that controlled activin gradients can drive the formation of nearly complete tadpole-like larvae, effectively recapitulating the processes of early embryogenesis. This system offers valuable insights into the mechanisms underlying axis formation and organogenesis, providing a promising platform for future research in developmental biology.

Celebrating the centennial of the most famous experiment in embryology: Hilde Mangold, Hans Spemann and the organizer

This Issue of Cells & Development celebrates the centennial of the Spemann-Mangold organizer experiment. This was the most famous experiment in embryology and its reverberations have greatly influenced developmental biology. This historical issue describes the impact of the discovery and is a prelude to the second volume of this Festschrift, which will consist of the proceedings of the international meeting to be held in Freiburg University, at the place where the organizer was discovered.