M. Daniela Candia Carnevali

Dynamic structure of the mesohyl in the sponge Chondrosia reniformis (Porifera, Demospongiae)

Abstract The common demosponge Chondrosia reniformis possesses the capacity to undergo an unusual creep process which results in the formation of long outgrowths from the parent body. These shape changes, which have been interpreted as adaptive strategies related to environmental factors, asexual reproduction or localised locomotor phenomena, are due mainly to the structural and mechanical adaptability of the collagenous mesohyl. This contribution describes the morphological correlates of mesohyl plasticisation in C. reniformis. The microscopic anatomy of the mesohyl was examined when it was in different physiological conditions: (1) standard ”resting” condition, (2) ”stiffened” condition and (3) dynamic ”creep” condition. In this last case four representative regions of the sponge body were analysed: the parent region, the elongation region, the transition region and the propagule region. The results show that the histological modification of the sponge mesohyl during plasticisation is limited and localised. The most significant structural changes involve mainly cytological features of specific cellular components characterised by granule inclusions (i.e. the spherulous cells) and the arrangement and density of the collagenous extracellular framework, though the integrity of the collagen fibrils themselves is not affected. Morphological and functional aspects of mesohyl plasticisation invite comparison with the mutable collagenous tissue of echinoderms. Possible functional analogies between these two tissues are hypothesised.

TISSUE DISTRIBUTION OF MONOAMINE NEUROTRANSMITTERS IN NORMAL AND REGENERATING ARMS OF THE FEATHER STAR ANTEDON MEDITERRANEA

Abstract.Crinoid echinoderms can completely and rapidly regenerate arms lost following self-induced or traumatic amputation. Arm regeneration in these animals therefore provides a valuable experimental model for studying all aspects of regenerative processes, particularly with respect to the nervous system and its specific contribution to regenerative phenomena. Taking into account the primary role of the nervous system in regeneration in other invertebrates, we have investigated the specific involvement of neural factors, viz. the monoamine neurotransmitters dopamine and serotonin, in arm regeneration of Antedon mediterranea. In the present work, the presence of classical monoamines has been revealed by employing specific immunocytochemical and histofluorescence tests in association with biochemical detection by means of high pressure liquid chromatography. The distribution pattern of these neurohumoral molecules at standard regenerative stages has been compared with that of normal non-regenerating arms. Results indicate that both dopamine and serotonin dramatically change in both their distribution and concentration during the repair and regenerative processes. Their remarkably enhanced pattern during regeneration and widespread presence at the level of both nervous and non-nervous tissues indicates that they are important neural growth-promoting factors in crinoid arm regeneration.

Muscle system organization in the echinoderms: II. Microscopic anatomy and functional significance of the muscle‐ligament‐skeleton system in the arm of the comatulids (Antedon mediterranea)

Comatulids are able to perform quick and complex movements of the arms which are used to swim, creep, walk, and also form a rigid, feeding‐filtration fan. To perform such versatile movements, the arms of these animals are equipped with a classical endoskeletal system, with joints, muscles, ligaments, and a hydroskeleton of three different coelomic channels. Light microscopic study of the detailed anatomical organization of the arm clarifies both the complex relations between the parts involved in the movements and their functional responsibilities. In particular, (1) the ventral muscle bundles show a heterogeneous structure that consists of different and variously arranged populations of fibers, which allows the different flexing movements of the arms (i.e., flexion and maintaining the flexed state); (2) the ligaments (both dorsal and interarticular) consist only of collagen fibrils and, therefore, have a passive function in binding the skeletal pieces together: their possible active engagement in the extending movements of the arms is thus excluded; (3) owing to the absence of other suitable antagonists to the flexor muscles, the only efficient antagonist system seems to be the coelomic cavities, which are well separated from each other and are also provided with muscular valves. They thus function as typical hydraulic systems, which allows the arm to perform both simple extensions and very complex combined movements and to maintain some rigid straight or twisted positions.

Microarchitecture and mechanics of the sea‐urchin peristomial membrane

Abstract The peristomial membrane is an area of flexible body wall which connects the test to the lantern or masticatory apparatus of regular sea‐urchins. The peristomial membrane of Paracentrotus lividus was examined by various light microscopical techniques and by scanning and transmission electron microscopy, and its mechanical properties were investigated by load‐deformation tests. Attention was focused on the collagenous dermis which consists mainly of crossed‐fibre arrays arranged in three histologically distinct sublayers. This microarchitectural arrangement gives rise to nonlinear load‐deformation characteristics: when subjected to vertical deformation such as would occur during lantern protraction, the peristomial membrane shows firstly low stiffness and then high stiffness phases. The response of the peristomial membrane to chemical agents suggests that the tensile properties of the dermis are under physiological control. Our results indicate that the collagenous fibres of the dermis belong to s...

Microstructure and mechanical design in the lantern ossicles of the regular sea‐urchin Paracentrotus lividusi A scanning electron microscope study

Abstract Echinoids are the echinoderm group which has explored and exploited most efficiently the potential of the endoskeleton in a range of extremely advanced and sophisticated adaptive solutions. The most ingenious of these adaptations are employed in the dental apparatus, whose different elements represent a striking example of the versatility of the skeletal tissue. The dental ossicles (jaws, rotulae, compasses and teeth), though having the same basic organization, show a wide range of structural and functional solutions: inorganic phase and organic stroma are variously combined and integrated in a very plastic and adaptable tissue, which is able to fulfil very specific mechanical requirements. On the whole, all the different arrangements shown by the dental elements can be considered as differentiations of the two limit‐models of skeletal microarchitecture, represented respectively by classical porous stereom and a composite lamellar structure. The first structural model is by far the most common: a...

Muscle system organization in the echinoderms: III. Fine structure of the contractile apparatus of the arm flexor muscles of the comatulids (Antedon mediterranea)

The crinoid arm muscles consist of obliquely striated fibers with striking differences in function and in ultrastructural features. These fibers can be distinguished mainly on the basis of different myofilament arrangements (A‐ and B‐type patterns) and are variously combined at different levels (proximal, intermediate, and distal portions) of the arm. Some rare smooth fibers (C‐type) are irregularly distributed in the periphery and in the core of the bundle. The characteristic features of the A‐ and B‐type obliquely striated fibers are (1) a continuous and homogeneous structure of the Z line and (2) a very heterogeneous arrangement of myosin filaments which vary widely in size, number, and distribution from section to section. The significance of such an atypical, obliquely striated muscle may be related to the double skeletal system combination (endoskeleton and hydroskeleton) of the crinoid arms.

Structure and ultrastructure of muscles in the priapulid Halicryptus spinulosus : functional and phylogenetic remarks

The Priapulida phylum is a rather homogeneous small group of marine invertebrates, living buried in sand and mud of cold waters. These worms present great difficulty in determining their systematic position: in the first instance, Priapulida were considered to be pseudo-coelomates, and classified among the Aschelminthes (Hyman, 1951). How ever, later studies (Shapeero, 1961) have pointed out that the body cavity is actually a coelom, and not a pseudo-coelom. Consequently it seems improper to include Priapulida among Aschelminthes, even if their real position still remains undefined. Today most authors tend to consider priapulids as the last relicts of an originally larger group, forming a separate phylum (van der Land, 1970). We can assume some connexions with other minor Protostoma, but not to an extent sufficient to consider a possible group affinity (Grasse, 1959). The general anatomical organization of a priapulid is now well known (van der Land, 1970), but the only detailed microscopical descriptions are those of the genus Priapulus (Mattisson, Nilsson & Fange, 1974) and the genus Tubilucus (Kirsteuer & van der Land, 1970). We have only incomplete data and summary descriptions concerning the other existing genus. Ultrastructural researches are even more scarce; electron microscope studies could clarify the submicroscopical organization of these animals and allow comparisons with other groups. Afzelius & Ferraguti (1978), for instance, have shown that the sperm of Priapulus caudatus present a very primitive pattern, resembling that of the sperm of many other marine invertebrates with external fertilization. The muscular systems are also useful for researches of phylogenetic correlations: it is, in fact, possible to correlate the ultrastructural organization of the muscles with the systematic position of their respective animals (Hanson & Loewy, 1960; Lanzavecchia, 1977). For the muscular systems of priapulids the only pertinent data are those of Mattisson et al. (1974) on Priapulus caudatus . A detailed analysis of the muscles of other species seems therefore necessary to set up a general picture of the situation in Priapulida. In these animals the muscles are organized to form a typical hydraulic system. The hydraulic systems present one of the most topical problems, from both the morphological and functional points of view (Hammond, 1970; Chapman, 1975).

Re‐growth, morphogenesis, and differentiation during starfish arm regeneration

The red starfish Echinaster sepositus is an excellent model for studying arm regeneration processes following traumatic amputation. The initial repair phase was described in a previous paper in terms of the early cicatrisation phenomena, and tissue and cell involvement. In this work, we attempt to provide a further comprehensive description of the later regenerative stages in this species. Here, we present the results of a detailed microscopic and submicroscopic investigation of the long regenerative phase, which can be subdivided into two subphases: early and advanced regenerative phases. The early regenerative phase (1–6 weeks p.a.) is characterized by tissue rearrangement, morphogenetic processes and initial differentiation events (mainly neurogenesis and skeletogenesis). The advanced regenerative phase (after 6 weeks p.a.) is characterized by further differentiation processes (early myogenesis), and obvious morphogenesis and re‐growth of the regenerate. As in other starfish, the regenerative process in E. sepositus is relatively slow in comparison with that of crinoids and many ophiuroids, which is usually interpreted as resulting mainly from size‐related aspects and of the more conspicuous involvement of morphallactic processes. Light and electron microscopy analyses suggest that some of the amputated structures, such as muscles, are not able to replace their missing parts by directly re‐growing them from the remaining tissues, whereas others tissues, such as the skeleton and the radial nerve cord, appear to undergo direct re‐growth. The overall process is in agreement with the distalization‐intercalation model proposed by Agata and co‐workers. Further experiments are needed to confirm this hypothesis.

An unusual Z-system in the obliquely striated muscles of crinoids: three-dimensional structure and computer simulations

SummaryThe peculiar functional structure of the Z-line in the obliquely striated muscles of some feather stars is described. It is known that cross-striated muscles are characterized by linear and continuous Z-bands, and obliquely striated muscles by disconnected, obliquely aligned Z-elements. Owing to this discontinuous organization, the sarcomere can perform wide active lengthenings, shortenings, and even ‘super-elongations’ in the helical fibres. In contrast, the obliquely striated fibres of crinoids show markedly continuous and homogeneous oblique Z-lines; such a structure is not compatible with ‘super-performances’ like sliding and shearing of the sarcomere elements, but instead could allow functions comparable to those characteristic of a cross-striated muscle (quick, short movements, mechanically amplifiable by bone levers). This odd situation, only interpretable in terms of evolutionary constraint, could be considered opposite and symmetrical to that of cross-striated ‘super-contracting’ muscles, where the Z-line is exceptionally fragmented to allow the sarcomere to super-contract.The possible architecture of a significant parameter such as the Z-line, which determines muscle fibre potential capacities, is analysed in detail: (1) through qualitative-quantitative evaluation of electron micrographs, supported by statistical analysis of the data; and (2) bycomputer simulations. The data obtained suggest that the most realistic conformation of the whole Z-complex in these muscles consists of a multiple system of continuous, ribbon-like helical planes running in parallel along the fibre from end to end and regularly cutting it with a constant thickness. The proposed model seems morphologically compatible with the experimentally verified situations and functionally compatible with the mechanical requirements for a normal contraction and for a balanced distribution of the involved strengths.

Wound repair during arm regeneration in the red starfish Echinaster sepositus

Starfish can regenerate entire arms following their loss by both autotomic and traumatic amputation. Although the overall regenerative process has been studied several times in different asteroid species, there is still a considerable gap of knowledge as far as the detailed aspects of the repair phase at tissue and cellular level are concerned, particularly in post‐traumatic regeneration. The present work is focused on the arm regeneration model in the Mediterranean red starfish Echinaster sepositus; to describe the early cellular mechanisms of arm regeneration following traumatic amputation, different microscopy techniques were employed. In E. sepositus, the repair phase was characterized by prompt wound healing by a syncytial network of phagocytes and re‐epithelialisation followed by a localized subepidermal oedematous area formation. Scattered and apparently undifferentiated cells, intermixed with numerous phagocytes, were frequently found in the wound area during these first stages of regeneration and extensive dedifferentiation phenomena were seen at the level of the stump, particularly in the muscle bundles. A true localized blastema did not form. Our results confirm that regeneration in asteroids mainly relies on morphallactic processes, consisting in extensive rearrangement of the existing tissues which contribute to the new tissues through cell dedifferentiation, redifferentiation, and/or migration.