Parameters of self-organization inHydraaggregates

Abstract

Self-organization has been demonstrated in a variety of systems ranging from chemical-molecular to ecosystem levels, and evidence is accumulating that it is also fundamental for animal development. Yet, self-organization can be approached experimentally in only a few animal systems. Cells isolated from the simple metazoan Hydra can aggregate and form a complete animal by self-organization. By using this experimental system, we found that clusters of 5-15 epithelial cells are necessary and sufficient to form de novo head-organizing centers in an aggregate. Such organizers presumably arise by a community effect from a small number of cells that express the conserved HyBra1 and HyWnt genes. These local sources then act to pattern and instruct the surrounding cells as well as generate a field of lateral inhibition that ranges up to 1,000 m. We propose that conserved patterning systems in higher animals originate from extremely robust and flexible molecular self-organizing systems that were selected for during early metazoan evolution.A nimal development is commonly explained in terms of hierarchical genetic cascades that begin with an initial asymmetry based on either maternal or external cues. In Drosophila, for instance, complex cellular interactions during oogenesis eventually lead to the local deposition of maternal messages in the oocyte that determine the body axes (1, 2). In the Xenopus embryo, a pre-established animal-vegetal asymmetry in the egg and sperm entry point determine the location of the anterior-posterior and the dorsoventral axes by locally initiating genetic cascades that lead to the formation of the future organizer (3, 4). Less clear is how an organizer is set up in tissue without inherent asymmetry or external cues.Only a few animal systems are amenable to the experimental analysis of self-organization during development (5-7). The simple metazoan Hydra is particularly useful in this context, because its body plan and any positional information can be completely destroyed and re-established in dissociationreaggregation studies. Hydra consists of a single axis with a head and foot at either end of a tubular body column. The axial pattern of the animal is maintained by a gradient of head formation competence, commonly referred to as the head activation gradient (8), or source density gradient (9). The term gradient of head formation competence, or more simply gradient of head competence, is in line with current terminology and will be used in this work. (The term head activation will be used for the actual process of head formation.) This head competence gradient reflects the ability of tissue of the body column to form a head either on bisection leading to head regeneration at the apical end of the lower half, or on transplantation of a piece of the body column to the body column of a second animal. The gradient is maximal near the head decreasing down the body column and is a relatively stable property of the body column (8). Head formation in the body column is prevented by a head inhibition...

The Evolution of the Wnt Pathway

“…Brachyury is a direct downstream target of Wnt signaling during posterior growth of vertebrates (Yamaguchi et al 1999), where it directly regulates Wnt signaling Kimelman 2008, 2009). At which point in animal evolution this positive Brachyury-Wnt loop was established is unknown so far (Martin and Kimelman 2008), but the coexpression of Wnt and Brachyury in basal metazoans (Technau et al 2000) suggests that it was established during early metazoans evolution.…”

“…We therefore name it the "Hydra Organizer (HO)." A group of 10-50 ectodermal and endodermal epithelial cells constitutes this signaling center (Technau et al 2000), and when transplanted to an ectopic site, the HO induces a secondary body axis by recruiting host tissue (Browne 1909;Broun and Bode 2002).…”

The Wnt code: cnidarians signal the way

“…Hydra's head organizer is located at the apical tip of the hypostome and was described for the first time by Browne (1909). When this hypostomal tissue is transplanted to ectopic positions along the body column of a host polyp it organizes a secondary body axis by recruiting host tissue (Technau et al, 2000;Broun and Bode, 2002;Broun et al, 2005). The hypostome is homologous to the blastopore of the cnidarian embryo and can be compared to the vertebrate organizer based on a conserved set of genes expressed in both organizers.…”

“…Hydra polyps can be dissociated into suspensions of single cells so that the original polarity of the animal is completely disorganized (Technau et al, 2000). Within 24 h ectoderm and endoderm form, after that new head organizers form de novo, organizing the surrounding tissue into new body axes that separate into individual polyps (Technau et al, 2000;Holstein et al, 2003). Similar to bud formation and head regenerates, hytcf and hyb-cat are homogeneously expressed before hywnt3a in early reaggregates (24 h) and are later downregulated except in regions where new head organizers are forming.…”

“…Similar to bud formation and head regenerates, hytcf and hyb-cat are homogeneously expressed before hywnt3a in early reaggregates (24 h) and are later downregulated except in regions where new head organizers are forming. hywnt3a is first expressed in small clusters of 10-20 endodermal cells, corresponding to the minimal number of cells that can act as an organizer Technau et al, 2000). These spots of hywnt3a increase in size, and finally become localized to the tip of the hypostome of new polyps Technau et al, 2000), a kinetics that reflects the self-establishment and stabilization of a signalling centre.…”