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

Introductory remarks.- Coelenterata.- The nervous systems of Cnidarians.- Platyhelminthes.- The flatworm nervous system: Pattern and phylogeny.- Nematoda.- Repeating patterns of motoneurons in nematodes: The origin of segmentation?.- Annelida.- Genesis of segmental identity in the leech nervous system.- Mollusca.- Comparative aspects of gastropod neurobiology.- The cephalopod nervous system: What evolution has made of the molluscan design.- Onychophora.- Common and special features of the nervous system of Onychophora: A comparison with Arthropoda, Annelida and some other invertebrates.- Arthropoda Chelicerata.- Comparative aspects of the chelicerate nervous systems.- Mandibulata Embryogenesis.- Conservation versus change in early axonogenesis in arthropod embryos: A comparison between myriapods, crustaceans and insects.- Motoneurones.- Homologous structures, exemplified by motoneurones of Mandibulata.- Sensory neurones.- Developmental mechanisms, homology and evolution of the insect peripheral nervous system.- Interneurones.- Lineage analysis as an analytical tool in the insect central nervous system: Bringing order to interneurons.- Peptidergic neurones.- Comparative aspects of peptidergic signaling pathways in the nervous systems of arthropods.- The arthropod brain.- Ground plans, evolutionary changes and homologies in decapod crustacean brains.- The arthropod mushroom body: Its functional roles, evolutionary enigmas and mistaken identities.- Is the evolution of arthropod brains convergent?.- Echinodermata.- The nervous systems of Echinodermata: Recent results and new approaches.- Methodological comment.- The homology concept-still alive.- Coda.- Are the main grades of brains different principally in numbers of connections or also in quality?.

The Nervous Systems of Invertebrates: An Evolutionary and Comparative Approach

Commercially important sea cucumbers of the world

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

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.

Ilia Torres-Avillán

Papers

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

FMRFamide-like immunoreactivity in cells and fibers of the holothurian nervous system.

Cells in the intestinal system of holothurians (Echinodermata) express cholecystokinin-like immunoreactivity.