Patterning processes in aggregates of Hydra cells visualized with the monoclonal antibody, TS19

Cellular and Molecular Mechanisms of Hydra Regeneration

Regeneration of lost body parts is essential to regain the fitness of the organism for successful living. In the animal kingdom, organisms from different clades exhibit varied regeneration abilities. Hydra is one of the few organisms that possess tremendous regeneration potential, capable of regenerating complete organism from small tissue fragments or even from dissociated cells. This peculiar property has made this genus one of the most invaluable model organisms for understanding the process of regeneration. Multiple studies in Hydra led to the current understanding of gross morphological changes, basic cellular dynamics, and the role of molecular signalling such as the Wnt signalling pathway. However, cell-to-cell communication by cell adhesion, role of extracellular components such as extracellular matrix (ECM), and nature of cell types that contribute to the regeneration process need to be explored in depth. Additionally, roles of developmental signalling pathways need to be elucidated to enable more comprehensive understanding of regeneration in Hydra. Further research on cross communication among extracellular, cellular, and molecular signalling in Hydra will advance the field of regeneration biology. Here, we present a review of the existing literature on Hydra regeneration biology and outline the future perspectives.

Parameters of self-organization in Hydra aggregates

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 microm. 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.

Spontaneous symmetry breaking turing-type pattern formation in a confined Dictyostelium cell mass

We have discovered a new type of patterning which occurs in a two-dimensionally confined cell mass of the cellular slime mold Dictyostelium discoideum. Besides the longitudinal structure reported earlier, we observed a spontaneous symmetry breaking spot pattern whose wavelength shows similar strain dependency to that of the longitudinal pattern. We propose that these structures are due to a reaction-diffusion Turing instability similar to the one which has been exemplified by CIMA (chlorite-iodide-malonic acid) reaction. The present finding may exhibit the first biochemical Turing structure in a developmental system with a controllable boundary condition.

Patterning in hydra cell aggregates without the sorting of cells from different axial origins

Aggregates of Hydra cells were studied to find out how the primary centers that form new heads are generated in a system of cells in which the original pattern has been destroyed. Since cells that originate near the heads (apical cells) temporarily maintain a high level of head activation potential during aggregate formation and may contribute to pattern formation, their distribution in the aggregates was investigated. The mutual distances between labeled epithelial cells were followed by vitally staining apical cells with DAPI. The distribution was random during early regeneration stages (3, 6, 24 hr). These results show that epithelial cells originating from apical regions do not sort. That is, dynamic cell movement to generate rudiments of new heads is not necessary for head formation in aggregates. A possible explanation of the mechanism is discussed.

Cell sorting during the regeneration of Hydra from reaggregated cells

The role of cell sorting in the reorganization of Hydra cell reaggregates was studied. We quantitatively labeled ectodermal and endodermal cells by incubating whole animals in fluorescent beads or by injecting the beads into the gastric cavity. Beads were stably incorporated into the cells by phagocytosis. Our data show that dramatic cell sorting processes drive the formation of ectoderm and endoderm within the first 12 hr of reaggregation. After the ectoderm is established, no further rearrangement could be observed. We also tested the ability of cells to sort out with respect to their original position in Hydra by dissociating labeled apical and basal pieces of Hydra and measuring the clumping of labeled cells during reorganization. There was no increase in the clumping of cells during reorganization indicating that cell sorting is not involved in the formation of early activation centers. There was also no preferential incorporation of apically derived (presumptive head) tissue into tentacles that subsequently formed, indicating that after dissociation into single cells there is no predisposition of erstwhile presumptive head tissue to form heads.

Simultaneous effects of head activator on the dynamics of apical and basal regeneration in Hydra vulgaris (formerly Hydra attenuata)

Previous studies have demonstrated that head activator (HA), an 11 amino acid peptide, stimulates head-specific differentiation processes in hydra. Additionally, HA enhances the differentiation of interstitial cells into nerve cells. This study investigated the effects of exogenous synthetic HA on the dynamics of both apical and basal regeneration in a piece of tissue excised from the body column of treated animals which comprised one-eighth of the original animal. The dynamics of apical and basal regeneration were monitored using the monoclonal antibody TS19. This antibody binds to apical and basal ectodermal tissue very early in the process of regeneration, before morphological structures are evident. Labeling is ultimately localized to the tentacles of the head and a ring above the basal disc. Thus, TS19 is a useful tool for analyzing the dynamics of both apical and basal patterning processes in the same regenerate simultaneously. Quantification of TS19 positive areas on regenerates over a time course of 72 hr revealed that HA treatment accelerated and amplified the dynamics of both apical and basal TS19 labeling. The specific basal effect was novel and was demonstrated to occur in the absence of a determined head independently of new nerve cell differentiation. It is proposed that the basal effect was the result of growth factor-like activity of HA.