All species of the Ophiuroidea have exceptional regenerative capabilities; in particular, they can replace arms lost following traumatic or self-induced amputation. In order to reconstruct this complex phenomenon, we studied arm regeneration in two diVerent ophiuroids, Ophioderma longicaudum (Retzius, 1805) and Amphiura Wliformis O. F. Müller, 1776, which are quite distantly related. These species present contrasting regeneration and diVerentiation rates and diVer in several ecological traits. The aim of this paper is to interpret the primary sequence of morphogenetic and histogenetic events leading to the complete reconstruction of a new arm, comparing the arm regenerative processes of these two ophiuroid species with those described in crinoids. Arm regeneration in ophiuroids is considered an epimorphic process in which new structures develop from a typical blastema formed from an accu-mulation of presumptive undiVerentiated cells. Our results showed that although very diVerent in some respects such as, for instance, the regeneration rate (0.17 mm/week for O. longicaudum and 0.99 mm/week for A. Wliformis), morphogenetic and histogenetic aspects are surprisingly similar in both species. The regenerative process presents similar characteristics and follows a developmental scheme which can be subdivided into four phases: a repair phase, an early regenerative phase, an intermediate regenerative phase and an advanced regenerative phase. In terms of histogenesis, the regenerative events involve the development of new structures from migratory pluripotent cells, which proliferate actively, in addition in both cases there is a signiWcant contribution from dediVerentiated cells, in particular ded-iVerentiating myocytes, although to varying extents. This evidence conWrms the plasticity of the regenerative phenomenon in echinoderms, which can apparently follow diVerent pathways in terms of growth and morphogenesis, but nevertheless involve both epimorphic and morphallactic contributions at the cellular level.
Echinoderms and sponges share a unique feature that helps them face predators and other environmental pressures. They both possess collagenous tissues with adaptable viscoelastic properties. In terms of morphology these structures are typical connective tissues containing collagen fibrils, fibroblast- and fibroclast-like cells, as well as unusual components such as, in echinoderms, neurosecretory-like cells that receive motor innervation. The mechanisms underpinning the adaptability of these tissues are not completely understood. Biomechanical changes can lead to an abrupt increase in stiffness (increasing protection against predation) or to the detachment of body parts (in response to a predator or to adverse environmental conditions) that are regenerated. Apart from these advantages, the responsiveness of echinoderm and sponge collagenous tissues to ionic composition and temperature makes them potentially vulnerable to global environmental changes.
Gonads of the mediterranean crinoid Antedon mediterranea were analyzed in order to reconstruct their histological organization. The tissue arrangement resembled that previously described in other crinoids. Five reproductive stages were identified in both males and females: recovery, growing, premature, mature, spent. Sexually dimorphic characters were observed at the gonopore level. Hermaphroditic individuals were never detected. There were novel findings concerning somatic accessory cells of the gonads. The reproductive cycle periodicity was indicated by analysis of reproductive stage frequencies in the period 2003-2005: spring and winter were the seasons with highest and lowest maturity levels, respectively. A role in the regulation of reproduction was hypothesized for testosterone and 17b-estradiol, which were found to be present in A. mediterranea tissues and to vary during the reproductive cycle, thus suggesting a correlation between steroid levels and morphologically recognizable reproductive stages.
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