Regeneration in Echinoderms and Ascidians

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Regeneration in Echinoderms and Ascidians

M Daniela Candia Carnevali, Università di Milano, Milano, Italy
Paolo Burighel, Università di Padova, Padova, Italy

Published online: September 2010

DOI: 10.1002/9780470015902.a0022102

Full Article on Wiley Online Library

Abstract
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Abstract

The regenerative potential is expressed to a maximum extent in echinoderms and ascidians. They provide unique and valuable deuterostome models, closely related to vertebrates (man included), for an integrated approach exploring regeneration from tissue repair to asexual cloning. The comparison of results derived from different experimental models of echinoderms and ascidians and employing different approaches, in vivo and in vitro, provides an insight on specificity of regulatory mechanisms and processes governing large‐scale pattern formation and information signalling storage between cells and tissues allowing a living system to reliably regenerate and maintain a complex morphology. Since in these animals, regenerative phenomena involve progenitor cells present in the circulating fluids or in the tissues, the crucial questions opened are those related to (1) stemness properties of responsible cells, in terms of origin and derivation (stem cells or dedifferentiated cells) and (2) activities (proliferation and/or migration), plasticity and differentiation potential (derived cellular phenotypes).

Key Concepts:

Regeneration is a regulative and conservative developmental process complementary to asexual reproduction.
Asexual reproduction (cloning) represents the highest expression of the regenerative potential of an organism.
Regeneration processes generally imply the key‐contribution of pluripotential cells (stem cells or reprogrammed cells).
Echinoderms utilise regeneration processes at all stages of their life cycle (embryo, larva and adult).
Echinoderms can regenerate body parts and even complete individual from a fragment following self‐induced or traumatic amputation processes.
In ascidians the potential for asexual development is expressed during colony formation by developing functional individuals from adult tissues.
Ascidians are the unique adult chordates able to regenerate completely the ablated nervous system.

Keywords: regeneration potential; asexual reproduction; stem cells; morphogenesis; differentiation; epimorphosis; blastema; arm regeneration; siphon regeneration

	Figure 1. Schematic phylogenetic tree of deuterostome groups.
	Figure 2. Schematic representation of the life cycle of ascidians. Solitary species reproduce only sexually (left), whereas in colonial ascidians both sexual and asexual reproduction are present. Modified from Ballarin and Burighel .
	Figure 3. Echinoderms. Overview of regenerative processes in the crinoid Antedon mediterranea. (a) Drawing of an adult specimen at natural size. The arrows indicate the main structures involved in amputation/regeneration phenomena. See for details in (b) and (d)–(e). Regenerating arms (red rectangle) are also shown. (b) Schematic presentation of the experimental model including arm and explants following autotomy processes. On the left it is shown as the donor arm, on the right the respective arm explants. When reamputated at its distal end, the explant undergoes regenerative processes (in green) in distal direction in parallel to regeneration of its donor arm (in green). (c) Stereomicroscopic view of a regenerating arm (2 weeks post‐amputation). Bar=1 mm. (d) Photographic detail of the aboral appendices (cirri and arrows), frequently subjected to amputation/regeneration. (e) Photographic detail of the lateral branchings of the arms (pinnulae and arrows) which are preferential sites of regeneration. (f) Side view of the central body (calyx), showing the visceral mass (encircled) surrounded by the bases of the arms. (g) Schematic drawing of the internal anatomy showing the visceral mass (in yellow) subjected to evisceration/regeneration. (h) Comprehensive top‐view of the regenerated visceral mass (2 weeks post‐evisceration) photographed at stereomicroscope. The mouth (green arrow) and the anal papilla (blue arrows) are already developed. Bar=2 mm. (i) and (j) Larval stages frequently subjected to regeneration. (i) Swimming larval stage (doliolaria) viewed at scanning electron microscope. Bar=50 μm; (j) sessile larval stage (pentacrinoid) viewed at stereomicroscope. Bar=800 μm. (k) Top‐view detail of the regenerated visceral mass (2 weeks post‐evisceration) at scanning electron microscope. Mouth (green arrow) and anal papilla (blue arrows). Bar=1 mm.
	Figure 4. Echinoderms. Microscopic aspects of arm regeneration in the crinoid Antedon mediterranea. (a)–(c) Schematic reconstruction showing the three main phases of arm regeneration. The arm is shown in sagittal and cross section, with indication about its basic anatomy. (a) repair phase (0–24 h pa); (b) early regenerative phase (24–72 h pa) and (c) advanced regenerative phase (72 h–4 weeks pa). (d)–(j) Regenerating samples seen with different microscopic methods and illustrating the main phases of arm regrowth in experimental conditions (after pseudo‐autotomic amputation). (d) and (g) Repair phase (24 h post‐amputation). Stereomicroscopy (d) overall view of the still flat stump amputation plane (arrows). Bar=240 μm. Scanning electron microscopy (g) gives more details about the wound healing in progress: different migratory cells, fibrous material and tissue debris form a largely incomplete cicatricial layer. Roundish cells can be identified as coelomocytes. Bar=40 μm. (e) and (h) Early regenerative phase (72 hr post‐amputation). Stereomicroscopic view (e) of the arm stump showing a well developed regenerative blastema (encircled). Bar=240 μm. Light microscopic detail (h) of the prominent regenerative blastema. The different coelomic compartments (cc) are developing inside the bud. The bud sagittal section is quite comparable to that shown in (b). Bar=100 μm. Compare with the distal regeneration blastema (1 week post‐amputation) developed by an explant ((i) inset). Bar=100 μm. (f) and (j) Advanced regenerative phase (72 h–3 weeks post‐amputation). Stereomicroscopic views of the stump and the regenerate. The progressive development of the new arm with its internal ossicles (arrows) is evident at different stages: 1 week (f), bar=240 μm; 3 weeks (i), bar=250 μm. (k) Light microscopy detail of the regenerative blastema (72 h post‐amputation). BrdU immunocytochemical protocol (ABC method) for monitoring cell proliferation. Many blastemal cells are strongly labelled (dark nuclei, arrows) as well as the epithelium of the regrowing coelomic canal (arrows), indicating extensive cell proliferation activity. Bar=20 μm. (l)–(o) Regeneration‐competent cells. Trasmission electron micrographs of some of the main migratory cytotypes involved in regeneration. The cells recruited display specific distinctive features. (l) Coelomocyte (presumptive stem cell). Bar=1 μm. (m) Phagocyte with large phagosomes (arrows). Bar=1 μm. (n) Granulocyte with massive number of dense cytoplasmic granules. Bar=3 μm. (o) Dedifferentiating myocyte with evident process of rearrangement/dedifferentiation of their contractile apparatus (arrows). Bar=2 μm. (p) Neural factors involved in crinoid arm regeneration. Substance‐P and TGFβ immunocytochemistry (Texas Red/Sub‐P and FITC/TGFβ double‐fluorescence). Standard arm regeneration (1 week post‐amputaion). Confocal detail of the brachial nerve in sagittal section. The double staining emphasises an highly developed network of neural elements strongly reactive for the two factors: varicose Sub‐P immunoreactive processes (red‐orange) and rectilinear TGFβ immunoreactive processes (green) can be recognised. The amputation plane is on the top. Bar=20 μm.
	Figure 5. Ascidian regeneration and asexual reproduction. (a) On the left, scheme of abdominal region of Clavelina (A) to show the digestive loop with the epicardium (in red), as well as the level of two body section planes (x,y). On the right, following section‐x, a new thorax (with branchia and stigmata) is regenerated exclusively by epicardium (B); after synchronous section‐x and section‐y, bipolar regeneration forms a thorax at the two extremities (C); if section‐y follows section‐x after 72 h, regeneration is unipolar and a complete zooid is regenerated (D) (from Brien, , modified). (b) Schematic illustration of the main modes of budding in ascidians: strobilation (A); stolonic (B); pyloric (C) and peribranchial and vascular (arrow) (D). In red: tissues involved in asexual reproduction (from Nakauchi, , modified).
	Figure 6. Ascidian asexual reproduction. (a) Detail of B. schlosseri colony as visible in vivo. The filtering clonal adults and their buds are embedded in the thin tunic penetrated by a network of defined ‘walled’ vessels (arrows) which terminate in numerous blind peripheral ampullae. (b) Triploblastic vesicular bud. The derivation of the blastozooid organs from the three bud layers is reported (from Brien, ; and Manni and Burighel 2006; modified). (c) Scheme of blastozooids with reference to multipotent cells (red) in representatives of the three suborders. Aplouso‐ and Phlebobranchiata have multipotent cells in epicardium (A, B) and stolonic septum (D). In Stolidobranchiata (C) the atrial epithelium is multipotent (from Kawamura et al., ; modified).

Article Title:	Regeneration in Echinoderms and Ascidians
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