The ventral nerve cord signals positional information during segment formation in an annelid (Ophryotrocha puerilis, Polychaeta)

Nerves and availability of mesodermal cells are essential for the function of the segment addition zone (SAZ) during segment regeneration in polychaete annelids

Most of annelids grow all over their asexual life through the continuous addition of segments from a special zone called "segment addition zone" (SAZ) adjacent to the posterior extremity called pygidium. Amputation of posterior segments leads to regeneration (posterior regeneration-PR) of the pygidium and a new SAZ, as well as new segments issued from this new SAZ. Amputation of anterior segments leads some species to regeneration (anterior regeneration-AR) of the prostomium and a SAZ which produces new segments postero-anteriorly as during PR. During the 1960s and 1970s decades, experimental methods on different species (Syllidae, Nereidae, Aricidae) showed that the function of SAZ depends on the presence and number of mesodermal regeneration cells. Selective destruction of mesodermal regeneration cells in AR had no effect on the regeneration of the prostomium, but as for PR, it inhibited segment regeneration. Thus, worms deprived of mesodermal regeneration cells are always able to regenerate the pygidium or the prostomium, but they are unable to regenerate segments, a result which indicates that the SAZ functions only if these regeneration cells are present during PR or AR. Additionally, during AR, nerve fibres regenerate from the cut nerve cord toward the newformed brain, a situation which deprives the SAZ of local regenerating nerve fibres and their secreted growth factors. In contrast, during PR, nerve fibres regenerate both during the entire regeneration phase and then in normal growth. This review summarizes the experimental evidence for mesoderm cell involvement in segment regeneration, and the differential impact of the digestive tube and the regenerated nerve cord during PR vs AR.

Neural regulation of body polarities in nereid worm regeneration

Nerve dependence in regeneration has been established more than 200 years ago but the mechanisms by which nerves are necessary to regeneration remain to be fully elucidated. Aside from their direct impact in stimulating cellular growth, nerves also have a role on the establishment of body polarities (antero-posterior and dorso-ventral patterns) and this has been particularly well studied in nereid annelid worms. Nereids can regenerate appendages (parapodia) and the tail (body segments). In both parapodia and tail regeneration, the presence of the nerve cord is necessary to the establishment of body polarities. In this review, we will detail the experimental procedures which have been conducted in nereids to elucidate the role of the nerve cord in the establishment of the antero-posterior and dorso-ventral polarities. Most of the studies reported here were published several decades ago and based on anatomical and histological analyses; this review should constitute a knowledgebase and an inspiration for needed modern-time explorations at the molecular levels to elucidate the impact of the nervous system in the acquisition of body polarities.

Comparative Aspects of Annelid Regeneration: Towards Understanding the Mechanisms of Regeneration

The question of why animals vary in their ability to regenerate remains one of the most intriguing questions in biology. Annelids are a large and diverse phylum, many members of which are capable of extensive regeneration such as regrowth of a complete head or tail and whole-body regeneration, even from few segments. On the other hand, some representatives of both of the two major annelid clades show very limited tissue regeneration and are completely incapable of segmental regeneration. Here we review experimental and descriptive data on annelid regeneration, obtained at different levels of organization, from data on organs and tissues to intracellular and transcriptomic data. Understanding the variety of the cellular and molecular basis of regeneration in annelids can help one to address important questions about the role of stem/dedifferentiated cells and "molecular morphallaxis" in annelid regeneration as well as the evolution of regeneration in general.

Regulation of dorso-ventral polarity by the nerve cord during annelid regeneration: A review of experimental evidence

An important goal for understanding regeneration is determining how polarity is conferred to the regenerate. Here we review findings in two groups of polychaete annelids that implicate the ventral nerve cord in assigning dorso-ventral polarity, and specifically ventral identity, to the regenerate. In nereids, surgical manipulations indicate that parapodia develop where dorsal and ventral body wall territories contact. Without a nerve cord at the wound site, the regenerate differentiates no evident polarity (with no parapodia) and only dorsal identity, while with two nerve cords the regenerate develops a twinned dorso-ventral axis (with four parapodia per segment instead of the normal two). In sabellids, a striking natural dorso-ventral inversion in parapodial morphology occurs along the body axis and this inversion is morphologically correlated with the position of the nerve cord. Parapodial inversion also occurs in segments in which the nerve cord has been removed, even without any segment amputation. Together, these data strongly support a role for the nerve cord in annelid dorso-ventral pattern regulation, with the nerve cord conferring ventral identity.

Regeneration of posterior segments and terminal structures in the bearded fireworm, Hermodice carunculata (Annelida: Amphinomidae)

Like many other annelids, bearded fireworms, Hermodice carunculata, are capable of regenerating posterior body segments and terminal structures lost to amputation. Although previous research has examined anterior regeneration in other fireworm species, posterior regenerative ability in fireworms remains poorly studied. As the morphology of the anal lobe (a small, fleshy terminal structure of unknown function) has been used to distinguish East and West Atlantic H. carunculata populations, there is a more imminent need to understand the morphology and organization of tissues in specimens undergoing posterior regeneration, and the timeframe in which significant developmental changes occur. To further investigate this phenomenon, we amputated the posterior segments of living H. carunculata specimens collected from the Gulf of Mexico and monitored posterior regeneration over a 6-month study period. Although many aspects of posterior regeneration in H. carunculata are consistent with the findings of other annelid regeneration studies, histological analysis revealed that once formed, anal lobe morphology remains relatively unchanged at all stages of posterior regeneration; East Atlantic morphotypes were not observed in the West Atlantic specimens studied here. Additionally, we found that the ventral nerve chord, which is partially responsible for the regeneration of lost body parts in polychaete annelids, terminates within the anal lobe, suggesting that this structure may play a role in the formation of new segments.

Activation of Hox genes during caudal regeneration of the polychaete annelid Platynereis dumerilii

The capability of regenerating posterior segments and pygidial structures is ancestral for annelids and has been lost only a few times within this phylum. As one of the three major segmented taxa, annelids enable us to monitor reconstruction of lost tissues and organs. During regeneration, regional identities have to be imprinted onto the newly formed segments. In this study, we show spatial and temporal localization of expression of nine Hox genes during caudal regeneration of the polychaete annelid Platynereis dumerilii. Hox genes are homeodomain genes encoding transcriptional regulators of axial patterning in bilaterian animals during development. We demonstrate that five Platynereis Hox genes belonging to paralog groups (PG) 1, 4, 5, 6, and 9-14 are expressed in domains of the regenerating nervous system consistent with providing positional information along the anteroposterior axis of the regenerate. We report that expression in regenerating neuromeres is limited to varying subsets of perikarya, called gangliosomes. Four of nine genes analyzed do not appear to be involved in axial patterning. Two genes, Pdu-Hox2 and Pdu-Hox3, are predominantly expressed in the growth zone region. For some Hox genes expression in newly formed coelomic epithelia can be observed. Platynereis Hox genes do not exhibit temporal or spatial colinearity. Although there are some similarities to previously reported expression patterns during larval and postlarval development in Nereididae (Kulakova et al. 2007), expression patterns observed during caudal regeneration also show unique patterns.

Immunohistochemical analysis of nervous system regeneration in chimeric individuals of Dorvillea bermudensis (Polychaeta, Dorvilleidae)

In regeneration experiments, 0.5% of the two- or five-segmented fragments of the polychaete Dorvillea bermudensis were found unexpectedly transplanted: two fragments of each that were lying close together during the initial period, fused and regenerated a chimeric individual. Of the three theoretical possibilities (i.e. fusion of (i). two posterior ends; (ii). one anterior and one posterior end; (iii). or two anterior ends) only the last two were realized. The similarly oriented fragments regenerated a normal animal while anterior-anterior fused ones produced two heads or a double head. Whether the ventral cords of the fragments are located vis-à-vis or adjacent, influences the course of regeneration as well. Immunohistochemical methods (anti-acetylated alpha-tubulin) in conjunction with confocal laser scanning microscopy were used to investigate the wiring pattern of the nervous systems of the grafts. In all cases, at least two supraesophageal ganglia were formed and palps, antennae and nuchal organs were innervated by the correct nerves but, in special cases, were innervated vice versa from the other brain. From these results it can be concluded that fusion of a regenerating connective with another connective results in formation of a new brain, irrespective of whether it belongs to the same nerve cord or not.

Symmetry and asymmetry in the development of inner organs in parabiotic twins of amphibians (Urodela)

Newt embryos of different developmental stages were combined to parabiotic twins in different positions. The exterior appearance and the symmetry relations, particularly of the internal organs (intestinal tract, heart, nuclei habenulae, and vitelline vein) were studied. Experimentally caused organ inversions allowed conclusions with respect to organ asymmetry and unilateral dominance. There was no direct correlation between appearance and symmetry of the exterior and the internal organs. All internal organs showed a continuous transition between normal and ideally inverse situs. The concordance of the organ situs differs greatly. The "left-hand side" or "right-hand side" dominance is not uniform. It depends on the type of fusion, i.e. the relative position of the parabiotic twins, and is often specific for a given organ. In some cases a non-genetic "symmetrisation factor" appears to be strongly active, depending on the fusion type and resulting in a dominant transindividual organ mirror image symmetry in the parabiotic twins. The older twin generally dominates the processes of determination and induction. The "symmetrisation factor" also acts on members of different families, i.e. genetically completely heterogeneous parabiotic twins. The development of organ asymmetry appears to be a process with several phases.

The atokous-epitokous border is determined before the onset of heteronereid development in Platynereis dumerilii (Annelida, Polychaeta)

In most nereids sexual maturation is accompanied by a dramatic reorganization of the body that enables swarming of the formerly benthic worms. However, a border exists between unchanged anterior (atokous) and metamorphosed posterior (epitokous) segments. The site of this atokous-epitokous border (a/e border) is different in sexually mature males and females of Platynereis dumerilii. There is no correlation between the total number of setigerous segments of a specimen and the location of the a/e border. The location of the a/e border and sexual development are affected neither by cutting off caudal segments of juveniles (including the prospective a/e border) nor by transecting the ventral nerve cord. When parapodia are transplanted from prospective epitokous regions to prospective atokous regions and vice versa, they maintain their original character during metamorphosis. The results presented here suggest that prospective atokous as well as epitokous characters are determined at or only very shortly after formation of the respective segments. Thus the a/e border is established well in advance of the onset of epitokous metamorphosis.

Antibody staining reveals novel aspects of segmentation within the leech central nervous system

The leech is a segmented annelid with a well characterized central nervous system. In this report, we use antibodies to map the distribution of neurons confined to selected segmental ganglia in the mud leech Haemopis marmorata. The distribution of these neurons suggest 3 novel aspects of segmentation in the leech nervous system: (1) neurons are assigned to even-numbered ganglia through a mechanism which effectively counts through the leech segmental body plan by units of 2, (2) neurons are assigned to ganglia 7 and 14 through a mechanism which effectively counts in units of 7 and (3) neurons are assigned to the 2nd and 4 fused head ganglia and to the 2nd of 21 unfused midbody ganglia through a mechanism which effectively counts units from the origin of these 2 ganglionic series. These 3 hypothetical counting mechanisms divide the central nervous system (CNS) into supersegmental units. Neurons used to define these supersegmental units have been injected with tracer and identified as interganglionic interneurons. Competitive interactions among embryonic precursors of these neurons may directly eliminate their homologs from intervening ganglia, and thus sculpture supersegmental patterns into the mature nervous system.