The aberrant, a morphological mutant of Hydra attenuata, has altered inhibition properties

Formation of the head organizer in hydra involves the canonical Wnt pathway

Stabilization of β-catenin by inhibiting the activity of glycogen synthase kinase-3β has been shown to initiate axis formation or axial patterning processes in many bilaterians. In hydra, the head organizer is located in the hypostome, the apical portion of the head. Treatment of hydra with alsterpaullone, a specific inhibitor of glycogen synthase kinase-3β,results in the body column acquiring characteristics of the head organizer, as measured by transplantation experiments, and by the expression of genes associated with the head organizer. Hence, the role of the canonical Wnt pathway for the initiation of axis formation was established early in metazoan evolution.

Characterization of the head organizer in hydra

A central process in the maintenance of axial patterning in the adult hydra is the head activation gradient, i.e. the potential to form a secondary axis, which is maximal in the head and is graded down the body column. Earlier evidence suggested that this gradient was based on a single parameter. Using transplantation experiments, we provide evidence that the hypostome, the apical part of the head, has the characteristics of an organizer in that it has the capacity to induce host tissue to form most of the second axis. By contrast, tissue of the body column has a self-organizing capacity, but not an inductive capacity. That the inductive capacity is confined to the hypostome is supported by experiments involving a hypostome-contact graft. The hypostome, but not the body column, transmits a signal(s) leading to the formation of a second axis. In addition, variations of the transplantation grafts and hypostome-contact grafts provide evidence for several characteristics of the organizer. The inductive capacity of the head and the self-organizing capacity of the body column are based on different pathways. Head inhibition, yya signal produced in the head and transmitted to the body column to prevent head formation, represses the effect of the inducing signal by interfering with formation of the hypostome/organizer. These results indicate that the organizer characteristics of the hypostome of an adult hydra are similar to those of the organizer region of vertebrate embryos. They also indicate that the Gierer-Meinhardt model provides a reasonable framework for the mechanisms that underlie the organizer and its activities. In addition, the results suggest that a region of an embryo or adult with the characteristics of an organizer arose early in metazoan evolution.

Interactions between the foot and bud patterning systems in Hydra vulgaris

In the freshwater coelenterate, hydra, asexual reproduction via budding occurs at the base of the gastric region about two-thirds of the distance from the head to the foot. Developmental gradients of head and foot activation and inhibition originating from these organizing centers have long been assumed to control budding in hydra. Much has been learned over the years about these developmental gradients and axial pattern formation, and in particular, the inhibitory influence of the head on budding is well documented. However, understanding of the role of the foot and potential interactions between the foot, bud, and head patterning systems is lacking. The purpose of this study was to investigate the role of the foot in the initiation of new axis formation during budding by manipulating the foot and monitoring effects on the onset of first bud evagination and the time necessary to reach the 50% budding point. Several experimental situations were examined: the lower peduncle and foot (PF) were injured or removed, a second PF was laterally grafted onto animals either basally (below the budding zone) or apically (above the budding zone), or both the head and PF were removed simultaneously. When the PF was injured or removed, the onset of first bud evagination was delayed and/or the time until the 50% budding point was reached was longer. The effects were more pronounced when the manipulation was performed closer to the anticipated onset of budding. When PF tissue was doubled, precocious bud evagination was induced, regardless of graft location. Removal of the PF at the same time as decapitation reduced the inductive effect of decapitation on bud evagination. These results are discussed in light of potential signals from the foot or interactions between the foot and head patterning systems that might influence bud axis initiation.

HyBra1, a Brachyury homologue, acts during head formation in Hydra

A homologue of the T-box gene, Brachyury, has been isolated from hydra. The gene, termed HyBra1, is expressed in the endoderm and is associated with the formation of the hypostome, the apical part of the head in four different developmental situations. In adults, which are continuously undergoing patterning, HyBra1 is continuously expressed in the hypostome. During budding, hydra's asexual form of reproduction, the gene is expressed in a small area that will eventually form the hypostome of the developing bud before any morphological sign of budding is apparent. The gene is also expressed very early during head regeneration and is confined to the region that will form the hypostome. During embryogenesis, HyBra1 is expressed shortly before hatching in the region that will form the apical end of the animal, the hypostome. The absence of expression at the apical end of decapitated animals of reg-16, a head formation-deficient mutant, provides additional evidence for a role of HyBra1 during head formation. Further, treatments that alter the head activation gradient have no effect on HyBra1 expression indicating the role of the gene is confined to head formation. Transplantation experiments indicate that the expression occurs before head determination has occurred, but expression does not irreversibly commit tissue to forming a head. A comparison of the function of the Brachyury homologues suggests an evolutionary conservation of a molecular mechanism that has been co-opted for a number of developmental processes throughout evolution.

A symbiogenetic theory for the origins of cnidocysts in Cnidaria

Did cnidarian cnidocysts originate from cnidocyst-bearing protoctistans living as symbiotic partners with an epithelial placula? If an increase in the fitness of symbiotic partners was "locked in" by an evolutionary stable strategy, co-evolution and compartmentalization could have led phyletically separate, eukaryotic symbionts to fuse and undergo nuclear merger. Traits originating in the symbiotic partners would have been brought to the "synthetic" organism and reworked through evolution into the development of an integrated organism. Support for the theory of symbiogenetic origins of Cnidaria rests on traces of symbiosis detected in the relationship of cnidarian epithelium to interstitial cells (I-cells), the precursors of cnidocyst-producing cnidoblasts: (1) epithelium and I-cell are autonomous and differ in morphology, cellular dynamics, the relationship of differentiation to proliferation and the variety of cell types formed; (2) hydras and planulas can be "cured" of I-cells and their derivatives, thereby creating "epithelial" animals which lack responsiveness but retain vegetative properties. (3) The reintroduction of I-cells into "epithelial" animals which lack responsiveness but retain vegetative properties. (3) The reintroduction of I-cells into "epithelial" animals restores missing differentiated cell and organismic characteristics. Symbiogenesis as a source of metazoan species has consequences for concepts of development, from the origins of cell lines to the evolution of differentiation.

Perturbations in morphogen gradients induce budding in hydra

Lateral grafting of small pieces of midregion tissue into different levels of the hydra body column was done to assess the influence of the host hypostome and basal disc (or, of the underlying morphogenetic gradients) in inducing secondary structures in the transplanted tissue; and also to identify the role, if any, of the induced secondary structures (or, perturbed morphogen gradients) on the pattern of the host. The same midpiece tissue differentiated to a basal disc when grafted near the host hypostome, and to a small hypostome with tentacles when grafted near the host basal disc. Chimeras with induced secondary basal discs showed a phenomenal increase in budding compared to the controls and to the chimeras having induced hypostomes. These results indicate a positive cross-reaction between both organizing regions during patterning in hydra.

Cell cycle length, cell size, and proliferation rate in hydra stem cells

We have analyzed the cell cycle parameters of interstitial cells in Hydra oligactis. Three subpopulations of cells with short, medium, and long cell cycles were identified. Short-cycle cells are stem cells; medium-cycle cells are precursors to nematocyte differentiation; long-cycle cells are precursors to gamete differentiation. We have also determined the effect of different cell densities on the population doubling time, cell cycle length, and cell size of interstitial cells. Our results indicate that decreasing the interstitial cell density from 0.35 to 0.1 interstitial cells/epithelial cell (1) shortens the population doubling time from 4 to 1.8 days, (2) increases the [3H]thymidine labeling index from 0.5 to 0.75 and shifts the nuclear DNA distribution from G2 to S phase cells, and (3) decreases the length of G2 in stem cells from 6 to 3 hr. The shortened cell cycle is correlated with a significant decrease in the size of interstitial stem cells. Coincident with the shortened cell cycle and increased growth rate there is an increase in stem cell self-renewal and a decrease in stem cell differentiation.

Putative intermediates in the nerve cell differentiation pathway in hydra have properties of multipotent stem cells

We have investigated the properties of nerve cell precursors in hydra by analyzing the differentiation and proliferation capacity of interstitial cells in the peduncle of Hydra oligactis, which is a region of active nerve cell differentiation. Our results indicate that about 50% of the interstitial cells in the peduncle can grow rapidly and also give rise to nematocyte precursors when transplanted into a gastric environment. If these cells were committed nerve cell precursors, one would not expect them to differentiate into nematocytes nor to proliferate apparently without limit. Therefore we conclude that cycling interstitial cells in peduncles are not intermediates in the nerve cell differentiation pathway but are stem cells. The remaining interstitial cells in the peduncle are in G1 and have the properties of committed nerve cell precursors (Holstein and David, 1986). Thus, the interstitial cell population in the peduncle contains both stem cells and noncycling nerve precursors. The presence of stem cells in this region makes it likely that these cells are the immediate targets of signals which give rise to nerve cells.

Neuron differentiation in hydra involves dividing intermediates

The neuron differentiation pathway in hydra is usually assumed to be the following. A multipotent stem cell among the large interstitial cells becomes committed to neuron differentiation and divides. The two daughter cells, which are postmitotic small interstitial cells, subsequently differentiate into neurons. Herein the neuron pathway of the lower peduncle of Hydra oligactis was examined in some detail. In this region a substantial amount of neuron differentiation takes place, but very few large interstitial cells are present. It was found that small interstitial cells, which are capable of dividing, differentiate into neurons. The minimum time required to traverse the pathway from S phase of the last proliferating intermediate to a neuron is 18 hr. Thus, the neuron differentiation pathway in the lower peduncle involves dividing intermediates and is therefore more complex than usually assumed. Evidence for dividing small interstitial cells in the head, where the highest rate of neuron differentiation occurs, suggests that this more complex pathway may be common to all regions of the animal. A consequence of this finding is that the body of evidence concerning the commitment of multipotent stem cells to neurons and the control of this commitment requires reinterpretation.

Pattern formation in hydra tissue without developmental gradients

Ring-shaped pieces of hydra tissue were excised from a specified position on a body column of 20-30 polyps and grafted together in tandem like a chain of beads. A "tandem graft" prepared in this way has the same basic tissue organization and same tube-like morphology as a normal hydra body column, but lacks the head, foot, and developmental gradients ordinarily present. Three major types of structures were formed along the length of the tandem graft: heads, buds, and feet. The relative number of these structures produced was strongly affected by the origin of the tissue used to prepare the tandem graft. Evidence was obtained which suggests that tissue originally located outside of the budding zone in intact hydra has a strong latent capacity to form a bud, and that the level of this capacity forms a gradient from the budding zone toward the hypostome. Evidence was also obtained which is consistent with the view that the head and foot forming mechanisms cross-react positively, increasing the chances for these two structures to be formed next to each other on a tandem graft.

Patterning of the head in hydra as visualized by a monoclonal antibody, II. The initiation and localization of head structures in regenerating pieces of tissue

The body column of hydra is polarized such that a new head will regenerate from the apical end when both extremities are removed. This is due to a graded property of the tissue termed the head activation gradient. The aim of the experiments presented here was to determine what events connect a two-dimensional segment of the activation gradient in an isolated piece of tissue with the formation of a head structure at a particular location. To this end, tissue pieces with three different shapes were excised and analyzed during and after regeneration. The most apical tissue of each piece was labeled with the DNA-intercalating dye, DAPI, and the area where developmental changes were occurring was monitored using the monoclonal antibody CP8 (Javois et al., 1986). First, it was shown that polarity of regeneration was maintained regardless of the fraction of body length included in the excised pieces. Second, while head structures usually formed from the original apical tissue, they could be located anywhere in the regenerate. This was an effect of the healing process which shaped the apical edge differently in different pieces. Third, early CP8 binding occurred in similarly shaped areas suggesting that patterning events were initiated in a contiguous manner wherever apical tissue was located. And finally, not all of the CP8-marked tissue successfully formed structures. Apparently some regions were favored to continue the patterning process, and these in turn extinguished the process in neighboring regions.

Gland cells arise by differentiation from interstitial cells in Hydra attenuata

The origin of the gland cells in asexually reproducing adult hydra is unclear. There is evidence suggesting that the gland cells are a self-renewing population as well as contrary evidence suggesting that they must arise from another cell type. We have reexamined the question and found the latter to be the case. Analysis of ectoderm/endoderm chimeras in which the ectoderm was labeled with [3H]thymidine indicates a precursor for gland cells in the ectoderm which migrates into the endoderm. Analysis of grafts between labeled lower halves and unlabeled upper halves of animals indicates the migratory precursor is either a large or a small interstitial cell. Measurement of the cell cycle times of the gland cells and the epithelial cells provided further support. The cell cycle time of the gland cells appears to be longer than that of the epithelial cells of the endoderm throughout the animal. This means that in the steady-state growth condition of hydra tissue, the gland cells cannot maintain their population size simply by cell division. These results and other data suggest the following dynamics for the gland cell population. Gland cells arise by differentiation from large interstitial cells, undergo a limited number of cell divisions, and then become postmitotic.

Selective disruption of gap junctional communication interferes with a patterning process in hydra

The cells that make up the body column of hydra are extensively joined by gap junctions, capable of mediating the rapid exchange of small hydrophilic molecules between the cytoplasms of neighboring cells. Both the rate of transfer of small molecules through the gap junctions and the rate of return of gap junction coupling after grafting experiments are sufficiently rapid to mediate events in the patterning of hydra tissue. Antibodies to the major rat liver gap junction protein (27,000 daltons) recognize a gap junction antigen in hydra and are effective in eliminating junctional communication between hydra cells. The antibodies perturb the head inhibition gradient in grafting operations, suggesting that cell-cell communication via gap junctions is important in this defined tissue patterning process.

Patterning of the head in hydra as visualized by a monoclonal antibody. I. Budding and regeneration

A monoclonal antibody, CP8, has been isolated which displays a position-specific binding pattern to epithelial cells of Hydra oligactis. Antibody binding is restricted to the head of adult animals. When a new head develops during the budding process, CP8 binding is present in the area which will form the head well before morphological signs of it. Similarly, following decapitation as a new head regenerates, CP8 label appears covering a domed area at the apical end of the regenerate before tentacles evaginate delineating the head. When bud development or regeneration is complete, CP8 label is restricted to the new head. Experiments indicate the appearance of CP8 label during the formation of a head correlates closely with the patterning events which result in the determination of the tissue to form a head. The usefulness of CP8 as a diagnostic tool for exploring the dynamics of head pattern formation in hydra is discussed.

Genetic analysis of developmental mechanisms in hydra. X. Morphogenetic potentials of a regeneration-deficient strain (reg-16)

Morphogenetic potentials of hydra tissue involved in head or foot formation were examined in a standard wild-type strain (105) and a mutant strain (reg-16) which has a very low head regenerative but a nearly normal foot regenerative capacity (T. Sugiyama and T. Fujisawa, 1977, J. Embryol. Exp. Morphol. 42, 65-77). Hydra tissue has two types of morphogenetic potentials to control head formation: the potential to form head structure (head-activation potential) and the potential to inhibit head formation (head-inhibition potential). It also has two types of morphogenetic potentials to control foot formation: foot-activation and foot-inhibition potentials. A lateral tissue grafting procedure (G. Webster and L. Wolpert, 1966, J. Embryol. Exp. Morphol. 16, 91-104), was used to examine and compare the relative levels of these potentials in the normal and the mutant strains. The potential levels were examined along the body axis of the intact animals and also in the regenerating animals after head removal. The results obtained show that the potentials involved in head formation are highly abnormal, whereas the potentials involved in foot formation are apparently normal in the mutant strain (reg-16). This suggests that the abnormal potentials are related in some way to, and may be responsible for, the reduced head regenerative capacity in the mutant strain reg-16.

Migration of I-cells from ectoderm to endoderm in Hydra attenuata Pall (Cnidaria, Hydrozoa) and their subsequent differentiation

The cellular composition of isolated ecto- and endoderm of the gastric column of Hydra attenuata Pall were recorded qualitatively and quantitatively. The endoderm contains a small population of I-cells ("basal cells") which give rise to the endodermal neurons. The recombination of live ecto- and endoderm, one of which had previously been [3H]thymidine labeled, revealed that the endodermal I-cells and their neural derivatives originate from ectodermal I-cells which migrate across the mesoglea. No other cell types were found to pass from one cell layer to the other. The experiments support the idea that the endodermal gland cells constitute an autoreproductive cell line independent of the pluripotent I-cells.

Genetic analysis of developmental mechanisms in hydra. IX. Effect of food on development of a slow-budding strain (L4)

A mutant hydra strain L4 produces buds at a much lower rate than the standard wild-type strain (105) when fed with brine shrimp nauplii (T. Sugiyama and T. Fujisawa, 1979b, Dev. Growth Differ. 21, 361-375). It was found that addition of a small amount of tubifex worm tissue to the normal brine shrimp diet significantly improved the budding rate of L4 but not of 105. Detailed examination and comparison of food effect on various developmental processes in L4 and 105 have provided the following observations. (1) L4 development is strongly affected by food and has significantly lower rates than 105 in all the developmental parameters examined which involve the bud initiation process. In contrast, such effects and differences are not observed in parameters not involving bud initiation. These observations suggest that L4 has a defect(s) in its bud initiating mechanisms and that the expression of this defect is somehow strongly affected by food. (2) Epithelial cells proliferate in L4 nearly as rapidly as in 105 and without food effect. Epithelial cells produced by cell division are mostly utilized to form buds in mature 105 polyps. These cells, however, are not fully utilized for this purpose in L4 polyps which bud very slowly. Instead, they appear to be somehow lost from tissue in these animals.