Muscle and neuronal differentiation in primary cell culture of larval Mytilus trossulus (Mollusca: Bivalvia)

In vitro proliferation of Mytilus edulis male germ cell progenitors

Our understanding of basic cellular processes has mostly been provided by mammalian cell culture, and by some non-mammalian vertebrate and few invertebrate cell culture models. Developing reliable culture conditions for non-model organisms is essential to allow investigation of more unusual cellular processes. Here, we investigate how cells isolated from different tissues of the marine mussel Mytilus edulis thrive and survive in vitro in the hope of establishing a suitable laboratory model for the investigation of cellular mechanisms specific to these bivalve mollusks. We found that cells dissociated from mantle tissue attached to the culture vessels and proliferated well in vitro, whereas cells isolated from gills, although remaining viable, did not maintain divisions over three to four weeks in culture. We used antibodies against the germ-line marker DEAD-box helicase 4 (DDX4), also known as VASA, and the epithelial cell marker cytokeratin to distinguish different cell types in culture. DDX4-positive cells were predominant in 25-day-old cultures from male mantles. Cells from other tissues remained in low numbers and did not seem to change in composition over time. Overall, the culture conditions described here allow an efficient selection of male germ cells that could be used to study specific cellular mechanisms in vitro.

Comparative Neuroanatomy of Pediveliger Larvae of Various Bivalves from the Sea of Japan

Here, we describe the nervous system structures from pediveligers of eight bivalve species (Callista brevisiphonata, Mactromeris polynyma, Crenomytilus grayanus, Kellia japonica, Mizuhopecten yessoensis, and Azumapecten farreri) with different modes of life in their adult stages, corresponding to the ecological niches that they occupy (burrowing, cemented, byssally attached, and mobile forms). We have identified neuromorphological features of the central and peripheral nervous systems in larval bivalves. We show that the unpaired sensory apical organ is still present in pediveligers along with the developing paired cerebral ganglia characteristic of an adult mollusk. Pediveligers have the pleural ganglia connected to the pedal ganglia via the pedal nerve cords and to the visceral ganglia via the lateral nerve cords. We have found a number of structures of the peripheral nervous system whose presence varies between pediveligers of different species. Mactromeris, Callista, and Pododesmus have 5-HT-immunopositive stomatogastric neurons, whereas the Yesso and Farrer's scallops have an FMRFamide-immunopositive enteric nervous system. The innervation of the anterior part of the velum is connected to a system of the apical organ and cerebral ganglia, and the innervation of the posterior part is connected to the visceral ganglia. Most differences in the structure of the peripheral elements of the nervous system are species-specific and weakly depend on the ecological niche that pediveligers occupy.

Presence of immune factors in freshwater mussel (Hyriopsis cumingii) entails autologous serum an essential component in the culture of mantle cells

Mussel cell culture is a challenging problem and serum serves a crucial biological role in cell culture as an autologous supply and an immunizing agent. In this study, the biology (calcium ions, total protein, pH, and osmotic pressure) of fetal bovine serum (FBS) and Hyriopsis cumingii serum (HCS) was investigated, and the development of Hyriopsis cumingii (H. cumingii) mantle cells in HCS and FBS systems was examined. The results showed that total protein, calcium ions, and osmotic pressure varied significantly (p<0.05). The activity of mantle cells was superior in the HCS culture system to that in the FBS culture system. The label-free technique was used to distinguish the two serum proteins to investigate the supportive effect of autologous serum on cell culture. These were examined for 109 unique proteins and 35 particular HCS proteins. Most differentially expressed proteins (DEPs) were involved in immune response, cell differentiation, and calcium ion binding. Furthermore, immune factors such as HSP, CALR, APOB, C3 were identified with significant differences. HSP was significantly more present in HCS than in FBS as an endogenous protective protein that regulates immune system function, cell differentiation, transport, and activity regulation. Parallel reaction monitoring (PRM) analysis was carried out to validate the expression levels of 19 DEPs, indicating high reliability of the proteomic results. This study reveals the important role of immune factors in mussel cell culture, providing a theoretical basis for explaining the applicability of autologous serum in cell culture. It is also helpful in improving the cell culture conditions of mussels.

Differentiation and Maturation of Muscle and Fat Cells in Cultivated Seafood: Lessons from Developmental Biology

Cultivated meat, also known as cultured or cell-based meat, is meat produced directly from cultured animal cells rather than from a whole animal. Cultivated meat and seafood have been proposed as a means of mitigating the substantial harms associated with current production methods, including damage to the environment, antibiotic resistance, food security challenges, poor animal welfare, and-in the case of seafood-overfishing and ecological damage associated with fishing and aquaculture. Because biomedical tissue engineering research, from which cultivated meat draws a great deal of inspiration, has thus far been conducted almost exclusively in mammals, cultivated seafood suffers from a lack of established protocols for producing complex tissues in vitro. At the same time, fish such as the zebrafish Danio rerio have been widely used as model organisms in developmental biology. Therefore, many of the mechanisms and signaling pathways involved in the formation of muscle, fat, and other relevant tissue are relatively well understood for this species. The same processes are understood to a lesser degree in aquatic invertebrates. This review discusses the differentiation and maturation of meat-relevant cell types in aquatic species and makes recommendations for future research aimed at recapitulating these processes to produce cultivated fish and shellfish.

Neurogenesis of the scallop Azumapecten farreri: from the first larval sensory neurons to the definitive nervous system of juveniles

Background

Scallops are among the best-studied bivalve mollusks. However, adult nervous system and neurogenesis studies of scallops are limited. Here, we studied the localization of neurotransmitters (serotonin/5-HT, FMRFamide, catecholamines) in adult ganglia and larvae of Azumapecten farreri using histochemical and immunohistochemical methods.

Results

We found peptide FMRFamide in all adult scallop ganglia, whereas 5-HT-like immunoreactive (lir) somata were exclusively detected in the cerebropleural, pedal, and accessory ganglia. Scallop larval neurogenesis starts with the emergence of the 5-HT-lir neurons, which are part of the apical organ (AO) at the early veliger stage. Near the AO, paired anlagen of cerebral ganglion (CG) developed. 5-HT-lir neurites of the CG innervate the velum, ventral, and dorsal parts of the larva at the late veliger stage. Scallop pediveligers possess 5-HT-lir CG, pleural ganglia, and immunopositive signals in the developing enteric nervous system. FMRFamide-lir is first detected in dorsal, ventral, and AO cells of early veligers. Later, FMRFamide-lir extends to the visceral nervous cord, all ganglia, as well as in the enteric nervous system in pediveligers. Catecholaminergic neurons are detected near the larval mouth, in the vellum, and in the stomach in veligers.

Conclusions

We described the distribution of neurotransmitters of the ganglia in adult scallops and the larval neurodevelopment in A. farreri. Immunostaining of neurotransmitters showed that the gross anatomy of adult scallop ganglia, in general, is similar to that in other bivalves, but complicated by the complexity of the structure of the ganglia and the appearance of additional ganglia not described in other molluscs. A comparison of larval neuromorphology suggests that 5-HT-lir structures are more conservative than FMRF-lir structures in Bivalvia. Notably, the latter are much more distributed in scallop A. farreri larvae than in other studied bivalves.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00468-7.

Status in molluscan cell line development in last one decade (2010–2020): impediments and way forward

Despite the attempts that have started since the 1960s, not even a single cell line of marine molluscs is available. Considering the vast contribution of marine bivalve aquaculture to the world economy, the prevailing viral threats, and the dismaying lack of advancements in molluscan virology, the requirement of a marine molluscan cell line is indispensable. This synthetic review discusses the obstacles in developing a marine molluscan cell line concerning the choice of species, the selection of tissue and decontamination, and cell culture media, with emphasis given on the current decade 2010-2020. Detailed accounts on the experiments on the virus cultivation in vitro and molluscan cell immortalization, with a brief note on the history and applications of the molluscan cell culture, are elucidated to give a holistic picture of the current status and future trends in molluscan cell line development.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-022-00539-x.

Amphioxus muscle transcriptomes reveal vertebrate-like myoblast fusion genes and a highly conserved role of insulin signalling in the metabolism of muscle

BACKGROUND

The formation and functioning of muscles are fundamental aspects of animal biology, and the evolution of 'muscle genes' is central to our understanding of this tissue. Feeding-fasting-refeeding experiments have been widely used to assess muscle cellular and metabolic responses to nutrition. Though these studies have focused on vertebrate models and only a few invertebrate systems, they have found similar processes are involved in muscle degradation and maintenance. Motivation for these studies stems from interest in diseases whose pathologies involve muscle atrophy, a symptom also triggered by fasting, as well as commercial interest in the muscle mass of animals kept for consumption. Experimentally modelling atrophy by manipulating nutritional state causes muscle mass to be depleted during starvation and replenished with refeeding so that the genetic mechanisms controlling muscle growth and degradation can be understood.

RESULTS

Using amphioxus, the earliest branching chordate lineage, we address the gap in previous work stemming from comparisons between distantly related vertebrate and invertebrate models. Our amphioxus feeding-fasting-refeeding muscle transcriptomes reveal a highly conserved myogenic program and that the pro-orthologues of many vertebrate myoblast fusion genes were present in the ancestral chordate, despite these invertebrate chordates having unfused mononucleate myocytes. We found that genes differentially expressed between fed and fasted amphioxus were orthologous to the genes that respond to nutritional state in vertebrates. This response is driven in a large part by the highly conserved IGF/Akt/FOXO pathway, where depleted nutrient levels result in activation of FOXO, a transcription factor with many autophagy-related gene targets.

CONCLUSION

Reconstruction of these gene networks and pathways in amphioxus muscle provides a key point of comparison between the distantly related groups assessed thus far, significantly refining the reconstruction of the ancestral state for chordate myoblast fusion genes and identifying the extensive role of duplicated genes in the IGF/Akt/FOXO pathway across animals. Our study elucidates the evolutionary trajectory of muscle genes as they relate to the increased complexity of vertebrate muscles and muscle development.

Cell type diversity in scallop adductor muscles revealed by single-cell RNA-Seq

Studies on cell atlas in marine invertebrates provide a better understanding of cell types, stem cell maintenance, and lineages of cell differentiation. To investigate the molecular features of various cell types in molluscan muscles, we performed single-cell RNA sequencing (scRNA-seq) to map cell types in scallop adductor muscles. We uncovered the cell type-specific features of 20 cell clusters defined by the expression of multiple specific molecular markers. These cell clusters are mainly classified into four broad classes, including mesenchymal stem cells, muscle cells, neurons, and haemolymph cells. In particular, we identified a diverse repertoire of neurons in the striated adductor muscle, but not in the smooth muscle. We further reconstructed the cell differentiation events using all the cell clusters by single-cell pseudotemporal trajectories. By integrating dual BrdU-PCNA immunodetection, neuron-specific staining and electron microscopy observation, we showed the spatial distribution of mesenchymal stem cells and neurons in striated adductor muscle of scallops. The present findings will not only be useful to address the cell type-specific gene expression profiles in scallop muscles, but also provide valuable resources for cross-species comparison of marine organisms.

Regulation of growth-related genes by nutrition in paralarvae of the common octopus (Octopus vulgaris)

The common octopus (Octopus vulgaris) is a species of great interest to the aquaculture industry. However, the high mortalities registered during different phases of the octopus lifecycle, particularly the paralarvae stage, present a challenge for commercial aquaculture. Improvement of diet formulation is seen as one way to reduce mortality and improve growth. Molecular growth-markers could help to improve rearing protocols and increase survival and growth performance; therefore, over a hundred orthologous genes related to protein balance and muscle growth in vertebrates were identified for the common octopus and their suitability as molecular markers for growth in octopus paralarvae explored. We successfully amplified 14 of those genes and studied their transcription in paralarvae either fed with artemia, artemia + zoea diets or submitted to a short fasting-refeeding procedure. Paralarvae fed with artemia + zoea had higher growth rates compared to those fed only with artemia, as well as a significant increase in octopus mtor (mtor-L) and hsp90 (hsp90-L) transcription, with both genes also up-regulated during refeeding. Our results suggest that at least mtor-L and hsp90-L are likely linked to somatic growth in octopus paralarvae. Conversely, ckip1-L, crk-L, src-L and srf-L had expression patterns that did not match to periods of growth as would be expected based on similar studies in vertebrates, indicating that further research is needed to understand their function during growth and in a muscle specific context.

Chemical modulation of apoptosis in molluscan cell cultures

This study focused on the alterations that occur in larval molluscan cells after administration of apoptotic inducers and inhibitors used in mammalian cells in response to cold stress. This is the first report on apoptosis modulation in molluscan cells assessed by flow cytometry. Mitochondrial activity, general caspase activation, and membrane integrity of control molluscan cells were compared to those processes in frozen-thawed molluscan cells, primary mouse embryonic fibroblasts, and human colon tumor cells prior to treatment and after incubation with apoptotic inducers or inhibitors. We tested three apoptotic inducers (staurosporine, camptothecin, and mitomycin C, routinely used for the chemical induction of apoptosis in different mammalian cells) and found that only staurosporine resulted in an evident apoptotic increase in molluscan cell cultures: 9.06% early apoptotic cells in comparison with 5.63% in control frozen-thawed cells and 20.6% late apoptotic cells in comparison with 10.68% in controls. Camptothecin did not significantly induce molluscan cell apoptosis but did cause a slight increase in the number of active cells after thawing. Mitomycin C produced similar results, but its effect was less pronounced. In addition, we hypothesize that the use of the apoptotic inhibitors could reduce apoptosis, which is significant after cryopreservation in molluscan cells; however, our attempts failed. Development in this direction is important for understanding the mechanisms of marine organisms' cold susceptibility.

From the raw bar to the bench: Bivalves as models for human health

Bivalves, from raw oysters to steamed clams, are popular choices among seafood lovers and once limited to the coastal areas. The rapid growth of the aquaculture industry and improvement in the preservation and transport of seafood have enabled them to be readily available anywhere in the world. Over the years, oysters, mussels, scallops, and clams have been the focus of research for improving the production, managing resources, and investigating basic biological and ecological questions. During this decade, an impressive amount of information using high-throughput genomic, transcriptomic and proteomic technologies has been produced in various classes of the Mollusca group, and it is anticipated that basic and applied research will significantly benefit from this resource. One aspect that is also taking momentum is the use of bivalves as a model system for human health. In this review, we highlight some of the aspects of the biology of bivalves that have direct implications in human health including the shell formation, stem cells and cell differentiation, the ability to fight opportunistic and specific pathogens in the absence of adaptive immunity, as source of alternative drugs, mucosal immunity and, microbiome turnover, toxicology, and cancer research. There is still a long way to go; however, the next time you order a dozen oysters at your favorite raw bar, think about a tasty model organism that will not only please your palate but also help unlock multiple aspects of molluscan biology and improve human health.

The death pathways in mussel larval cells after a freeze-thaw cycle

We analyzed cell viability, caspase activity, plasma membrane alterations and cell ultrastructure morphology to estimate the morphological and biochemical alterations that occur in bivalve molluscan cell cultures during cryopreservation. The use of 5% dymethyl sulfoxide as a cryoprotectant resulted in greater cell survival and a scarcity of destroyed cells lacking cytosol among dead cells. In this case, almost all cells died through necrosis or apoptosis, which appeared to increase in mussel cell cultures after a freeze-thaw cycle. Apoptosis was not a main death pathway in mussel cells, but it was induced in a significant part of these cells (up to 24%) immediately after thawing and depended mostly on the cryoprotectant used. Regardless of the type of the used cryoprotectant, we observed some nuclear aberrations in cells after freezing-thawing, such as few multipolar mitoses or the absence of a division spindle in mitotic cells. After analyzing different methods for assessing cell damage, the best results were obtained from optimal approaches that could provide information regarding the cell disruption level after freezing-thawing and could be considered for future studies.

Culture of neural cells of the eyestalk of a mangrove crab is optimized on poly-L-ornithine substrate

Although there is a considerable demand for cell culture protocols from invertebrates for both basic and applied research, few attempts have been made to culture neural cells of crustaceans. We describe an in vitro method that permits the proliferation, growth and characterization of neural cells from the visual system of an adult decapod crustacean. We explain the coating of the culture plates with different adhesive substrates, and the adaptation of the medium to maintain viable neural cells for up to 7 days. Scanning electron microscopy allowed us to monitor the conditioned culture medium to assess cell morphology and cell damage. We quantified cells in the different substrates and performed statistical analyses. Of the most commonly used substrates, poly-L-ornithine was found to be the best for maintaining neural cells for 7 days. We characterized glial cells and neurons, and observed cell proliferation using immunocytochemical reactions with specific markers. This protocol was designed to aid in conducting investigations of adult crustacean neural cells in culture. We believe that an advantage of this method is the potential for adaptation to neural cells from other arthropods and even other groups of invertebrates.

β Integrin-like protein-mediated adhesion and its disturbances during cell cultivation of the mussel Mytilus trossulus

In this study, we focus on the specific contribution of β integrin-like protein to adhesion-mediated events in molluscan larval cells in culture that could not have been investigated within the whole animal. An analysis of disturbances to cell-substratum adhesion, caused by the integrin receptor inhibiting Arg-Gly-Asp-Ser (RGDS)-peptide, the Ca(2+)/Mg(2+)-chelators and the stress influence of freezing-thawing, reveals that all these factors resulted in the partial destruction of the integrin-extracellular matrix (ECM) interaction in culture and, in particular, changes in the distribution and relative abundance of β integrin-positive cells. The experiments, carried out on selected substrates, found that β integrin-positive cells demonstrate different affinities for the substrates. This finding further supports the assumption that epithelial differentiation in cultivated cells of larval Mytilus may be mediated by β integrin-like proteins via binding to laminin; direct binding to other components of the ECM could not be demonstrated. The mussel β integrin-positive cells are not involved in myogenic or neuronal differentiation on any of the substrates but part of them has tubulin-positive cilia, forming some epithelia-like structures. Our data indicate that β integrin-positive cells are able to proliferate in vitro which suggests that they could participate in renewing the digestive epithelium in larvae. The findings provide evidence that the distribution pattern of β integrin-like protein depends on the cell type and the factors influencing the adhesion.

Identification of Mytilus edulis genetic regulators during early development

Understanding the mechanisms that enable growth and survival of an organism while driving it to the full range of its adaptation is fundamental to the issues of biodiversity and evolution, particularly regarding global climatic changes. Here we report the Illumina RNA-sequencing (RNA-seq) and de novo assembly of the blue mussel Mytilus edulis transcriptome during early development. This study is based on high-throughput data, which associates genome-wide differentially expressed transcript (DET) patterns with early activation of developmental processes. Approximately 50,383 high-quality contigs were assembled. Over 8000 transcripts were associated with functional proteins from public databases. Coding and non-coding genes served to design customized microarrays targeting every developmental stage, which encompass major transitions in tissue organization. Consequently, multi-processing pattern exploration protocols applied to 3633 DETs helped discover 12 unique coordinated eigengenes supposedly implicated in various physiological and morphological changes that larvae undergo during early development. Moreover, dynamic Bayesian networks (DBNs) provided key insights to understand stage-specific molecular mechanisms activated throughout ontogeny. In addition, delayed and contemporaneous interactions between DETs were coerced with 16 relevant regulators that interrelated in non-random genetic regulatory networks (GRNs). Genes associated with mechanisms of neural and muscular development have been characterized and further included in dynamic networks necessary in growth and functional morphology. This is the first large-scale study being dedicated to M. edulis throughout early ontogeny. Integration between RNA-seq and microarray data enabled a high-throughput exploration of hidden processes essential in growth and survival of microscopic mussel larvae. Our integrative approach will support a holistic understanding of systems biology and will help establish new links between environmental assessment and functional development of marine bivalves.

Molluscan cells in culture: primary cell cultures and cell lines

In vitro cell culture systems from molluscs have significantly contributed to our basic understanding of complex physiological processes occurring within or between tissue-specific cells, yielding information unattainable using intact animal models. In vitro cultures of neuronal cells from gastropods show how simplified cell models can inform our understanding of complex networks in intact organisms. Primary cell cultures from marine and freshwater bivalve and gastropod species are used as biomonitors for environmental contaminants, as models for gene transfer technologies, and for studies of innate immunity and neoplastic disease. Despite efforts to isolate proliferative cell lines from molluscs, the snail Biomphalaria glabrata Say, 1818 embryonic (Bge) cell line is the only existing cell line originating from any molluscan species. Taking an organ systems approach, this review summarizes efforts to establish molluscan cell cultures and describes the varied applications of primary cell cultures in research. Because of the unique status of the Bge cell line, an account is presented of the establishment of this cell line, and of how these cells have contributed to our understanding of snail host-parasite interactions. Finally, we detail the difficulties commonly encountered in efforts to establish cell lines from molluscs and discuss how these difficulties might be overcome.

Extracellular matrix is required for muscle differentiation in primary cell cultures of larval Mytilus trossulus (Mollusca: Bivalvia)

Components of the extracellular matrix may modulate the growth factor effects that play important roles in the proliferation and differentiation of precursor cells. We developed an in vitro cultivation protocol for cells of the larval marine bivalve Mytilus trossulus to study the role that extracellular matrix components may play in myodifferentiation and replication-mediated DNA synthesis using immunofluorescence and confocal laser scanning microscopy. Here, we demonstrate that the extracellular matrix regulates the expression of muscle proteins, leading to their assembly and the terminal muscle differentiation of larval cells during cultivation. We further show that the myogenesis process progresses in cells cultivated on fibronectin, carbon or poly-L-lysine but is inhibited in cells grown on a collagen carpet. Consistent with a decrease in muscle protein expression in cells cultivated on collagen, we demonstrate an increase in the number of BrdU-positive cells in comparison with cells cultured on other substrates during the entire cultivation period. Moreover, we demonstrate that the matrix-dependent myogenic differentiation of larval mussel cells is reversible. Round-shaped cells cultivated on collagen were able to differentiate into muscle cells after reseeding on fibronectin, carbon or poly-L-lysine. In addition, cells cultured on collagen and then transplanted to fibronectin exhibited distinct cross-striation and contractile activity. Taken together, our data suggest that the extracellular matrix participates in the regulation of the proliferation and myodifferentiation of mussel trochophore progenitor cells and validate novel approaches for successfully culturing cells from bivalves over extended periods.

Development, organization, and remodeling of phoronid muscles from embryo to metamorphosis (Lophotrochozoa: Phoronida)

BACKGROUND

The phoronid larva, which is called the actinotrocha, is one of the most remarkable planktotrophic larval types among marine invertebrates. Actinotrochs live in plankton for relatively long periods and undergo catastrophic metamorphosis, in which some parts of the larval body are consumed by the juvenile. The development and organization of the muscular system has never been described in detail for actinotrochs and for other stages in the phoronid life cycle.

RESULTS

In Phoronopsis harmeri, muscular elements of the preoral lobe and the collar originate in the mid-gastrula stage from mesodermal cells, which have immigrated from the anterior wall of the archenteron. Muscles of the trunk originate from posterior mesoderm together with the trunk coelom. The organization of the muscular system in phoronid larvae of different species is very complex and consists of 14 groups of muscles. The telotroch constrictor, which holds the telotroch in the larval body during metamorphosis, is described for the first time. This unusual muscle is formed by apical myofilaments of the epidermal cells. Most larval muscles are formed by cells with cross-striated organization of myofibrils. During metamorphosis, most elements of the larval muscular system degenerate, but some of them remain and are integrated into the juvenile musculature.

CONCLUSION

Early steps of phoronid myogenesis reflect the peculiarities of the actinotroch larva: the muscle of the preoral lobe is the first muscle to appear, and it is important for food capture. The larval muscular system is organized in differently in different phoronid larvae, but always exhibits a complexity that probably results from the long pelagic life, planktotrophy, and catastrophic metamorphosis. Degeneration of the larval muscular system during phoronid metamorphosis occurs in two ways, i.e., by complete or by incomplete destruction of larval muscular elements. The organization and remodeling of the muscular system in phoronids exhibits the combination of protostome-like and deuterostome-like features. This combination, which has also been found in the organization of some other systems in phoronids, can be regarded as an important characteristic and one that probably reflects the basal position of phoronids within the Lophotrochozoa.

Localization of αvβ3-like integrin in cultivated larval cells of the mussel Mytilus trossulus during neuronal and muscle differentiation

Using immunofluorescence phenotyping, the expression of αvβ3-like integrin was examined during neuronal and muscle differentiation in cell cultures derived from trochophore larvae of the mussel Mytilus trossulus. We have demonstrated that some mussel cells grown on fibronectin in vitro express the extracellular matrix (ECM) αvβ3 integrin-like receptor. At the same time, the distribution of αvβ3-like integrin is not ubiquitous, i.e. it depends on the cell type and the time of cultivation. Using immunohistochemical staining, we have found that only in some cells this integrin is co-localized with molluscan neuronal markers, neurotransmitters serotonin (5-HT) or Phe-Met-Arg-Phe-NH(2) neuropeptide (FMRFamide), and also with filament actin but not with paramyosin. Although we have previously shown that an integrin-dependent mechanism is involved in cell adhesion and differentiation of muscle cells of Mytilus, in this study, αvβ3-like integrin has not been found to participate in fibronectin adhesion of muscle cells but may be a linking agent between the ECM and the neuron-like cells.