On the role of the proventricle region in reproduction and regeneration in Typosyllis antoni (Annelida: Syllidae)
© The Author(s). 2016
Received: 24 January 2016
Accepted: 28 September 2016
Published: 4 October 2016
Syllids are a species rich annelid family possessing remarkable regenerative ability, which is not only the response after traumatic injury, but also a key step during the life cycle of several syllid taxa. In these animals the posterior part of the body becomes an epitoke and is later detached as a distinct unit named stolon. Such a sexual reproductive mode is named schizogamy or stolonization. The prostomium and the proventricle, a modified foregut structure, have been proposed to have a control function during this process, though the concrete mechanisms behind it have never been elucidated.
By using different experimental set-ups, histology and immunohistochemistry combined with subsequent cLSM analyzes, we investigate and document the regeneration and stolonization in specimens of Typosyllis antoni that were amputated at different levels throughout the antero-posterior body axis. The removal of the anterior end including the proventricle implies an incomplete anterior regeneration as well as severe deviations from the usual reproductive pattern, i.e. accelerated stolonization, masculinization and the occurrence of aberrant stolons. The detailed anatomy of aberrant stolons is described. A histological study of the proventricle revealed no signs of glandular or secretory structures. The ventricle and the caeca are composed of glandular tissue but they are not involved in the reproductive and regenerative processes.
As in other investigated syllids, the proventricle region has a significant role during stolonization and reproduction processes in Typosyllis antoni. When the proventricle region is absent, anterior and posterior regeneration are considerably deviated from the general patterns. However, proventricle ultrastructure does not show any glandular component, thereby questioning a direct involvement of this organ itself in the control of reproduction and regeneration. Our findings offer a comprehensive starting point for further studies of regeneration and reproductive control in syllids as well as annelids in general.
Annelids provide excellent model organisms for investigating regenerative processes. Their regenerative capabilities are remarkable. They are able to renew worn structures such as chaetae or opercula [1, 2]. Furthermore, some annelids redevelop a complete body originated only in one or a few segments [3–5]. Studies concerning developmental biology in annelids and their regeneration patterns have been possible because the life-cycles of some of them are well-known and have been reproduced successfully under laboratory conditions. Such is the case of species of the clitellates Enchytraeus, Helobdella and Pristina, and polychaetes Alitta, Capitella, Hydroides, Platynereis, among others [6–15].
The regeneration patterns of syllids and their reproductive methods were studied during the 20th century by Abeloos , Albert , Allen [22, 23], Berrill , Boilly [25–32], Boilly & Thibaut , Caullery , Deyle , Durchon [36–38], Durchon & Wissocq , Franke [18, 40–45], Gidholm , Hauenschild , Heacox & Schroeder , Izuka , Langhammer , Malaquin , Mesnil , Mesnil & Caullery , Michel , Müller & Kreischer , Okada [56–62], Pruvot , Schiedges , Verger-Bocquet [65–67], and Wissocq [68–73]. However, these studies did not continue during recent decades, and therefore new microscopic techniques could not be used. Some of the most relevant results suggested an endocrine function of the proventricle (a specific structure in the digestive tube) during the cyclic reproductive activity of Typosyllis spp. [38, 40–42, 44, 48, 68].
The proventricle is a muscular structure with radially arranged striated muscle cells surrounding the gut [74–77]. These cells consist of usually only one or two sarcomeres with up to 100 μm length, being the longest known sarcomeres within the Metazoa [78, 79]. Within the muscular fibers there are membrane-bound granules containing high amounts of calcium and phosphorus, contributing to the calcium metabolism of the muscle cells . Several authors [38, 40, 69] proposed that the proventricle produces a hormone that, in high concentrations, attenuates the stolonization while promoting the regeneration of the posterior end. In later studies, Franke [41, 42, 44] as well as Heacox & Schroeder  suggested the prostomium as the main control core that manages the proventricle endocrine activity. However, these studies did not specify the hormone’s nature and the exact location for its production. Recently, Aguado et al.  suggested that the hormones might be produced by adjacent digestive structures that follow the proventricle, the ventricle and caeca, which are supposed to have glandular functions. However, a detailed histological study of these structures has not been performed and the specific control mechanisms of stolonization and regeneration continue to be unknown.
In this study, we investigate the anterior and posterior regeneration and the removal effect of the proventricle region in Typosyllis antoni (Syllinae) involving recent microscopical techniques. We used a variety of experimental set-ups, histology and immunohistochemistry combined with subsequent confocal laser scanning microscopy (cLSM) in order to investigate the relationships between regeneration, reproduction and the proventricle function.
Specimens and experimental setup
For histology, specimens were treated as described previously , with the following modifications. Specimens were anesthetized in 7 % MgCl2 dissolved in artificial seawater, fixed overnight in Bouin's fixative (saturated aqueous picric acid, 37 % formaldehyde, glacial acetic acid; 15:5:1 by volume), washed in 70 % ethanol and dehydrated in an ascending ethanol series. Azan stains the neuropil of the nervous system gray, the extracellular matrix blue, and the musculature orange. The color of nuclei is variable based on their content.
Immunohistochemistry was performed as described previously . For this study, we used combined stainings of anti-α-tubulin or anti-serotonin with phalloidin-rhodamine. For anti-α-tubulin staining, a mixture of anti-acetylated α-tubulin (monoclonal anti-tubulin, acetylated antibody, produced in mouse, ascites fluid, Sigma-Aldrich, St. Louis, MO, USA; dilution 1:500 in PBST-NGS) and anti-tyrosinated α-tubulin (monoclonal anti-tubulin, tyrosine antibody, produced in mouse, ascites fluid, Sigma-Aldrich, St. Louis, MO, USA; dilution 1:250 in PBST-NGS) was used. The labeled α-tubulin is a structural component of microtubules, which are amongst others present in axons, while serotonin (=5-HT) is a neurotransmitter. Phalloidin-rhodamine labels filamentous muscular actin (f-actin) .
Neutral red staining, light microscopy and image processing
For neutral red staining, a specimen was placed for 2 min in neutral red solution (2 mg/ml in artificial seawater) and afterwards anesthetized in 3.5 % MgCl2 dissolved in artificial seawater. Light microscopic pictures of neutral red staining and regenerating specimens were taken using a Leica (Leica Microsystems, Wetzlar, Germany) DM1000 microscope with attached Leica DFC295 camera and the Leica LAS v3.6 software. All drawings, images and final image plates were processed and compiled using Adobe (San Jose, CA, USA) Photoshop CS6 and Illustrator CS6.
Proventricle morphology and function
Regeneration differs across amputation sites
In order to investigate if anterior regeneration varies depending on the presence of the proventricle region and to exclude an involvement of the subsequent structures (ventricle + caeca), we tested three cutting sites (1, 2 and 3). Amputation site 1 (Fig. 2) was located directly anterior to the proventricle, so that the posterior body part kept the latter feature, but no additional parts of the pharynx. Amputation site 2 (Fig. 2) was at the border between the proventricle and the ventricle plus caeca, thus the posterior part lacks the proventricle, but kept the other structures. Amputation site 3 (Fig. 2) was between chaetiger 35 and 36, clearly behind the modified foregut.
Amputation site 1 - posterior regeneration. The anterior ends of specimens dissected in front of the proventricle regenerate slowly and are incomplete over the period of observation: After 14 dpa, about one third of the anterior body ends remained in the invagination stage without any signs of posterior regeneration (e.g., blastema formation). The remaining ones regenerated some few segments, but less than specimens amputated at sites 2 or 3. The maximum number of regenerated segments was four, first seen in a specimen at ten dpa.
Amputation site 2 - anterior regeneration: The anterior regeneration in specimens amputated between proventricle and ventricle bears striking differences in comparison with the other two sites. At first, two or usually three segments were visible at once (first seen 6 dpa) and no more segments were added until 16 dpa. At 18 dpa, one specimen showed in total four and the other one five regenerated segments. Thus, additional segments were added sequentially.
Amputation site 2 - posterior regeneration: Posterior regeneration in specimens amputated between proventricle and ventricle is faster than in site 1. The maximum number of regenerated segments observed was 16 in a specimen at 18 dpa. However, not all the individuals regenerated at the same rate and some remained in the invagination stage for more than one week without any signs of blastema formation. In addition, during the experiment, a redevelopment of the ventricle and caeca was not observable.
Summary of regeneration in Typosyllis antoni after dissecting at site 2 and 3
amputation site 2 – anterior regeneration (observation only every 2 nd day)
invagination of 1st remaining segment at wound site
prostomium with developing antenna, palps and eyes
two or three segments
sequential addition of further segments
amputations site 2 – posterior regeneration (observation only every 2 nd day)
0 + 2
invagination of 1st remaining segment at wound site
pygidium with developing anal cirri and median papilla
sequential addition of segments
amputation site 3 – anterior regeneration
invagination of 1st remaining segment at wound site
prostomium with developing antenna, palps and eyes; two or three segments with tentacular cirri regenerating in 1st segment
growth of segments and structures
amputation site 3 – posterior regeneration
invagination of 1st remaining segment at wound site
pygidium with developing anal cirri and median papilla
addition of first new segment
addition of further new segments followed by their development
Amputation site 3 - posterior regeneration: The posterior regeneration in specimens amputated between chaetiger 35 and 36 is comparable to amputation site 2.
Stolonization and stolon morphology
The effect of prostomium and proventricle region removal during stolonization
In order to get a better understanding of potential relationships between regeneration and stolon development, 12 additional individuals were amputated between chaetiger 35 and 36 (site 3, Fig. 2). The anterior and posterior ends (latter ones without a proventricle) were monitored over a longer period to investigate the production of stolons.
Posterior regeneration - During posterior regeneration (anterior body parts), all 12 specimens showed an invagination at 1 dpa. One specimen was not found after 2 dpa and another specimen remained at this stage until the end of the experiment (25 dpa) without any signs of regeneration. The other ten regenerated their posterior ends, but in three specimens, after two weeks, the posteriormost part was lost and regenerated again. The maximum number of segments regenerated was 24 (without loss of segments; 25 dpa), the minimum 11 (23 dpa). In those specimens, which lost their posterior end, a total of 30 segments were regenerated. None of the examined specimens produced stolons. The lost posterior ends are certainly traumatic losses, as we did not observe gametes, attached stolons, or detached stolons in those specimens and their bowls.
Accelerated stolonization, masculinisation and the occurrence of aberrant stolons has been also observed in the specimens amputated at site 2. In contrast, none of these changes was observable in specimens amputated at site 1.
Morphology of aberrant stolons
Removal of the proventricle region has strong effects on the processes of stolonization and posterior regeneration. Our results are in agreement with previous studies [37, 40, 48, 69] and clearly support an additional regulatory function of this region. The presence of a proventricle has been proposed as a synapomorphy for Syllidae, and its ancestral function is seen in the role of a sucking pump [81, 82]. Comparable structures have not been found in the digestive tract of any other group of annelids . The proposed sucking function may be particularly useful in many syllids that feed on hydroids, bryozoans or other invertebrates [75, 84]. However, many other syllids show a different kind of feeding preferences [85–87], e.g., algae as in the T. antoni diet. These different feeding strategies are widely spread especially within Syllinae, where the individuals reproduce by schizogamy. Thus, the proventricle might have been coopted from a plesiomorphic sucking pump function into an additional stolonization control purpose. This might explain its presence in a group of annelids that show a high diversity in reproductive modes, but do not seem to primarily rely on it for feeding [17, 81]. Alternatively, structures located in the proventricle region (e.g., nervous system) might be responsible for the regulation of reproductive and regenerative processes.
Therefore, a relationship between the proventricle and stolonization-regeneration can be assumed. However, the specific way the proventricle controls these processes is not clearly established. Franke [41, 42] proposed an endocrine function of the proventricle itself, controlled by the prostomium. Two factors might be involved: the "stolonization-inhibiting hormone" released by the proventricle; and the "stolonization-promoting hormone" mediated by the prostomium. Additionally, the proventricle hormone would not be exclusively inhibiting stolonization, but a low concentration might be also necessary for timing reproductive processes. As well, the prostomial signal might not only target the proventricle, but also other structures involved in reproduction . A major reproductive control function originating in the prostomium is also described in other Phyllodocida such as Eulalia viridis, Harmothoe imbricata, or Hediste diversicolor [88–90]. A recent study on Nereidae revealed a high complexity of reproduction control, described by a multi-hormone model .
However, morphology does not support an endocrine function of the proventricle itself. The proventricle of syllids is basically a muscle structure. It is composed of large sarcomeres [74–76, 78, 92, 93] and granules with high amounts of phosphorus and especially calcium ions . Our histological survey has revealed that there is no other feature associated in the proventricular region, and there are no signs of glandular tissue through the whole proventricular structure. However, the proventricle muscle cells contain large plasmatic areas and remarkably huge nuclei (Fig. 4c, d), suggesting a high metabolic activity.
It has been suggested that the muscle cells of the proventricle in Syllis spongiphilia have a myoepithelial origin [76–78]. Myoepithelial cells have been described with secretory functions in other groups of organisms . However, our results and that of other previous authors contradict this possible myoepithelial origin. The muscle cells have no epithelial part and are demarcated with an extracellular matrix (presumably basal lamina) on both sides, close to the coelom as well as to the epithelium that surrounds the gut lumen (Fig. 4d). Additionally, the presence of an inner and an outer epithelium encircling the proventricle musculature in Syllis gracilis has been demonstrated using transmission electron microscopy . Obviously, the use of the term “myoepithelial” was either a misinterpretation or a misuse. The proventricle muscle cells are characteristically striated muscle cells with no secretory function.
The proventricle muscle cells contain high amounts of calcium ions. Recently, it has been demonstrated that intracellular calcium ion signalling is essential in the regulation of cell activity in intestinal stem cells of Drosophila . However, it is likely that the calcium in the syllid proventricle is related to the muscular function as well, in particular because extracellular calcium ion signalling has not been found [79, 95].
On the other hand, in Myrianida prolifera the stolonization process seems to be normal in females when only the proventricle is extirpated, not the complete proventricle region . However, the proventricle in Typosyllis spp., has repeatedly shown to be the control core through successive extirpation and reimplantation experiments, both in male and female specimens [37, 40–42, 48]. Additionally, the histological analyses performed herein clearly reveal that proventricular segments do not show any structural difference to other segments, apart from the proventricle itself.
In a previous study we suggested that the ventricle and caeca might be responsible for the endocrine control role during stolonization . Indeed, our histological sections clearly show glandular and secretory tissues in these structures (Fig. 4e-g). However, specimens amputated between the proventricle and the ventricle (Fig. 2, amputation site 2) show the same differences in stolonization as those amputated far behind the complete modified foregut (Fig. 2, amputation site 3). This suggests that the ventricle and the caeca have no influence on the stolonization and thus, only a digestive function seems to be likely for these structures. Considering all of the evidence together, the exact role of the proventricle during reproduction and regeneration still remains obscure.
Our results further suggest that the removal of the proventricle region is influencing the anterior regeneration. When the proventricle region is present in the posterior end, the anterior regeneration seems to be a gradual process with the possible participation of an anterior segment addition zone. A similar pattern has been reported for Branchiomma luctusosum, Dorvillea bermudensis or Syllis gracilis [33, 96, 97]. However, when the proventricle region is removed, the regeneration process shows deviations from the general pattern. In this case the first segments appear simultaneously. Such a pattern of regeneration has also been found in Cirratulus cf. cirratus, Enchytraeus japonensis, Euyrthoe complanata, Cirrineris sp. or Timarete cf. punctata, amongst others [5, 98–102]. There are two different explanations for the simultaneous development of segments after the removal of the proventricle region. Firstly, the segments may have been determined one after another by an anterior segment addition zone, but too fast to observe a difference in timing. Secondly, the regeneration process might have begun with the redevelopment of tissue followed by seperation into different segments. In the latter case, the presence of a segment addition zone is not necessary and segments are formed simultaneously. In any case, the proventricle region seems to play a control function not only in posterior regeneration and stolonization, but also influences anterior regeneration.
The syllid proventricle is a structure composed of striated muscle cells with giant sarcomeres and prominent calcium concretions, but without any signs of putative glandular tissue. All experimental data so far clearly suggests that the proventricle (region) influences regenerative and reproductive processes of these animals. Effects of proventricle region removal include stolonization acceleration, limited posterior segment regeneration after stolonization, masculinization, and appearance of aberrant stolons (Fig. 9). Proventricle region removal also seems to influence anterior regeneration, which either could be mediated by a segment addition zone or simultaneous formation of segments. Future experiments including dissection and regrafting of the proventricle, repeated amputation of the prostomium, as well as gene expression studies will allow to establish the robustness of our results and will provide a comprehensive understanding of syllid stolonization, regeneration, and their relationship.
The authors are grateful to Martin Schlegel and Thomas Bartoloameus for providing materials and facilities. We additionally thank Thomas Bartolomaeus for discussions concerning the nature of the proventricle musculature.
MW thanks the University of Leipzig for funding by scholarship (U00048). MTA was supported by the Spanish Government (Programa José Castillejo, CAS14/00409) for a research stay at the University of Leipzig. CB is supported by a “Ramón y Cajal” fellowship by the MINECO, project n° RYC-2014-15615. This study is a contribution of the project “Macroevolutionary transitions in Syllidae” CGL2015-63593-P supported by the MINECO/FEDER, UE funds. We acknowledge support from the German Research Foundation (DFG) and Universität Leipzig within the program of Open Access Publishing.
Availability of data and materials
The data sets supporting the results of this article are available in the morphdbase repository , https://www.morphdbase.de/, under accessions https://www.morphdbase.de/?M_Weidhase_20160818-M-7.1, https://www.morphdbase.de/?M_Weidhase_20160818-M-8.1, https://www.morphdbase.de/?M_Weidhase_20160818-M-9.1, https://www.morphdbase.de/?M_Weidhase_20160818-M-10.1, https://www.morphdbase.de/?M_Weidhase_20160818-M-11.1, https://www.morphdbase.de/?M_Weidhase_20160818-M-12.1, and https://www.morphdbase.de/?P_Beckers_20160817-M-91.1.
MW, CB and MTA designed the project. MW cultured the animals, performed all regeneration experiments and immunohistochemical stainings including cLSM analyses and generated the figures. PB did all histological sections, staining as well as appending image acquisition. MW, CB and MTA drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
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