Ultrastructural Studies of Calcium Location during the “Catch” Contraction of Clam Smooth Adductor Muscle Cells

Shell-adductor muscle attachment and Ca2+ transport in the bivalves Ostrea stentina and Anomia ephippium

Among bivalve muscles, the adductors are particularly important for animal survival because they control valve closure. Most studies have addressed the type and morphology of this muscle in bivalves but few have focused on the mechanism that anchors it to the shell myostracum layer. Moreover, the possible calcium transport mechanism through the adductor muscle cells to the myostracum shell layer, which is necessary for bivalve biomineralisation, has never been addressed. Our results indicate that the muscle cell-shell attachment is mediated by the outer mantle epithelial cell layer, here termed tendon cells. These cells are modified at the muscle scar zone by the presence of actin cytoskeletal bundles, which anchor cells to the extracellular matrix via focal adhesion (or focal contact) junctions at the basal side and to extrapallial matrix at the apical side, both rich in collagen. From apical focal adhesions, bundles of collagen-rich fibres cross the extrapallial space and penetrate the myostracum shell layer. The latter constitutes one of the strongest anchoring structures among invertebrates. Numerous vesicles protrude from the tendon cells into the extrapallial space. TEM-EDX analysis reveals the presence of Ca²⁺ inside some of these vesicles both in tendon cells and in the extrapallial space. This suggests a potential mechanism for calcium transport from cells to the myostracum. STATEMENT OF SIGNIFICANCE: The interfaces between bivalve shells and muscular attachments are unique and of special interest as adhesive functional biomaterials, being one of the strongest invertebrate anchoring structures. We present an updated ultrastructural model of the adductor muscle-shell attachment. Muscle cells connect with the shell through epithelial `tendon cells`, which have a cytoskeleton of actin microfilaments that connect to the extracellular matrix via focal adhesions. Collagen-rich fibres arise from apical focal adhesions, cross the nanometric extrapallial space and penetrate the myostracum where they form an organic network. Calcium is present inside vesicles that are released into the extrapallial space. The lack of direct cellular control on secretion restricts the myostracal microstructure to prismatic aragonitic similar to its inorganic counterpart.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Identification and origin of the germline stem cells as revealed by the expression of nanos-related gene in planarians

The planarian's remarkable regenerative ability is thought to be supported by the stem cells (neoblasts) found throughout its body. Here we report the identification of a subpopulation of neoblasts, which was revealed by the expression of the nanos-related gene of the planarian Dugesia japonica, termed Djnos. Djnos-expressing cells in the asexual planarian were distributed to the prospective ovary or testes forming region in the sexual planarian. During sexualization, Djnos-expressing cells produce germ cells, suggesting that in the asexual state these cells were kept as germline stem cells for the oogonia and spermatogonia. Interestingly, the germline stem cells were indistinguishable from the neoblasts by morphology and X-ray sensitivity and did not seem to contribute to the regeneration at all. Germline stem cells initially appear in the growing infant planarian, suggesting that germline stem cells are separated from somatic stem cells in the planarian. Thus, planarian neoblasts can be classified into two groups; somatic stem cells for regeneration and tissue renewal, and germline stem cells for production of germ cells during sexualization. However, Djnos-positive cells appeared in the newly formed trunk region from the head piece, suggesting that somatic stem cells can convert to germline stem cells.

Isolation of planarian X-ray-sensitive stem cells by fluorescence-activated cell sorting

The remarkable capability of planarian regeneration is mediated by a group of adult stem cells referred to as neoblasts. Although these cells possess many unique cytological characteristics (e.g. they are X-ray sensitive and contain chromatoid bodies), it has been difficult to isolate them after cell dissociation. This is one of the major reasons why planarian regenerative mechanisms have remained elusive for a long time. Here, we describe a new method to isolate the planarian adult stem cells as X-ray-sensitive cell populations by fluorescence-activated cell sorting (FACS). Dissociated cells from whole planarians were labeled with fluorescent dyes prior to fractionation by FACS. We compared the FACS profiles from X-ray-irradiated and non-irradiated planarians, and thereby found two cell fractions which contained X-ray-sensitive cells. These fractions, designated X1 and X2, were subjected to electron microscopic morphological analysis. We concluded that X-ray-sensitive cells in both fractions possessed typical stem cell morphology: an ovoid shape with a large nucleus and scant cytoplasm, and chromatoid bodies in the cytoplasm. This method of isolating X-ray-sensitive cells using FACS may provide a key tool for advancing our understanding of the stem cell system in planarians.

Distribution of the stem cells (neoblasts) in the planarian Dugesia japonica

It has been postulated that the high regeneration ability of planarians is supported by totipotent stem cells, called neoblasts. There have been a few reports showing the distribution of neoblasts in planarians. However, the findings were not completely consistent. To determine the distribution of neoblasts, we focused on proliferating cell nuclear antigen (PCNA), which is present in proliferative cells. We cloned and sequenced the cDNA of PCNA from the planarian Dugesia japonica and produced an antiserum recognizing the gene product. X-ray irradiation caused rapid loss of all PCNA-positive cells and loss of the neoblasts (which were morphologically defined by the presence of the chromatoid body), strongly suggesting that all PCNA-positive cells were true neoblasts. Using the antiserum, we were successful in identifying the neoblasts more clearly than any previous work. In addition to their dispersed distribution in the dorsal and ventral mesenchyme, the neoblasts were distributed as clusters along the midline and bilateral lines in the dorsal mesenchyme. We also examined the behavior of the neoblasts after decapitation. Decapitation did not seem to affect the migration of neoblasts far from the wound. We demonstrated here that DjPCNA is a powerful tool for identifying planarian neoblasts.

Intercalary regeneration in planarians

How can a planarian regenerate its entire body from a small portion of its body? Neoblasts, the totipotent stem cells of planarian, are assumed to be able to produce all missing cell types. However, we do not know how the cell fate of these cells is controlled during regeneration. Our recent studies with molecular markers suggest that intercalary regeneration is the fundamental principle in planarian regeneration. Here, we introduce the intercalation induced by ectopic grafting along the anteroposterior (A-P), dorsoventral (D-V), and left-right (L-R) axes. Blastema formation is evoked by ectopic D-V interactions after wound closure. Intercalation between the blastema and stump induces rearrangement of the positional identities along the A-P axis. Consequently, totipotent stem cells change their differentiation patterns according to the newly rearranged positional identities along the A-P, D-V, and L-R axes. According to the classic view, the blastema is regarded as the place where undifferentiated cells accumulate and regenerative events occur. Here, we propose a new interpretation, i.e., that the blastema may work as a signaling center inducing intercalary regeneration. Also, the roles of molecules and genes involved in intercalary regeneration are discussed.

Mediolateral intercalation in planarians revealed by grafting experiments

We investigated how planarians organize their left-right axis by using ectopic grafting. Planarians have three body axes: anteroposterior (A-P), dorsoventral (D-V), and left-right (L-R). When a small piece is implanted into an ectopic region along the A-P and D-V axes, intercalary structures are always formed to compensate for positional gaps. There are two hypotheses regarding L-R axis formation in this organism: first, that the left and right sides of the animal may be recognized as different parts, and L-R intercalation can induce midline structures (asymmetry hypothesis); second, that both sides may have symmetrical positional values, and mediolateral (M-L) intercalation creates positional values along the L-R axis (symmetry hypothesis). We performed ectopic grafting experiments in the head region of the planarian, Dugesia japonica, to examine these hypotheses. A left lateral fragment containing a left auricle was implanted into the medial region of the host. Ectopic structures were always formed only on the left side of the graft, where lateral tissues abutted onto the medial tissues. However, no morphologic change was induced on the right side of the graft, where left-sided tissues faced onto right-sided tissues. Molecular marker analyses indicated that ectopic structures formed on the left side of the graft were induced by M-L intercalation, supporting the "symmetry hypothesis." When the midline tissues were implanted into a lateral region, they induced a complete ectopic head, demonstrating that M-L intercalation may be sufficient to establish the L-R axis in planarians.

Cultivation and characterization of planarian neuronal cells isolated by fluorescence activated cell sorting (FACS)

Studies using molecular markers have revealed that planarians possess a highly organized brain. Here we separated brain neurons from dissociated planarian head cells by fluorescence activated cell sorting (FACS), and characterized them by single cell PCR analysis and cell culture. Dissociated cells were labeled with three different fluorescent dyes, Hoechst 33258, Merocyanine 540, and Propidium Iodide (PI), and fractioned by FACS. Interestingly, we have succeeded in identifying a cell fraction specific to the head, which we have named the head-abundant cell fraction (HAC). Most of the HAC expressed neuron-specific genes and proteins. When they were cultured in vitro, they showed an ability to extend neurites on several types of extracellular matrices (ECMs), and, depending on the ECM type used, presented a high level of plasticity in morphology and gene expression.