Fine structure and differentiation of Ascidian muscle. I. Differentiated caudal musculature of Distaplia occidentalis tadpoles

Regulation and evolution of muscle development in tunicates

For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.

Mutations affecting tail and notochord development in the ascidian Ciona savignyi

Ascidians are among the most distant chordate relatives of the vertebrates. However, ascidians share many features with vertebrates including a notochord and hollow dorsal nerve cord. A screen for N-ethyl-N-nitrosourea (ENU)-induced mutations affecting early development in the ascidian Ciona savignyi resulted in the isolation of a number of mutants including the complementing notochord mutants chongmague and chobi. In chongmague embryos the notochord fails to develop, and the notochord cells instead adopt a mesenchyme-like fate. The failure of notochord development in chongmague embryos results in a severe truncation of tail, although development of the tail muscles and caudal nerve tracts appears largely normal. Chobi embryos also have a truncation of the tail stemming from a disruption of the notochord. However, in chobi embryos the early development of the notochord appears normal and defects occur later as the notochord attempts to extend and direct elongation of the tail. We find in chobi tailbud embryos that the notochord is often bent, with cells clumped together, rather than extended as a column. These results provide new information on the function and development of the ascidian notochord. In addition, the results demonstrate how the unique features of ascidians can be used in genetic analysis of morphogenesis.

Cross-coupling between voltage-dependent Ca2+ channels and ryanodine receptors in developing ascidian muscle blastomeres

1. Ascidian blastomeres of muscle lineage express voltage-dependent calcium channels (VDCCs) despite isolation and cleavage arrest. Taking advantage of these large developing cells, developmental changes in functional relations between VDCC currents and intracellular Ca2+ stores were studied. 2. Inactivation of ascidian VDCCs is Ca2+ dependent, as demonstrated by two pieces of evidence: (1) a bell-shaped relationship between prepulse voltage and amplitude during the test pulse in Ca2+, but not in Ba2+, and (2) the decay kinetics of Ca2+ currents (ICa) obtained as the size of tail currents. 3. During replacement in the external solution of Ca2+ with Ba2+, the inward current appeared biphasic: it showed rapid decay followed by recovery and slow decay. This current profile was most evident in the mixed bath solution (2 % Ca2+ and 98 % Ba2+, abbreviated to '2Ca/98Ba'). 4. The biphasic profile of I2Ca/98Ba was significantly attenuated in caffeine and in ryanodine, indicating that Ca2+ release is involved in shaping the current kinetics of VDCCs. After washing out the caffeine, the biphasic pattern was reproducibly restored by depolarizing the membrane in calcium-rich solution, which is expected to refill the internal Ca2+ stores. 5. The inhibitors of endoplasmic reticulum (ER) Ca2+-ATPase (SERCAs) cyclopiazonic acid (CPA) and thapsigargin facilitated elimination of the biphasic profile with repetitive depolarization. 6. At a stage earlier than 36 h after fertilization, the biphasic profile of I2Ca/98Ba was not observed. However, caffeine induced a remarkable decrease in the amplitude of I2Ca/98Ba and this suppression was blocked by microinjection of the Ca2+ chelator BAPTA, showing the presence of caffeine-sensitive Ca2+ stores at this stage. 7. Electron microscopic observation shows that sarcoplasmic membranes (SR) arrange closer to the sarcolemma with maturation, suggesting that the formation of the ultrastructural machinery underlies development of the cross-coupling between VDCCs and Ca2+ stores.

Development of the liver in the chicken embryo. I. Hepatic cords and sinusoids

Hemopoiesis in the liver of the chicken embryo begins on day 7 of incubation (Hamburger and Hamilton Stage 30) and peaks on day 14 (Stage 40). During this time frame, the differentiation of hepatic cells was examined by light microscopy, transmission and scanning electron microscopy, and morphometry. The avian liver is a closely packed mass of dendriform cords and discontinuous sinusoids. Hepatocytes are pyramidal in shape, and they ring the bile canaliculi which run through the centers of the cords. Semithin sections, made possible by infiltration and embedding in glycol methacrylate, were stained with hematoxylin and eosin to assess the general architecture of the organ and the lipid content of the hepatocytes and by the periodic acid-Schiff reaction and hematoxylin to visualize the cytoplasmic stores of glycogen. The number of hepatocytes with demonstrable glycogen fluctuates erratically in early hemopoiesis, and the proportion of glycogen-containing cells progressively increases as hemopoiesis climbs to a peak. Most differentiating hepatocytes are devoid of lipid droplets until Stages 39 and 40. From Stage 30 to 35, hepatocyte volume falls to its lowest value. Subsequently (Stages 36 to 40), cell volume increases and hepatocytes achieve a relatively uniform size. Ultrastructural changes in the differentiating hepatocytes, including alterations to the mitochondria, endoplasmic reticulum, and Golgi apparatus, are documented. These morphological and morphometric findings on the prehepatocyte population and hepatic vasculature cover 2 of the 3 elements deemed critical to hepatic hemopoiesis in many vertebrates.

Multiple actin genes encoding the same alpha-muscle isoform are expressed during ascidian development

Ascidian embryos develop rapidly into tadpole larvae containing striated tail muscle cells. We have isolated and characterized five actin cDNA clones from a Styela clava tailbud-stage library. The nucleotide sequences of these clones and genomic Southern blot analysis indicate that they represent at least four different muscle actin genes, which are designated ScTb1, ScTb24, ScTb30, and ScTb12/34. The derived protein sequences of these genes indicate that they encode the same alpha-muscle actin which has features related to each of the three different classes of vertebrate alpha-muscle actins. Northern and in situ hybridization with probes prepared from the 3' untranslated region (UTR) of several of the ScTb clones showed that these muscle actin genes are expressed in different temporal and spatial patterns during development. ScTb1 was detected in eggs, embryos, and adults, ScTb24 and ScTb12/34 were detected in embryos and adults, and ScTb30 was detected only in embryos. The maternal transcripts disappeared shortly after fertilization and zygotic mRNAs were first detected during gastrulation and continued to accumulate during subsequent tail muscle differentiation. ScTb30 mRNA, which is expressed in the embryo, peaks during the tailbud stage and is present at low levels in the tadpole larva. In contrast, ScTb1, ScTb24, and ScTb12/34 mRNAs, which are expressed in embryos and adults, peak during the late tailbud stage and are present in substantial quantities in the larva. The ScTb24 gene was detected only in tail muscle cells, whereas the ScTb30 gene was detected in embryonic tail muscle, mesenchyme, epidermal, and neural cells. The ScTb24 mRNA also accumulates primarily in vascular tissue in the branchial sac and mantle of adults. The existence of a gene family encoding the same alpha-muscle actin isoform is unique among the chordates and may function to maximize muscle actin production during the rapid differentiation phase of ascidian larval muscle cells.

Developmental changes in delayed rectifier K+ currents in the muscular- and neural-type blastomere of ascidian embryos

1. Developmental changes in the amplitude, kinetic properties, tetraethyl-ammonium (TEA) sensitivity, and ion selectivity of the delayed rectifier K+ currents were investigated in differentiating muscular-type (M) and neural-type (N) blastomeres isolated from the early cleavage-arrested ascidian embryos, using conventional two-microelectrode voltage clamp techniques. 2. No voltage-sensitive outward K+ currents were found in either type of blastomere during the first 35 h of development at 9 degrees C. Thereafter the delayed rectifier K+ current became apparent. The peak amplitude of the K+ current in the M-blastomere increased abruptly from 50 to 60 h and tended to plateau after 60 h, while in the N-blastomere it continued to increase after initial emergence at around 35 h. 3. The threshold potential level of the K+ current in the M-blastomere was initially about -10 mV in a standard external solution (1 mM-K+ solution), but shifted towards the hyperpolarized direction until it reached a steady level at 45 h after fertilization. At the fully differentiated stages, the threshold was around -32 mV and -26 mV in the M- and N-blastomeres, respectively. 4. Throughout development, the reversal potential of the tail current changed with the external K+ concentration in both M- and N-blastomeres as expected for a K(+)-electrode. There was no significant difference in the selectivity ratios for the K+ channel between the two types of blastomeres. The relative selectivities were K+ (1.000): Rb+ (0.774): NH4+ (0.122): Na+ (0.074) and K+ (1.000): Rb+ (0.724): NH4+ (0.155): Na+ (0.074) in the M- and N-blastomeres, respectively. 5. Modified Scatchard plots of TEA-sensitivity data indicated a one-to-one reaction between TEA and the K+ channel. These plots revealed the presence of TEA-resistant K+ channels in addition to TEA-sensitive K+ channels in the M-blastomere, but revealed only TEA-sensitive K+ channels in the N-blastomere. The dissociation constant (Ki) values of these three types of K+ channel did not change during development. In the M-blastomere, the Ki of the TEA-sensitive K+ channel was 1.29 +/- 0.05 mM (mean +/- S.E.M., n = 31) and that of the TEA-resistant K+ channel was 1.4 +/- 0.1 M (mean +/- S.E.M., n = 31) at a test potential of 45 mV. The Ki value of the neural-type K+ current was 1.38 +/- 0.03 mM (mean +/- S.E.M., n = 20) at 45 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural study of the hyaline layer of the starfish embryo, Pisaster ochraceus

The hyaline layer (HL) is a multilayered extracellular matrix (ECM) that coats the external surfaces of sea urchin and starfish embryos. It is thought to protect and lubricate the embryo, stabilize the blastomeres during morphogenesis, and regulate nutrient intake. Ultrastructural studies of chemically fixed embryos have shown the HL to consist of two to four sublayers. However, since chemical fixatives may cause collapse and alter the positions and antigenicity of the extracellular components, fixation methods that exclude chemicals may reveal a picture of the HL closer to what is present in vivo. Freeze substitution, a fixation method whereby tissues are rapidly frozen and dehydrated at low temperatures, has proved useful for fixing material rich in ECM. In this study, embryos of the starfish Pisaster ochraceus were fixed for microscopy using freeze substitution and three chemical methods in order to determine, as accurately as possible, the structure of the HL. Embryos appear to be best preserved by freeze substitution and demonstrate a HL consisting of at least six distinct sublayers. Based on staining with anionic dyes, most sublayers appear to contain glycosaminoglycans. Freeze substituted embryos, which were also stained with monoclonal antibodies raised against their ECM, revealed that some molecules are common to all six sublayers, whereas other molecules may be restricted to specific sublayers. This suggests that each sublayer could have a different function. Additional evidence suggests that microvillus associated bodies, present in other marine invertebrate embryos, may anchor the asteroid HL to the cell surface microvilli.

Bryozoan larvae as mosaics of multifunctional ciliary fields: Ultrastructure of the sensory organs of Bugula solonifera (Cheilostomata: Cellularioidea)

The ultrastructure of ciliated sensory and effector organs has been examined by light microscopy and by scanning and transmission electron microscopy in larvae of the cellularioid cheilostome, Bugula stolonifera. The larval surface is formed by a mosaic of ciliary fields, each distinctive in organization and function. The most extensive ciliary field consists of the motile cilia of the corona, which collectively constitute the primary larval locomotory organ and form most of the larval surface. The corona is innervated basally by a nerve ring that circumscribes the larval equator. A circular field of immotile shorter cilia at the aboral pole arise from bands of radial cells in the apical disc. The radial ciliated cells form chemical synapses with underlying nerves of the paired aboral nerve cords, which extend to an equatorial nerve plexus on the median anterior side of the larva, beneath the pyriform organ. The pyriform organ is a complex of ciliated and glandular regions aligned along the anterior midline of the larva. It consists of, in aboral to oral sequence: the superior glandular field, the vibratile plume, the inferior glandular field, and the ciliated cleft. The vibratile plume is distinctive from the surrounding ciliation by its length and pattern of beating. It consists of four bundles of cilia approximately 64 μm long that beat rhythmically in synchrony into the ciliated cleft. Each bundle of cilia arises from a single cell which is further distinguished by apical pits of stout microvilli. The glandular fields and the vibratile plume are bordered by a variety of previously undescribed ciliated cells, longitudinal myoepithelial cells, and a pair of antero-medial eyespots. Another type of ciliated sensory cell is located between cornal cells. These intercoronal cells form synapses directly with the adjacent coronal cells. On the basis of these ultrastructural observations, the radial ciliated cells of the apical disc and the intercoronal cells are classified as primary sensory cells with different motor pathways. The radial ciliated cells are associated with the various effector organs of the larva via the nervous system. The intercoronal cells may combine sensory and motor functions by synapsing directly with the larval locomotory organ.

Development of a muscle actin specified by maternal and zygotic mRNA in ascidian embryos

In this investigation, we characterize the embryonic and adult actins and describe the embryonic expression of a muscle actin in the ascidian Styela. Two-dimensional polyacrylamide gel electrophoresis showed that embryos, tadpole larvae, and adult organs contain three major and two minor isoforms of actin. Two of the major isoforms, which are present in the mantle, branchial sac, alimentary tract, and gonads of adults and in eggs, embryos, and heads and tails of tadpoles, are likely to be cytoplasmic actins. The third major isoform, which was enriched in the mantle and branchial sac of adults and localized primarily in the tails of tadpoles, is a muscle actin. The muscle actin isoform was not detected in eggs and early embryos. Radioactivity incorporation studies showed that the cytoplasmic actins were synthesized throughout early development, but muscle actin synthesis was first detected between the 16- and 64-cell stages, 2-3 hr after fertilization. Two lines of evidence indicate that embryonic muscle actin synthesis is directed in part by maternal mRNA. First, poly(A)+ RNA isolated from unfertilized eggs directed the synthesis of muscle actin in an mRNA-dependent reticulocyte lysate. Second, muscle actin was synthesized in anucleate egg fragments. Arguments are also presented that muscle actin synthesis is not directed exclusively by maternal mRNA. It is concluded that embryonic and adult Styela exhibit actin heterogeneity, that one of the actin isoforms is a muscle actin, and that the muscle actin is synthesized during embryogenesis under the direction of maternal and zygotic mRNA.

The ontogeny of shell secretion in Terebratalia transversa (Brachiopoda, Articulata). II. Formation of the protegulum and juvenile shell

The fine structure of the shell and underlying mantle in young juveniles of the articulate brachiopod Terebratalia transversa has been examined by electron microscopy. The first shell produced by the mantle consists of a nonhinged protegulum that lacks concentric growth lines. The protegulum is secreted within a day after larval metamorphosis and typically measures 140-150 micron long. A thin organic periostracum constitutes the outer layer of the protegulum, and finely granular shell material occurs beneath the periostracum. Protegula resist digestion in sodium hypochlorite and are refractory to sectioning, suggesting that the subperiostracal portion of the primordial shell is mineralized. The juvenile shell at 4 days postmetamorphosis possesses incomplete sockets and rudimentary teeth that consist of nonfibrous material. The secondary layer occuring in the inner part of the juvenile shell contains imbricated fibers, whereas the outer portion of the shell comprises a bipartite periostracum and an underlying primary layer of nonfibrous shell. Deposition of the periostracum takes place within a slot that is situated between the so-called lobate and vesicular cells of the outer mantle lobe. Vesicular cells deposit the basal layer of the periostracum, while lobate cells contribute materials to the overlying periostracal superstructure. Cells with numerous tonofibrils and hemidesmosomes differentiate in the outer mantle epithelium at sites of muscle attachments, and unbranched punctae that surround mantle caeca develop throughout the subperiostracal portion of the shell. Three weeks after metamorphosis, the juvenile shell averages about 320 micron in length and is similar in ultrastructure to the shells secreted by adult articulates.

The ontogeny of shell secretion in Terebratalia transversa (Brachiopoda, Articulata). I. Development of the mantle

The morphology of the mantle in free-swimming and metamorphosing larvae of the articulate brachiopod Terebratalia transversa has been examined by scanning and transmission electron microscopy. The mantle begins to form approximately 2 days after fertilization and subsequently develops into a skirtlike lobe that encircles the middle region of the larval body. A simple epithelium covers both the outer surface of the mantle lobe and the inner side situated next to the pedicle lobe of the larva. During metamorphosis, the mantle lobe is everted over the anterior end of the larva. Thus, the epithelium covering the outer part of the mantle lobe in the larva subsequently becomes the inner epithelium of the juvenile mantle. Similarly, the inner epithelium of the larval mantle lobe represents the future outer epithelium of the juvenile mantle. In free-swimming larvae, the prospective outer mantle epithelium contains two types of cells, called "lobate" and "vesicular" cells. Lobate cells initially deposit a thin layer of amorphous material, and vesicular cells produce ovoid multigranular bodies. Following settlement at about 5 days postfertilization, the vesicular cells secrete an electron-dense sheet that constitutes the basal layer of the developing periostracum. Within several hours to a day thereafter, reversal of the mantle lobe is rapidly effected, apparently by contractions of the pedicle adjustor muscles.

Developmental autonomy of muscle fine structure in muscle lineage cells of ascidian embryos

We have observed ultrastructural features of muscle differentiation in the muscle lineage cells of cleavage-arrested whole embryos and partial embryos of ascidians. Whole embryos of Ciona intestinalis and Ascidia ceratodes were cleavage-arrested with cytochalasin B at the 8-cell stage and reared to an age equivalent to several hours after hatching; these embryos formed extensive myofilaments which were often further organized into myofibrils of different sizes and densities in the peripheral cytoplasm of the two muscle lineage blastomeres (B4.1 pair). Developing myofibrils in cleavage-arrested embryos resembled the muscle elements observed in normal hatched larvae, but were less uniformly organized. A similar development of myofilaments and myofibrils occurred in the muscle lineage cells of multicellular partial embryos reared to "hatching" age. These partial embryos resulted from the isolated muscle lineage pair (B4.1) of blastomeres of the 8-cell stage (Ciona and Ascidia), and from a muscle lineage blastomere pair (B5.2) isolated at the 16-cell stage (Ascidia). Muscle lineage cells in the partial embryos were readily identified by the dense aggregates of mitochondria in their cytoplasm. Taken together, these results from the two kinds of partial embryo effectively eliminate inductive interactions with embryonic tissues other than mesodermal as a necessary factor in the onset of self-differentiation in muscle lineage cells. The relative complexity of muscle phenotype expressed in cleavage-arrested and partial embryos attests to an unusually strong developmental autonomy in the ascidian muscle lineages. This autonomy lends further support to the theory that a localized and segregated egg cytoplasmic determinant is responsible for larval muscle development in ascidian embryos.

Development of acetylchilinesterase during embryogenesis of the ascidian Ciona intestinalis

We have characterized the embryonic muscle cell cholinesterase of the solitary ascidian, Ciona intestinalis (L.). The effects of selective enzyme inhibitors and the inhibition of enzyme activity at high concentrations of substrate suggest that the muscle cell enzyme is an acetylcholinesterase (E.C. 3.1.1.7). After gastrulation and before hatching, acetylcholinesterase activity increased 35- to 40-fold; after hatching (18 hours postfertilization) this activity continued to increase, leveling off at about 36 hours of development. Histochemical observations showed that before hatching acetylcholinesterase was located principally in the muscle cells of the tail and, after hatching, it began to develop in cells of the adult musculature and brain. Inhibition of protein syntnesis by puromycin and of RNA synthesis by actinomycin D, suggest that both protein and RNA synthesis were required for the increase in acetylcholinesterase activity observed in unhatched embryos. Although the continued increase in enzyme activity duirng embryonic development was sensitive to puromycin at all times tested, the actinomycin D sensitivity of this increase was restricted to a discrete time that was completed by about 11 hours of development.

An electron microscopic study of early developmental stages, myogenesis, oogenesis and cnidogenesis in the anthomedusa, Podocoryne carnea M. Sars

This descriptive electron microscopic study of the blastogenetic medusa development of Podocoryne carnea focuses on the earliest stages of primordium formation, myogenesis, oogenesis and cnidogenesis. The events which take place at the cellular level prior to the formation of stage 1 (Frey, '68), which are characterized by ecto- and entodermal accumulations of undifferentiated I-cells and the beginning of transdifferentiation of epithelial cells, have been subdivided into four distinct stages (U1-U4). The genealogy of cells participating in medusa differentiation indicates that some cell types of the medusa are derived by transdifferentiation from the polyp's epithelial cells, while others originate by differentiation from I-cells. The myogenesis of the subumbrellar muscle cells resembles vertebrate myocardiac differentiation in many respects.

Electron microscopic studies on the myotomes of larval lamprey, Lampetra japonica

Myotomes of the caudal one-third of the body of 26-day-old larval lampreys, Lampetra japonica, were studied by electron microscopy. Each myotome consists of horizontally stacked muscle lamellae. The myotomes are covered laterally by a single layer of flattened cells called here "lateral cells", and the other aspect is covered by an external lamina. The myotomes are midsegmentally innervated. Each muscle lamella usually contains two single cortical layers of myofibrils along the dorsal and ventral sarcolemma with a nucleus and mitochondria interposed between two layers. Numerous peripheral couplings are observed with relatively less developed triads. There are no membrane specializations to connect adjacent muscle lamellae within a myotome. Intermyotomal junctions are, however, noted between tips of cytoplasmic processes of muscle lamellae of adjoining myotomes. They resemble tight or gap junctions. No myofibrils are present in these cytoplasmic processes. Myotendinous junctions, with "terminal couplings" (Nakao, '75), are seen under development at the myoseptal ends of muscle lamellae. Lateral cells contain only ordinary organelles and no special structures such as myofibrils are found in the cytoplasm. They are connected to each other and to muscle lamellae by primitive desmosomes. They generally have no external lamina investment.

The fine structure of developing locomotor muscles of the pelagic tunicate, cyclosalpa affinis (Thaliacea: Salpidae)

Salps are free-swimming tunicates whose peculiar life history renders them ideal for developmental studies. The solitary salp reproduces asexually by budding a stolon containing the complete developmental sequence of the aggregate generation. The ultrastructure of developing locomotor muscle of the aggregate generation of Cyclosalpa affinis was studied. The early muscle contains essentially non-striated myofibrils. However, in transverse sections, , areas indicating early I-band A-bands can be recognized. As development continues, the number of fibrils increases, the Z-line appear, and the fibrils contain more recognizable striations. The fully developed muscle has the caracteristic structure of striated muscle. Longitudinal sections show sarcomeres with irregular and discontinuous (perforated) Z-lines; H-zones are not apparent. No M-lines are seen. Throughout development, the ratio of thin to thick myofilaments is always 2:1, the ratio found in all vertebrate striated muscle. Other finding in C affinis suggest that: (1) multinucleated muscle cells are formed by the fusion of mononucleated cells, (2) membranes of adjacement mononucleated cells destined to fuse form myelin figures, and (3) these myelin figures become closely associated with mitochondria.

An electron microscopic study of the neuromuscular junction in the myotomes of larval lamprey, Lampetra japonica

The neuromuscular junction in the myotomes of larval and adult lampreys, Lampetra japonica, was studied with the electron microscope. In larval lampreys of 26 days after artificial insemination, the myotome consists of triangular lamellae of muscle cell with their bases laterally and apexes medially oriented and placed one on the other. The lateral aspect of the myotome is covered by a layer of flattened cell, and the other aspect is covered by an external lamina which does not extend into the intercellular space between adjacent cells within a myotome. A bundle of thin axons was found in a depression at the middle of medial edge (apex) of each muscle lamella of the myotome and neuromuscular junction was formed here. No nerve endings were found at the myoseptal ends or at the lateral borders of the muscle lamellae. Enlarged axon terminals contained numerous clear vesicles with a few cored vesicles, mitochondria and neurofilaments. The presynaptic axolemma was separated from the postsynaptic sarcolemma by an interspace (50-55 mmu wide) with an interposed external lamina. In the trunk musculature of adult lampreys, nerve endings were found on the surface of the parietal fibers whereas they were seen on both the myoseptal ends near the myotendinous junction along the medial border of each central fiber. Thickening and enhancement in the electron density and accumulation of filamentous material on the sarcoplasmic surface of the postsynaptic sarcolemma were noted in both the larval and adult lampreys. Any other specific alterations were not found on both the pre- and postsynaptic membrane in the neuromuscular junctions of larval and adult lampreys. A brief comment was made on the relationship in development of the characteristic muscle units and patterns of the motor innervation in the larval and adult lampreys. The significance of the neuromuscular junction in the very young larvae reported here was also discussed in respect to the evolution of the neuromuscular junction, particularly in lower chordates, and it was presumed to be the most primitive pattern of innervation in the vertebrate skeletal muscle.

T system in ascidian muscle: organization of the sarcotubular system in the caudal muscle cells of Botryllus schlosseri tadpole larvae

The organization of the sarcotubular system has been examined in the caudal muscle cells of the ascidian. Botryllus schlosseri. At variance with striated muscle of other protochordates. Botryllus muscle cells are endowed with a well-developed T system, which has a peculiar laminar structure. The thin T laminae are in continuity with the plasma membrane and extend longitudinally in the intermyofibrillar spaces. At the level of the I-band the T laminae are focally associated with SR cisternae in dyad junctions similar to those observed in invertebrate muscles. These findings are discussed in relation to the origin of the sarcotubular system in vertebrate muscle.

Scanning electron microscope observations on the muscle innervation of Oikopleura dioica Fol (Appendicularia, Tunicata) with notes on the arrangement of connective tissue fibres

Critical point dried and fractured appendicularia of the species Oikopleura dioica have been examined in the scanning electron microscope. The dorsal nerve cord with ganglion cells and peripheral nerve fibres could easily be observed. Thick peripheral nerve fibres leave the nerve cord as bilateral pairs at constant intervals along the tail. Most of these fibres branch from the naked nerve cord, but some evidently originate in ganglion perikarya bulging out from the nerve cord itself. These paired peripheral nerves always have elaborate end-arborizations on the medial surface of the lateral muscle cells. They are accordingly interpreted as motor axons. Some thinner peripheral nerve fibres originate at irregular intervals from both the nerve cord and the ganglion cells. Due to the numerous extracellular fibrils that connect the bilateral layers of the epidermal fins and the muscle cells to each other, these thin nerve fibres can seldom be traced to their termination. A few ones can, however, be traced ventrally between the notochord and the muscle cells and seem to end in singular bulb-like expansions. Clusters of synaptic vesicles are present in transmission electron micrographs of such nerves, and they are accordingly believed to carry efferent impulses. The extracellular fibrils are arranged in a highly ordered pattern with thick bundles crossing the gap between the structures to be interconnected and with numerous radiating insertions on the surface of the tissues.

On the structure and innervation of the muscle bands of Doliolum (Tunicata: Cyclomyaria)

The ultrastructure of the muscle fibres composing the circular muscle bands of Doliolum is described; these muscle fibres are obliquely striated. Each fibre is elongate and multinucleate; peripherally the myofibrillar array borders the fibre, centrally there are long mitochondria with tubular cristae, and vesiculated sarcoplasm containing glycogen particles. Neither a sarcoplasmic reticulum, nor invaginations of the sarcolemma are present; in both gonozooid and oozooid stages myelin figures are frequent in mitochondria and sarcolemma. On atrial and external faces of the fibres there are nerve terminals containing electron-lucent vesicles some 50 nm in diameter. The junctional gap is ca . 10-20 nm wide and is devoid of the dense layer found in the vertebrate synaptic cleft. Each fibre probably receives a number of nerve terminals. The significance of the organization of the muscle fibres is discussed in relation to their role in the locomotion of the animal, and to the structure of other tunicate muscle fibres.