Yoko Shiroya

Fine Structural Changes in the Acrosome Reaction of the Japanese Abalone, Haliotis disus

The spermatozoon of the Japanese abalone, Haliotis discus, and its structural changes during the acrosome reaction were observed by electron microscopy. The spermatozoon has a huge acrosome in the shape of a hanging bell or a forefinger with a deep fossa at the posterior end being filled with a bundle of microfilaments. The membranes of the acrosomal apex, the so‐called trigger region, are structurally discernible from those of other acrosomal regions. Following the trigger region, a unique structure under the acrosomal membrane covers the surface of the acrosomal content in the form of a truncated cone.

Ultrastructural Changes of the “Truncated Cone” during the Acrosome Reaction in Japanese Abalone Spermatozoa

An electron‐dense structure termed the “truncated cone” covers the apical surface of the acrosomal contents except for the trigger region in Haliotis discus spermatozoa. The truncated cone, having a slant height of 0.3 μm and diameters of circular top and base of 0.3 and 0.6 μm, shows striations with a periodicity of 6.6 to 8.0 nm. During the acrosome reaction, the truncated cone elongates simultaneously with the protrusion of the acrosomal process through the truncated cone. As the growth of the acrosomal process further proceeds, the truncated cone transforms into a cylindrical shape and eventually reaches 1 μm in length and 0.2 μm in diameter. The elongated truncated cone is characterized by regularlly helical striations with a periodicity of 19 to 21 nm with an inclination of 40° to 46°. These results may suggest that the truncated cone is composed of coiled filaments, which coil up further during the acrosome reaction causing the truncated cone to slenderize and elongate. The elongation is also achieved by stretching of the coil. In H. discus hannai Ino, structural changes in the truncated cone show close homology to those in H. discus. No such morphologically unique organelle has been found in other species thus far.

The effect of chronic exercise on the rat pancreas

SummaryBackground. We recently demonstrated that chronic physical exercise increases pancreatic protein content and basal amylase secretion. It is unknown whether chronic exercise causes hypertrophy or proliferation of pancreatic acinar cells. Methods. Female F344 rats (age, 6 wk) were divided into control (n= 7) and exercise (n=6) groups. Food consumption was matched between the 2 groups. Rats in the control group were kept sedentary. Rats in the exercise group were exercised for 60 min, 5 d/wk during the experiment. After 8 wk, the pancreas and hindlimb muscles were rapidly excised and weighed. Protein and DNA content and enzyme activity in pancreatic tissue were measured. Pancreatic tissues from control and exercised rats were also prepared for transmission electron microscopy. Results. Inhibition of growth and hypertrophy of hindlimb muscles were exhibited by the exercise group. In the exercise group, pancreatic wet weight, protein content, and amylase and lipase activities, but not DNA content, were significantly higher than those in the control group. Electron micrographs clearly revealed that acinar cells were hypertrophied and zymogen granules were increased in number in exercised rats.Conclusion. Chronic endurance exercise increases pancreatic weight, protein content and enzyme activity through hypertrophy of acinar cells.

Organization of Actin Filaments in the Axial Rod of Abalone Sperm Revealed by Quick Freeze Technique

An axial rod in abalone (Haliotis discus) sperm is a structure composed of a bundle of actin filaments, which elongates anteriorly to form the acrosomal process during the acrosome reaction. The ultrastructure of the actin filament bundle constituting the axial rod was examined using quick freeze technique followed by either freeze‐substitution or deep‐etch electron microscopy. Thin sections of quick freeze and freeze‐substituted sperm revealed that the actin filaments in the axial rod are hexagonally packed in a paracrystalline array through its almost entire length with an average center‐to‐center spacing of 12 nm. Periodic transverse bands were also observed across the actin filament bundle, which may reflect the cross‐bridges interconnecting the adjacent filaments. Quick‐freeze deep‐etch analysis provided the three‐dimensional view of the axial rod. Actin filaments exhibiting 5.5–6 nm spaced striations were observed to run in parallel with each other inside the axial rod. The existence of cross‐bridging structures was also displayed between adjacent filaments. These results suggest that the actin filaments in the axial rod are probably held together by regularly spaced cross‐bridges to form a well ordered hexagonally packed bundle, and also cross‐linked by fibrous structure to the lateral inner acrosomal membrane which closely surrounds the anterior half of the actin filament bundle.

The Exercise-Induced Improvement in hyperglycemia is Mediated by DHT Produced in the Skeletal Muscle of Zucker Diabetic Fatty Rats

The ability of exercise to improve hyperglycemia by enhancing glucose metabolism in the skeletal muscle of type 2 diabetic patients is well established. We reported sex steroid hormones can be locally synthesized in skeletal muscle and decrease fasting blood glucose levels in obese rats. Here, we determined whether exercise-induced production of sex steroid hormones in skeletal muscle could directly reverse hyperglycemia in the Zucker diabetic fatty rat model using osmotic mini pump. Thirty Zucker diabetic fatty rats were randomly assigned to the following groups: control, exercise, or exercise with continuous infusion of 5α-reductase inhibitor. The results indicated 6 weeks of exercise significantly reduced serum insulin and fasting glucose levels compared to control group. Dehydroepiandrosterone, 5α-dehydrotestosterone, and 5α-reductase levels were all significantly higher in skeletal muscle of the exercise group. Moreover, exercise increased glucose transporter-4 translocation with a concomitant upregulation of phosphorylated phosphoinositide 3-kinase, protein kinase B and C-ζ/λ. Furthermore, significant correlations were observed between fasting glucose and muscular DHT levels. Interestingly, the observed exercise-induced improvements in serum insulin and fasting glucose levels were all suppressed by administration of 5α-reductase inhibitor. These results indicated the exercise-induced improvements in glucose metabolism signaling and glucose levels may be directly attributed to the increased levels of sex steroid hormones within skeletal muscles.

Coiled filamentous structure in the sperm acrosome of gastropoda, Sulculus aquatilis and Turbo cornutus

A helically coiled filamentous structure, termed the truncated cone originally identified in Haliotis discus, was demonstrated to exist in the apex of the acrosome subjacent to the outer acrosomal membrane of the sperm in two species of Gastropoda, Sulculus aquatilis and Turbo cornutus. Thin section and whole mount preparations revealed that in intact sperm this filamentous structure consisted of about 12 helically arranged filaments with a diameter of 10–12 nm which were tightly packed in a truncated shape. This truncated cone elongated anteriorly from the acrosomal opening and transformed into a cylinder which closely surrounds the acrosomal process during the acrosome reaction. In S. aquatilis and T. cornutus, the truncated cone elongated more than 3 and 1.5 times as long as the original height, respectively. The elongated truncated cone was characterized by striations with increase in its periodic spacing and inclinations in thin sections. The truncated cone in both the species was fundamentally analogous to that of Haliotis discus, further suggesting that the truncated cone plays a role in fertilization as a common cytoskeletal structure among the species of Gastropoda.

Distribution of Intramembrane Particles and Its Changes during the Acrosome Reaction in Spermatozoa of the Japanese Abalone

The distribution of intramembrane particles in the plasma and acrosomal membranes of sperm of the Japanese abalone, Haliotis discus, and its changes during the acrosome reaction were studied by the freeze‐fracture replica technique. The P face of the plasma membrane covering the acrosome has sparse membrane particles except in the apical region, which includes the trigger and ‘truncated cone’ regions. Large particles with an average diameter of 10 nm are located in this apical region. The E face of the plasma membrane has only a few particles. On the outer acrosomal membrane, many particles are randomly distributed throughout the P face, but only a small number of particles are found on the E face. Numerous particles on the P face of the inner acrosomal membrane show a regular arrangement as a dense lattice or with a concentric circular pattern. The initial change in the acrosome reaction is clearance of membrane particles from both the P and E faces of the plasma and outer acrosomal membranes around the apical region, where fusion of the two membranes occurs. As the acrosomal process elongates, the dense arrangement of particles on the inner acrosomal membrane changes via a loose lattice arrangement to a patchy distribution with particle‐free areas. Then the arrangement is further disorganized becoming a sparse, random distribution.

Characterization of fat metabolism in the fatty liver caused by a high-fat, low-carbohydrate diet: A study under equal energy conditions

The pathology of fatty liver due to increased percentage of calories derived from fat without increased overall caloric intake is largely unclear. In this study, we aimed to characterize fat metabolism in rats with fatty liver resulting from consumption of a high-fat, low-carbohydrate (HFLC) diet without increased caloric intake. Four-week-old male Sprague-Dawley rats were randomly assigned to the control (Con) and HFLC groups, and rats were fed the corresponding diets ad libitum. Significant decreases in food intake per gram body weight were observed in the HFLC group compared with that in the Con group. Thus, there were no significant differences in body weights or caloric intake per gram body weight between the two groups. Marked progressive fat accumulation was observed in the livers of rats in the HFLC group, accompanied by suppression of de novo lipogenesis (DNL)-related proteins in the liver and increased leptin concentrations in the blood. In addition, electron microscopic observations revealed that many lipid droplets had accumulated within the hepatocytes, and mitochondrial numbers were reduced in the hepatocytes of rats in the HFLC group. Our findings confirmed that consumption of the HFLC diet induced fatty liver, even without increased caloric intake. Furthermore, DNL was not likely to be a crucial factor inducing fatty liver with standard energy intake. Instead, ultrastructural abnormalities found in mitochondria, which may cause a decline in β-oxidation, could contribute to the development of fatty liver.