Hydra are remarkable because they are immortal. Much of immortality can be ascribed to the asexual mode of reproduction by budding, which requires a tissue consisting of stem cells with continuous self‐renewal capacity. Emerging novel technologies and the availability of genomic resources enable for the first time to analyse these cells in vivo. Stem cell differentiation in Hydra is governed through the coordinated actions of conserved signaling pathways. Studies of stem cells in Hydra, therefore, promise critical insights (...) of general relevance into stem cell biology including cellular senescence, lineage programming and reprogramming, the role of extrinsic signals in fate determination and tissue homeostasis, and the evolutionary origin of these cells. With these new facts as a backdrop, this review traces the history of studying stem cells in Hydra and offers a view of what the future may hold. (shrink)

The convergence of morphogenesis into viable organisms under variable conditions suggests closed‐loop dynamics involving multiscale functional feedback. We develop the idea that morphogenesis is based on synergy between mechanical and bio‐signaling processes, spanning all levels of organization: molecular, cellular, tissue, up to the whole organism. This synergy provides feedback within and between all levels of organization, to close the loop between the dynamics of the morphogenesis process and its robust functional outcome. Hydra offer a powerful platform to explore this (...) direction, thanks to their simple body plan, extraordinary regeneration capabilities, and the accessibility and flexibility of their tissues. Our recent experiments show that structural inheritance of the actomyosin organization directs body‐axis formation during Hydra regeneration. Morphogenesis is then stabilized through dynamic cytoskeletal reorganization induced by the inherited structure. The observed cytoskeletal stability and reorganization capabilities suggest that mechanical feedback integrates with biochemical processes to establish viable patterns and ensure canalization. (shrink)

Scholarship has grown increasingly nuanced in its grappling with the intersections of religion, science, and technology but requires a new paradigm. Contemporary approaches to specific technologies reveal a wide variety of perspectives but remain too often committed to typological classification. To be vigilant of our obligation to understand and reveal, scholars in the study of religion, science, and technology can adopt a hydra‐logical stance: we can recognize that there are cultural monsters possessing scientific, technological, and religious heads. These heads (...) may work with a common agenda or they might not. They might disagree, pulling their shared body back and forth in a public commotion that lays waste to their surroundings. They might see past one another or move in tandem—purposively or not. Evaluations of climate response and AI benefit from seeing how the various heads are inseparable: indeed, cutting one off simply promotes the growth of new heads. Methodological and analytical clarity, therefore, emerges in the transition from schemes of classification to the recognition of hydras. (shrink)

Deianira complains that Hercules, as a slave of Omphale, did not refrain from telling to the Lydian queen his famous labours; among them, the Hydra: quaeque redundabat fecundo vulnere serpens fertilis et damnis dives ab ipsa suis ‘It will be admitted that redundabat, which usually means to “overflow”’ can only be applied to the Hydra by a very strong metaphor; but it is not only a strong one, it is quite unexampled: so A. Palmer in The Academy 49 (...) , 160. But his own emendation rebellabat has not convinced anybody – rightly, as I think. Still, his remark was not otiose: redundabat refers to the ‘growing again’ of Hydra's heads, that seem to ‘spring’ from the wounds like water's spurts; it is not impossible, but nevertheless it is strange. Why does Ovid use this metaphor ‘a fluctibus desumpta’ here? I suggest that there is a precise reason: Ovid does not mention the name of the serpens, but of course his reader knows it and also knows why the Hydra is so called: πò τν δτων. (shrink)

The introductory personal remarks refer to my motivations for choosing research projects, and for moving from physics to molecular biology and then to development, with Hydra as a model system. Historically, Trembley’s discovery of Hydra regeneration in 1744 was the begin¬ning of developmental biology as we understand it, with passionate debates about preformation versus de novo generation, mechanisms versus organisms. In fact, seemingly conflicting bottom-up and top-down concepts are both required in combination to understand development. In modern terms, (...) this means analysing the molecules involved, as well as searching for physical principles underlying development within systems of molecules, cells and tissues. During the last decade, molecular biology has provided surprising and impressive evidence that the same types of mol¬ecules and molecular systems are involved in pattern formation in a wide range of organisms, including coelenterates like Hydra, and thus appear to have been “invented” early in evolution. Likewise, the features of certain systems, especially those of developmental regulation, are found in many different organisms. This includes the generation of spatial structures by the interplay of self-enhancing activation and “lateral” inhibitory effects of wider range, which is a main topic of my essay. Hydra regeneration is a particularly clear model for the formation of defined patterns within initially near-uniform tissues. In conclusion, this essay emphasizes the analysis of development in terms of physical laws, including the application of mathematics, and insists that Hydra was, and will continue to be, a rewarding model for understanding general features of embryogenesis and regeneration. -/- -/- . (shrink)

Cells isolated from this freshwater polyp can aggregate and form a complete new animal. Experiments with the system lend support to a physico-chemical scheme for the creation of biological pattern.

The microbiome of human hair follicles (HFs) has emerged as an important player in different HF and skin pathologies, yet awaits in‐depth exploration. This raises questions regarding the tightly linked interactions between host environment, nutrient dependency of host‐associated microbes, microbial metabolism, microbe‐microbe interactions and host immunity. The use of simple model systems facilitates addressing generally important questions and testing overarching, therapeutically relevant principles that likely transcend obvious interspecies differences. Here, we evaluate the potential of the freshwater polyp Hydra, to (...) dissect fundamental principles of microbiome regulation by the host, that is the human HF. In particular, we focus on therapeutically targetable host‐microbiome interactions, such as nutrient dependency, microbial interactions and host defence. Offering a new lens into the study of HF – microbiota interactions, we argue that general principles of how Hydra manages its microbiota can inform the development of novel, microbiome‐targeting therapeutic interventions in human skin disease. (shrink)

Almost nobody favors long consent forms for prospective research subjects. Almost everybody thinks they interfere with informed consent’s purpose—good decisions. Nevertheless, almost everybody believes consent forms have long been getting longer. Years ago, Paul Appelbaum lamented the “tendency to cram ever more information into consent forms.”1 Weeks ago, Ilene Albala and her colleagues (one of them Appelbaum) reported in IRB: Ethics & Human Research that the length of one institutional review board’s forms “increased roughly linearly by an average of 1.5 (...) pages per decade. In the 1970s, the average consent form was less than one page long and often only a paragraph or two, but by the mid-1990s the .. (shrink)

We define a direct translation from finite rooted trees to finite natural functions which shows that the Worm Principle introduced by Lev Beklemishev is equivalent to a very slight variant of the well-known Kirby-Paris' Hydra Game. We further show that the elements in a reduction sequence of the Worm Principle determine a bad sequence in the well-quasi-ordering of finite sequences of natural numbers with respect to Friedman's gapembeddability. A characterization of gap-embeddability in terms of provability logic due to Lev (...) Beklemishev is also presented. (shrink)

This article is the introduction of the volume The Many-Headed Hydra. The Hidden History of the Revolutionary Atlantic, to be published by Amsterdam, a historical investigation of the multi-ethnique class which formed the cheap workforce which made possible the rising of capitalism and of modern transatlantic economy, since the beginning of 17th century. A motley crowd made by merchants, pirates, workers, women, soldiers, convicted criminals, religious radicals, etc. developed forms of resistence and mutual cooperations, in order to escape the (...) deep exploitation of the workforce, taking place in an economy already globalised. The authors tell the story of the role played by these wretched of modern world and of their fight for liberty. (shrink)

• Aggregates of previously isolated cells of Hydra are capable, under suitable solvant conditions, of regeneration forming complete animals. In a first stage, ecto- and endodermal cells sort out, producing the bilayered hollow structure characteristic of Hydra tissue; thereafter, heads are formed (even if the original cell preparation contained no head cells), eventually leading to the separation of normal animals with head, body column and foot. Hydra appears to be the highest type of organism that allows for (...) regeneration of the entire structure from random cell aggregates. The system is particularly useful for studying cell interactions, tissue polarity, pattern formation, and cell differentiation. (shrink)

The body wall of Hydra is organized as an epithelial bilayer with an intervening extracellular matrix (ECM). Molecular and biochemical analyses of Hydra ECM have established that it contains components similar to those seen in more complicated vertebrates such as human. In terms of biophysical parameters, Hydra ECM is highly flexible; a property that facilitates continuous movements along the organism's longitudinal and radial axis. A more rigid ECM, as in vertebrates, would not be compatible with this degree (...) of movement. The flexible nature of Hydra ECM can now be explained in part by the unique structure of the organism's collagens. Interestingly, some aspects of the structural features of Hydra collagens mimic what is seen in Ehlers‐Danlos syndrome, an inherited condition in humans that results in an abnormally flexible ECM that can be debilitating in extreme cases. This review will focus on structure–function relationships of the ECM of Hydra. BioEssays 23:716–724, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Ever since it was discovered in hydra, regeneration has remained a stimulating question for developmental biologists. Cellular approaches have revealed that, within the first few hours of apical or basal hydra regeneration, differentiation and determination of nerve cells are the primary cellular events detectable. The head and foot activators (HA, FA), neuropeptides that are released upon injury, are signaling molecules involved in these processes. In conditions where it induces cellular differentiation or determination, HA behaves as an agonist of (...) the cyclic AMP (cAMP) pathway involving the modulation of CREB nuclear transcription factor activity. This cascade would be required for proper regeneration, regardless of whether the polarity involved is apical or basal. Modulations of the protein kinase C pathway, which have been shown to affect apical or basal positional values, might signal to bring about this polarity; however, endogenous ligands responsible for this modulation are as yet unknown. (shrink)

Here we evaluate our current understanding of the function of the nervous system in Hydra, a non‐bilaterian animal which is among the first metazoans that contain neurons. We highlight growing evidence that the nervous system, with its rich repertoire of neuropeptides, is involved in controlling resident beneficial microbes. We also review observations that indicate that microbes affect the animal's behavior by directly interfering with neuronal receptors. These findings provide new insight into the original role of the nervous system, and (...) suggest that it emerged to orchestrate multiple functions including host‐microbiome interactions. The excitement of future research in the Hydra model now relies on uncovering the common rules and principles that govern the interaction between neurons and microbes and the extent to which such laws might apply to other and more complex organisms. (shrink)

Common Law Theory of Judicial Review: The Living Tree, Wil Waluchow neatly sidesteps the critique of judicial review based on the con- tention that constitutional rights are unacceptably indeterminate by arguing that it is this very indeterminacy that makes a common law method of legal interpretation appropriate. However, his contention that judges are able to ‘discover’ the underlying ‘authentic’ moral views of citizens is insufficiently grounded to meet the objection that common law reasoning utilising such unspecific material will result in (...) controversial decisions that reflect the val- ues and experience of an unrepresentative social and professional elite. He also ignores the important fact that electoral democracy is partly about the protection of vulnerable majorities against powerful minorities. However, Waluchow’s approach could be rendered more acceptable where there is the practical possibility of effective legislative on law style decision-making in the arena of human rights.Resumen:En su libro A Common Law Theory of Judicial Review: The Living Tree, Wil Waluchow de manera elegante y sutil evita la crítica al judicial review con el argumento de que los derechos constitucionales son irremediablemen te indeterminados y sostiene que precisamente por este aspecto de inde terminación un método basado en el common law es adecuado para la in terpretación de los derechos. Sin embargo, su postura de que los jueces son capaces de “descubrir” las perspectivas morales “auténticas” de los ciudadanos no está bien fundamentada como para hacer frente a la crítica de que el método del common law, al utilizar dicho material poco específico, da como resultado decisiones controvertidas que reflejan los valores y experiencia de una elite profesional y social no representativa. También el argumento de Waluchow ignora que la democracia electoral tiene como objetivo proteger a mayorías vulnerables de poderosas minorías. No obstante lo anterior, la tesis de Waluchow puede ser más viable donde exista la posibilidad práctica de una revisión efectiva de las leyes a través de las decisiones propias del método del common law en el ámbito de los derechos humanos. (shrink)

It has been convincingly shown that peptides play important roles in the regulation and maintenance of a variety of tissues and organs in living animals. However, little is known concerning the potential role of peptides as signaling molecules in developmental processes. In Hydra, there is circumstantial evidence that small diffusible molecules act as morphogens in the regulation of patterning processes. In order to view the entire spectrum of peptide signaling molecules, we initiated a project aiming at the systematic identification (...) of peptide signaling molecules in Hydra. In this review, we describe three peptide signaling molecules and one family of peptides that function as signaling molecules in the processes of axial pattern formation and neuron differentiation in Hydra. These peptides are produced by epithelial cells and are therefore termed “epitheliopeptides”. We discuss the importance of epitheliopeptides in developmental processes within a subset of hydrozoans. (shrink)

We first note that Gentzen's proof-reduction for his consistency proof of PA can be directly interpreted as moves of Kirby-Paris' Hydra Game, which implies a direct independence proof of the game . Buchholz's Hydra Game for labeled hydras is known to be much stronger than PA. However, we show that the one-dimensional version of Buchholz's Game can be exactly identified to Kirby-Paris' Game , by a simple and natural interpretation . Jervell proposed another type of a combinatorial game, (...) by abstracting Gentzen's proof-reductions and showed that his game is independent of PA. We show that this Jervell's game is actually much stronger than PA, by showing that the critical ordinal of Jervell's game is φω = ϵ0) in the Veblen hierarchy of ordinals. (shrink)

In 1975 E. O. Wilson called for biologists to appropriate ethics.1 Few philosophers worried deeply about this potential usurpation because they felt firmly ensconced on the other side of the Humean wall from the biologists. Science can provide neither guidance nor values. Perhaps nowhere is this more clear than in the crowning question of ethics; namely, what is the meaning of life? Since evolution proposes an ateleological account of the natural world, biologists can dismiss the question to which we all (...) desperately want an answer as a category mistake. If pressed to understand the question metaphorically, biologists begrudgingly reply that the meaning of life is to get as much of your genetic material. (shrink)

We shall give a direct proof of the independence result of a Buchholz style-Hydra Game on labeled finite trees. We shall show that Takeuti-Arai's cut-elimination procedure of $(\Pi^{1}_{1}-CA) + BI$ and of the iterated inductive definition systems can be directly expressed by the reduction rules of Buchholz's Hydra Game. As a direct corollary the independence result of the Hydra Game follows.

En este breve comentario discuto algunos aspectos de la interpretación de la epistemología de Davidson que sugiere Willian Duica en su reciente libro. Luego de una presentación somera del libro me centro en tres asuntos centrales de la interpretación de Duica. En primer lugar, argumento que su lectura de la crítica de Davidson al dualismo esquema/contenido es muy restrictiva y deja abierta la posibilidad de un realismo directo empirista. En segundo lugar, argumento que en su lectura el propio Duica se (...) compromete inadvertidamente con un empirismo de este tipo y, de este modo, su interpretación entra en tensión con el coherentismo de Davidson. Finalmente, discuto algunos aspectos de la interpretación que hace Duica de la tesis davidsoniana de la triangulación. In this short comment I discuss some aspects of William Duica's interpretation of Davidson's epistemology in a recent book. After a brief review of the book, I focus on three central issues of Duica's interpretation. First, I argue that his reading of Davidson's criticism of the scheme/content dualism is too restrictive and leaves open the possibility of an empiricist direct realism. Second, I argue in his reading Duica inadvertently commits himself to an empiricism of this sort and, as a result, his interpretation is in tension with Davidson's own coherentism. Finally, I discuss some aspects of Duica's interpretation of Davidsonian triangulation. (shrink)

In Diderot’s philosophy, the nature of the eighteenth-century, Isis veiled, is constituted of the same substance as the metaphor, the analogy and the hieroglyph. To show that, this article takes into consideration the naturalistic inquiry on Trembley’s Hydra. This animal, which is at the heart of the philosophical interest of the period, seems to shape itself starting from the mythological imagination, but at the same time it becomes the model that, for Diderot, defines the faculty of thinking and its (...) features. The reversibility between word and image, nature and culture, representation and the represented thing provides the observation point on a philosophy – Diderot’s philosophy – that develops itself along with the human physiology and imagination, its own faculty. (shrink)

Understanding aging and how it affects an organism's lifespan is a fundamental problem in biology. A hallmark of aging is stem cell senescence, the decline of functionality, and number of somatic stem cells, resulting in an impaired regenerative capacity and reduced tissue function. In addition, aging is characterized by profound remodeling of the immune system and a quantitative decline of adequate immune responses, a phenomenon referred to as immune‐senescence. Yet, what is causing stem cell and immune‐senescence? This review discusses experimental (...) studies of potentially immortal Hydra which have made contributions to answering this question. Hydra transcription factor FoxO has been shown to modulate both stem cell proliferation and innate immunity, lending strong support to a role of FoxO as critical rate‐of‐aging regulator from Hydra to human. Constructing a model of how FoxO responds to diverse environmental factors provides a framework for how stem cell factors might contribute to aging. (shrink)

The paper addresses the formation of striking patterns within originally near-homogenous tissue, the process prototypical for embryology, and represented in particularly purist form by cut sections of hydra regenerating, by internal reorganisation of the pre-existing tissue, a complete animal with head and foot. The essential requirements are autocatalytic, self-enhancing activation, combined with inhibitory or depletion effects of wider range – “lateral inhibition”. Not only de-novo-pattern formation, but also well known, striking features of developmental regulation such as induction, inhibition, and (...) proportion regulation can be explained on this basis. The theory provides a mathematical recipe for the construction of molecular models with criteria for the necessary non-linear interactions. It has since been widely applied to different developmental processes. (shrink)

While many different mechanisms contribute to the generation of spatial order in biological development, the formation of morphogenetic fields which in turn direct cell responses giving rise to pattern and form are of major importance and essential for embryogenesis and regeneration. Most likely the fields represent concentration patterns of substances produced by molecular kinetics. Short range autocatalytic activation in conjunction with longer range “lateral” inhibition or depletion effects is capable of generating such patterns (Gierer and Meinhardt, 1972). Non-linear reactions are (...) required, and mathematical criteria were derived to design molecular models capable of pattern generation. The classical embryological feature of proportion regulation can be incorporated into the models. The conditions are mathematically necessary for the simplest two-factor case, and are likely to be a fair approximation in multi-component systems in which activation and inhibition are systems parameters subsuming the action of several agents. Gradients, symmetric and periodic patterns, in one or two dimensions, stable or pulsing in time, can be generated on this basis. Our basic concept of autocatalysis in conjunction with lateral inhibition accounts for self-regulatory biological features, including the reproducible formation of structures from near-uniform initial conditions as required by the logic of the generation cycle. Real tissue form, for instance that of budding Hydra, may often be traced back to local curvature arising within an initially relatively flat cell sheet, the position of evagination being determined by morphogenetic fields. Shell theory developed for architecture may also be applied to such biological processes. (shrink)

.A term rewrite system is called simply terminating if its termination can be shown by means of a simplification ordering. According to a result of Weiermann, the derivation length function of any simply terminating finite rewrite system is eventually dominated by a Hardy function of ordinal less than the small Veblen ordinal. This bound had appeared to be of rather theoretical nature, because all known examples had had multiple recursive complexities, until recently Touzet constructed simply terminating examples with complexities beyond (...) multiple recursion. This was established by simulating the Hydra battle for all ordinal segments below the proof-theoretic ordinal of Peano arithmetic. By extending this result to the small Veblen ordinal we prove the huge bound of Weiermann to be sharp. As a spin-off we can show that total termination allows for complexities as high as those of simple termination. (shrink)

The Liar paradox raises foundational questions about logic, language, and truth (and semantic notions in general). A simple Liar sentence like 'This sentence is false' appears to be both true and false if it is either true or false. For if the sentence is true, then what it says is the case; but what it says is that it is false, hence it must be false. On the other hand, if the statement is false, then it is true, since it (...) says (only) that it is false. -/- How, then, should we classify Liar sentences? Are they true or false? A natural suggestion would be that Liars are neither true nor false; that is, they fall into a category beyond truth and falsity. This solution might resolve the initial problem, but it beckons the Liar's revenge. A sentence that says of itself only that it is false or beyond truth and falsity will, in effect, bring back the initial problem. The Liar's revenge is a witness to the hydra-like nature of Liars: in dealing with one Liar you often bring about another. -/- JC Beall presents fourteen new essays and an extensive introduction, which examine the nature of the Liar paradox and its resistance to any attempt to solve it. Written by some of the world's leading experts in the field, the papers in this volume will be an important resource for those working in truth studies, philosophical logic, and philosophy of language, as well as those with an interest in formal semantics and metaphysics. (shrink)

Communication plays a vital and unique role in society-often blamed for problems when it breaks down and at the same time heralded as a panacea for human relations. A sweeping history of communication, _Speaking Into the Air_ illuminates our expectations of communication as both historically specific and a fundamental knot in Western thought. "This is a most interesting and thought-provoking book.... Peters maintains that communication is ultimately unthinkable apart from the task of establishing a kingdom in which people can live (...) together peacefully. Given our condition as mortals, communication remains not primarily a problem of technology, but of power, ethics and art." —Antony Anderson, _New Scientist_ "Guaranteed to alter your thinking about communication.... Original, erudite, and beautifully written, this book is a gem." —_Kirkus Reviews_ "Peters writes to reclaim the notion of authenticity in a media-saturated world. It's this ultimate concern that renders his book a brave, colorful exploration of the hydra-headed problems presented by a rapid-fire popular culture." —_Publishers Weekly_ What we have here is a failure-to-communicate book. Funny thing is, it communicates beautifully.... _Speaking Into the Air_ delivers what superb serious books always do-hours of intellectual challenge as one absorbs the gradually unfolding vision of an erudite, creative author." —Carlin Romano, _Philadelphia Inquirer_. (shrink)

Communication plays a vital and unique role in society-often blamed for problems when it breaks down and at the same time heralded as a panacea for human relations. A sweeping history of communication, _Speaking Into the Air_ illuminates our expectations of communication as both historically specific and a fundamental knot in Western thought. "This is a most interesting and thought-provoking book.... Peters maintains that communication is ultimately unthinkable apart from the task of establishing a kingdom in which people can live (...) together peacefully. Given our condition as mortals, communication remains not primarily a problem of technology, but of power, ethics and art." —Antony Anderson, _New Scientist_ "Guaranteed to alter your thinking about communication.... Original, erudite, and beautifully written, this book is a gem." —_Kirkus Reviews_ "Peters writes to reclaim the notion of authenticity in a media-saturated world. It's this ultimate concern that renders his book a brave, colorful exploration of the hydra-headed problems presented by a rapid-fire popular culture." —_Publishers Weekly_ What we have here is a failure-to-communicate book. Funny thing is, it communicates beautifully.... _Speaking Into the Air_ delivers what superb serious books always do-hours of intellectual challenge as one absorbs the gradually unfolding vision of an erudite, creative author." —Carlin Romano, _Philadelphia Inquirer_. (shrink)

Cover Photograph: Transgenic Hydra provide insights of general relevance into stem cell biology. The image shows a mass culture of transgenic Hydra expressing EGFP in all of their ectodermal epithelial cells. See article by Thomas C. G. Bosch pp. 478–486.