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Home , Regeneration (biology)

Protective mechanisms in (Dermal skeleton, evisceration, )

Dermal Skeleton

The skeleton is dermal but nonetheless conspicuous in echinoderms, with the exception of most holothurians, and forms an effective armour. Each skeletal unit (ossicle) usually consists of two parts, a living tissue (stroma) and a complex lattice (stereom) of mineral calcium carbonate, or calcite, which is derived from the stroma. In living echinoderms, certain properties of calcite are not evident in the stereom because of its latticed structure and the presence of soft stroma. In fossils, however, the stroma may be replaced by secondary calcite (i.e., calcite laid down in continuity with the original skeletal calcite), and recognition of fragments of skeletons in fossil strata is easier because no other group has the same type of skeleton. Each ossicle is formed from granules in the dermal layer that, after secretion from special lime- secreting cells, enlarge, branch, and fuse to build up a three-dimensional network of calcite. Parts of the skeleton enlarge as an animal grows, and resorption and of the skeleton may occur.

Echinoderms exhibit a variety of skeletal structures. In the echinoids, a hollow test (skeleton) consisting of 10 columns of plates bears large and small spines as well as pincerlike organs (pedicellariae) used in defense and in the removal of unwanted particles from the body. Pedicellariae, also found in the asteroids, are absent from crinoids, ophiuroids, and holothurians. The complex feeding apparatus (Aristotle’s lantern) of echinoids consists of 40 ossicles held together by muscles and collagenous sutures.

Crinoids have a hollow sheath (theca or calyx) composed of two or three whorls, each consisting of five skeletal plates; the stalk and the slender appendages (cirri) of unstalked forms consist of a series of drum-shaped ossicles. The asteroid skeleton is composed of numerous smooth or spine- bearing ossicles of various shapes held together by muscles and ligaments, permitting flexibility. The arms of asteroids are hollow, those of ophiuroids solid, with the central axis of each arm consisting of elongated ossicles called vertebrae. The microscopically sized ossicles of holothurians are highly variable in form, ranging from flat lattice plates with holes to exquisitely symmetrical wheels, and are usually numerous; one tropical , for example, has more than 26,000,000 ossicles in its body wall. A ring of plates, called the calcareous ring, surrounds the tube leading from the mouth to the stomach (i.e., the esophagus) of holothurians. Although located in a similar position to that of the echinoid Aristotle’s lantern, the calcareous ring functions as a point of insertion for muscles, not as a feeding apparatus.

Evisceration

Sea cucumbers (a class of echinoderms) exhibit a high capacity for regeneration, such that, following ejection of inner organs in a process called evisceration, the lost organs regenerate. There are two ways by which evisceration occurs in sea cucmber species: from the mouth (anterior) or the anus (posterior). Intriguingly, regenerating tissues are formed at both the anterior and posterior regions and extend toward the opposite ends, and merge to form a complete digestive tract. From the posterior side, the digestive tube regenerates extending a continuous tube from the cloaca, which remains at evisceration. In posteriorly-eviscerating species, the esophagus remains in the body, and a new tube regenerates continuously from it. However, in anterior-eviscerating species, no tubular tissue remains in the anterior region, raising the question of how the new digestive tube forms in the anterior regenerate.

Regeneration

Regeneration is the re-growth of missing or damaged tissues or organs in adults or even in larvae or , and often comparable to embryogenesis in that they involve . In embryogenesis, the whole develops various tissues in a coordinated manner, whereas during regeneration, an animal that lost a part of their body restores that part only without affecting other parts. Thus, regeneration is a self-organizing phenomenon, and its underlying mechanism may differ from that of normal embryogenesis. Although many studies have been published, the field of regeneration research has many unanswered questions, such as how regeneration is possible, what molecules are triggers for regeneration, and why the ability to regenerate differs among .

Echinoderms, consisting of five classes, regenerate efficiently and are also the subject of study. For example, sea stars, brittle stars, and crinoids have high potential for regeneration, and are able to self-amputate (autotomize) their arms, but then are able to completely regrow them. Some sea stars are known to regenerate an entire body even from an arm, and some crinoids also regenerate their viscera.

Sea cucumbers are known to exhibit extensive regenerative ability and regenerate a wide spectrum of body parts, e.g. their body wall, the nervous system, and internal organs, such as the digestive system, reproductive organs, and respiratory trees. Ejection and regeneration of the digestive system has been the object of studies. Evisceration, the ejection of almost the entire set of internal organs, happens under natural conditions. In evisceration, the digestive tract first autotomizes at the posterior or anterior ends, and is ejected from either the mouth (anterior evisceration) or the anus (posterior evisceration). Sea cucumbers of the orders Dendrochirotida (sea cucumbers that have branched tentacles and are mostly filter-feeders) and Aspidochirotida (sea cucumbers with flattened tentacles) are known to eviscerate. Dendrochirotid and aspidochirotid sea cucumbers mainly eviscerate anteriorly or posteriorly, respectively. In either case, the mesentery remains, which connects the internal lining (mesothelium) of the body cavity and the digestive tract. Animals that eviscerate anteriorly release the entire digestive tract, except for the cloaca, and loses the entire set of anterior end structures. Those that eviscerate posteriorly release the intestine between the esophagus and the cloaca. In posterior eviscerating sea cucumbers, the wound at the end of the remaining esophagus and cloaca due to autotomy are closed and anterior and posterior blind gut rudiments are formed, and these tubes grow and eventually join to form a continuous digestive tract. By contrast, in anterior eviscerating species, a blind tube is formed continuously from the remaining cloaca, but in the anterior part, a rudiment of a mass of cells is first formed and then the tube somehow regenerates and extends posteriorly. The regenerative potential is expressed to a maximum extent in echinoderms.

Key Concepts:

• Regeneration is a regulative and conservative developmental process complementary to . • Asexual reproduction (cloning) represents the highest expression of the regenerative potential of an . • 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.

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