Hideki Ishida

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

Abstract Contraction of molluscan adductors has been classified into three states; 1) resting state, 2) contracted state, 3) prolonged “catch” state. Among these, the “catch” state is considered a peculiar state, which requires little expenditure of energy, but in which contraction can be maintained for long periods. It is not yet known whether “catch” muscle contraction is regulated by Ca, or where Ca translocates during resting state through “catch” state, if the muscle contraction is regulated by Ca. We attempted to observe Ca translocation in muscle cells during contraction by the K-pyroantimonate method. We fixed “catch” muscle cells for electron microscopy with fixative including K-pyroantimonate, and observed where electron-dense precipitates, in which Ca is concentrated, were located in the muscle cells in the three states of contraction. At the resting state, precipitate was located at cell peripheries, in positions such as at the inner surface of cell membranes and in sarcoplasmic reticular systems (SRs). In the contracted state, they were located within the cytoplasm. At the “catch” state, they were found in both the cytoplasm and at peripheries, although the number of precipitates in peripheral areas was small. Thus, we show that calcium translocates in the cells during resting-contraction-catch cycles of “catch” muscle contraction.

Wound healing ability of Xenopus laevis embryos. II. Morphological analysis of wound marginal epidermis

We previously showed that bisectional wounds made in Xenopus laevis embryos at the primary eye vesicle stage were rapidly closed. In this study, microscopic analyses, including scanning electron microscopy, on the morphology of the epidermis were conducted during wound closure in the half embryos. Bright fluorescence of Texas red‐phalloidin showing actin filaments started to be visualized at the cut edge 10 min after wounding. It increased with time, forming a distinguished, though discontinuous, bundle along the wound margin. The wound closure was completely inhibited by 20 µm cytochalasin B, and almost completely by 50 mm 2,3‐butanedione 2‐monoxime, an inhibitor to myosin ATPase activity. Scanning electron microscopy revealed that the outer epidermal cells became extensively elongated in the radial direction, and the contour of the closing wound edge did not become smoother but remained ragged. Thus, a representative embryonic type of wound closure may be driven in Xenopus embryos by a complex mechanism, involving not only the actin ‘purse‐string’ but also an inward movement of individual cells. Anyhow, the wound closure is a movement of the epidermal sheet maintaining cell–cell contact, and not involving locomotion of single cells separated from the wound edge.

Two kinds of thick filament in smooth muscle cells in the adductor of a clam, Chlamys nobilis

The ultrastructure of the opaque portion of the adductor muscle in the pecten Chlamys nobilis was investigated. The opaque portion was composed of smooth muscle cells that contained thin and thick filaments. The thick filaments were classified into two kinds, thinner and thicker, according to the statistical analysis of diameters. They were also classified as being shorter and longer, when isolated native filaments were examined. The thick filaments may consequently be classified into two kinds: thinner and shorter filaments, and thicker and longer ones. The thinner and shorter filaments were about 26.5 nm in diameter and 7.5 mum in length, and the thicker and longer ones were about 42.0 nm in diameter and 13.0 mum in length, respectively. A regular periodicity was apparent on the surface of the core after removal of myosin molecules from its surface. The periodicity seemed similar for the two kinds of thick filament.

Effect of Mg2+ on Ca2+-dependent contraction of a Spirostomum cell model

Summary A detergent-extracted cell model of the heterotrichous ciliate Spirostomum ambiguum showed ATP-independent contraction on addition of > 10−7 M Ca2+, which is caused by contraction of the myoneme. To further understand the mechanism of cell contraction, the effects of Mg2+ and other cations were examined. Among various kinds of cations tested, Ba2+, Mn2+, Sr2+ and Cd2+ induced slight contraction, but they required higher concentrations (> 10−3 M). Contraction of the cell model was not induced by Mg2+. However, it shifted the threshold of Ca2+ concentration for inducing contraction to higher levels. The Hill constant, denoted as the number of calcium ions cooperatively bound to a contractile element in the myoneme, was 2–3 at 0–1 mM Mg2+, while it decreased with increasing Mg2+ concentration. The Ca2+-induced contraction was not influenced by changing ionic strength of the reactivation medium. These results suggest that Mg2+ ions interact with the Ca2+-binding sites which induce contraction of the myoneme.

Glycerinated Catch Apparatus of Sea Urchin Spines: the Effects of Cations on its Mechanical Properties and Ultrastructure

Abstract Sea urchin spinal ligaments (the catch apparatus) were extracted with glycerin, and electron microscopic observations comfirmed that no cell membranes remained intact after glycerination. We studied the effects of cations (Na+, K+, Ca2+, Mg2+) on the mechanical properties of the glycerinated ligaments. Monovalent cations decreased whereas divalent cations increased the viscosity of the ligaments. The ion dependencies were similar to previous results with detergent-extracted holothurian dermis, which suggests that the echinoid ligament shares a similar mechanism for changes in mechanical properties with other catch connective tissues. This provides evidence against the hypothesis of del Castillo et al (1995) that muscles in the catch apparatus are responsible for the changes in mechanical properties of the ligament. Fine projections cross-bridging collagen fibrils were observed in the glycerin-extracted ligaments as well as in the intact ligaments. They were found in all the ionic conditions studied.

Sound Production in the Aquatic Isopod Cymodoce japonica (Crustacea: Peracarida)

A vast variety of acoustic behaviors and mechanisms occur in arthropods. Sound production, in particular, in insects and decapod crustaceans has been well documented. However, except for a brief, anecdotal statement, there has been no report on the acoustic behavior of aquatic isopods. We present the first empirical evidence in aquatic Isopoda that males of Cymodoce japonica produce sound by stridulation, or the rubbing together of body parts. Sound production was associated with tail-lifting behavior, suggesting that stridulation occurs on thoracic and/or abdominal somites. Acoustic analysis revealed that syllable length was similar throughout the stridulation, at a mode of 2500–3000 Hz. With a scanning electron microscope, we identified file-like structures on the inner surface of the dorsal exoskeleton. Each file consisted of 188 ± 11.1 ridges at about 0.5 μm intervals; the theoretical frequency (number of ridges per syllable length) was estimated to be 2208–3646 Hz. This finding suggests that the stridulation sounds arose from these structures. Laboratory observations show that stridulation may play a role in the threatening of other males in the context of territorial and/or reproductive competitions.

Ecology and Distribution of Protists in Brackish Water Lakes

A brackish water area means water that is a little salty, indicating salinity between fresh and marine waters. While it is known as the habitat for many diversified organisms, it is a harsh environment due to changes in salinity. Although it has been speculated for many years that biodiversity is lowered at a salinity of 5–8 ‰ within the salinity gradient in the brackish water area, planktons such as planktonic protists, in particular, have been recently found to rather increase their diversity in this salinity range. Planktonic protists are not likely to create new species easily that inhabit only specified areas, because their small size makes it easy for them to expand their habitat areas and less likely for them to be extinguished. In reality, endemic species are found in many places including brackish water areas, but we do not have a clear explanation for this discrepancy. Lake Shinji and Lake Nakaumi are the prominent brackish water lakes in Japan. In these lakes, a halocline is formed, as in many other brackish water lakes. This chapter provides an overview of the survey of protist diversity in these lakes, with an attempt to determine the environmental factors influencing the composition of species in the brackish water area.