Significant progress has recently been made in our understanding of animal regenerative biology, spurred on by the use of a wider range of model organisms and an increasing ability to use genetic tools in traditional models of regeneration. This progress has begun to delineate differences and similarities in the regenerative capabilities and mechanisms among diverse animal species, and to address some of the key questions about the molecular and cell biology of regeneration. Our expanding knowledge in these areas not only provides insights into animal biology in general, but also has important implications for regenerative medicine and stem-cell biology.

Bridging the regeneration gap: genetic insights from diverse animal models

Autotomy is defined herein as the shedding of a body part, where (1) the loss of the body part is defensive (autotomy helps prevent the whole animal from being compromised and is in response to external stimuli); (2) shearing occurs by an intrinsic mechanism along a breakage plane (there has been selection for certain body parts to be pulled off easily); and (3) the loss is controlled - the animal moves away from the trapped limb, the loss is under some form of central control (neural or hormonal), or the body part is detached quickly. Autotomy (under this defensive definition) has evolved independently for a diverse array of body parts in many taxa; we have summarised available information for over 200 invertebrate species. The advantages of autotomy include escape from entrapment, an effective form of attack, expulsion of an infected body part or in limiting wounding. We discuss how the incidence of autotomy may therefore be correlated with various traits such as limb function, sex differences, other defence mechanisms, habitat disturbance, and sociality. There are also costs associated with autotomy. Short-term costs include loss of a specialised appendage or organ, reduced speed and stability, or even death. Long-term costs include compromised foraging and feeding (often leading to reduced growth), altered anti-predator, competitive or reproductive behaviour, and even defective development. Regenerating lost appendages may also incur significant costs for the individual. We examine the costs and benefits of autotomy, and discuss the evolutionary selective pressures that contribute to the prevalence and effectiveness of autotomy in invertebrates.

Leave it all behind: a taxonomic perspective of autotomy in invertebrates.

Regenerative potential is expressed to a maximum extent in echinoderms. It is a commonphenomenon in all the classes, extensively employed to reconstruct external appendages and internalorgans often subjected to amputation, self-induced or traumatic, rapidly followed by completesuccessful re-growth of the lost parts. Regeneration has been studied in adult individuals as well as inlarvae. In armed echinoderms, regeneration of arms is obviously frequent: in many cases, the detached body fragments can undergo phenomena of partial or total regeneration independently of the donor animal, and, in a few cases (asteroids), the individual autotomised arms can even regenerate to produce new complete adults, offering superb examples of cloning strategies. In the species examined so far most results throw light on aspects related to wound healing, growth, morphogenesis and differentiation, even though in most cases many crucial questions remain unanswered. The present paper provides an overview of the current understanding of the phenomenon and covers the main biological aspects of regeneration giving an idea of the “state of the art” across the phylum in terms of experimental approaches and representative models.

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Regeneration in Echinoderms: repair, regrowth, cloning

'Autotomy' refers to the adaptive detachment of animal body parts where this serves a defensive function, is achieved by an intrinsic mechanism, and is nervously mediated. With regard to each echinoderm class, this article itemises those structures that are autotomous, evaluates the extent to which autotomy precedes regeneration in natural populations, reviews current knowledge of the morphology of autotomy planes and mechanisms that effect fracture at autotomy, and comments on autotomy-related issues arising from studies of the cellular events of regeneration. Each autotomy plane can be regarded as an assemblage of breakage zones traversing the individual anatomical components of the autotomous structure. In any one autotomy plane some breakage zones are permanent sites of weakness that are fractured by external forces and some are potential sites of weakness that undergo a loss of tensile strength only at the time of autotomy. The latter occur predominantly in mutable collagenous structures, although there are a few examples of muscles that undergo an endogenous rupturing process. Available evidence indicates that autotomy is by far the commonest proximate cause of structural loss in echinoderms. Most echinoderm regeneration is therefore necessitated by autotomy and proceeds from the retained side of a fractured autotomy plane. Due to a lack of relevant research there is as yet little evidence for or against the presence of specific regeneration-promoting adaptations at autotomy planes, although it is argued that an autotomy plane designed primarily to effect rapid detachment would by itself increase regenerative efficiency.

Autotomy as a prelude to regeneration in echinoderms

Stem cell-based growth, regeneration, and remodeling of the planarian intestine.

Echinoderms are the deuterostome group with the most striking capacity to regenerate lost body parts. In particular, members of the class Holothuroidea are able to regenerate most of their internal organs following a typical evisceration process. Such formation of new viscera in an adult organism provides a unique model to study the process of organogenesis. We have studied this process in the sea cucumber Holothuria glabberrima by describing the spatial and temporal pattern of cellular events that occur during intestine regeneration following chemically induced evisceration. Regeneration begins as a thickening of the mesenteries that supported the autotomized organs to the body wall. The mesenterial thickening consists of tissues where most of the cellular populations found in the normal intestine are already present. However, the cell numbers differ, particularly those of hemocytes and amoebocytes, suggesting that some of these cells play an important role in the formation of the solid rod of hypertrophic mesentery that characterizes the intestinal primordia. The appearance of the luminal epithelium, together with the formation of the lumen, occurs during the second week of regeneration by proliferation and extensive migration of cells from the esophagus and cloacal ends into the thickenings. At this stage all tissue layers are present, but it takes an additional week for them to exhibit the proportions typical of the normal organ. Cell division, as determined by BrdU labeling, mainly occurs in the coelomic epithelia of the hypertrophic mesentery and in the regenerating luminal epithelium. Our study provides evidence that the process of new organ formation in holothurians can be described as an intermediate process showing characteristics of both epimorphic and morphallactic phenomena.

Roberto Santiago

Papers

Cellular mechanisms of intestine regeneration in the sea cucumber, Holothuria glaberrima Selenka (Holothuroidea:Echinodermata)