Yuichi Ozaki

Lipid content and fatty acid composition of muscle, liver, ovary and eggs of captive-reared and wild silver Japanese eel Anguilla japonica during artificial maturation

Changes in lipid content and fatty acid composition of muscle, liver and ovary of captive-reared and wild silver Japanese eel Anguilla japonica were examined during artificial maturation induced by salmon pituitary homogenate (SPH) injections. Although the relative levels of n-3 and n-6 highly unsaturated fatty acids (HUFA) in liver and ovary were higher than in muscle in both captive and wild silver eels before SPH injection, these tended to decrease with maturation. The relative levels of n-6 HUFA in muscle, liver, ovary and eggs of wild silver eels were remarkably higher than those in captive eels. Therefore, we attempted to alter the ratio of n-6 HUFA in eggs by feeding eels a diet supplemented with linoleic acid-rich plant oil. Although the percentage of n-6 polyunsaturated fatty acids to total fatty acids in eggs of eels fed the supplemented diet was similar to wild silver eels, the percentage of n-6 HUFA remained remarkably lower than in wild silver eels. Hence, it appears that the supplemented diet affected the fatty acid composition of eggs, but did not result in much conversion of linoleic acid to its higher homologs in eels.

Immunohistochemical changes in production of pituitary hormones during artificial maturation of female Japanese eel Anguilla japonica

Specific antibodies against follicle-stimulating hormone β subunit (FSHß), prolactin (PRL), and somatolactin (SL) of the Japanese eel Anguilla japonica were produced. These antibodies, as well as antibodies against luteinizing hormone β subunit (LHß) and growth hormone (GH) produced previously, were used to examine changes in the production of pituitary hormones in female eels during maturation induced by salmon pituitary homogenate (SPH) injection. Immunohistochemical observations showed a decrease in FSH production after SPH injection, suggesting that SPH inhibits FSH production. In contrast, LH production increased markedly with maturation. The number of GH producing cells decreased gradually during maturation, possibly because of inhibition by exogenous GH present in the SPH and/or endogenous insulin-like growth factor-I produced by the stimulation of salmon GH. Although changes in the number of PRL producing cells with maturation were not evident, the number of SL producing cells showed a peak at the late vitellogenic stages, and thereafter decreased to the migratory nucleus stage. These results suggest that GH and SL are involved in sexual maturation in SPH injected eels, as in other fishes.

Fine structure and differentiation of the alimentary canal in captive-bred Japanese eel Anguilla japonica preleptocephali

The fine structure of the alimentary canal in preleptocephali produced by artificially matured Japanese eel was examined. At 1 day posthatch (dph), the alimentary canal was found only above the dorsal side of the yolk mass, and the epithelium was composed of a single layer of epithelial cells. By 5 dph, the alimentary canal was divided into three segments based on the structure of the epithelial cells: foregut, midgut and hindgut, corresponding to the future esophagus, intestine and rectum, respectively. After 7 dph, the epithelium in the foregut was surrounded by a circular muscle layer, suggesting a role in the transportation of food materials. The epithelial cells of the midgut exhibited well-developed membranous structures, which are deduced to be invaginations of the cytoplasmic membrane. Pinocytotic invaginations and vacuoles were observed in the epithelial cells of the hindgut; this observation suggests that this region is involved in the uptake of food. Significant changes in morphological features of the epithelial cells in each segment were observed until 7 dph; however, these were not evident between 7 dph and 13 dph. Consequently, the differentiation of the alimentary canal was completed by 7 dph, and preleptocephalus had developed the ability to absorb food by 7 dph.

17 beta-HSD Type 12-Like Is Responsible for MaturationInducing Hormone Synthesis During Oocyte Maturation in Masu Salmon

The maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (DHP) was first identified in the amago salmon. Although carbonyl reductase-like 20β-hydroxysteroid dehydrogenase (CR/20β-HSD) was reported to convert 17α-hydroxyprogesterone (17OHP) to DHP in rainbow trout, we previously found that CR/20β-HSD messenger RNA (mRNA) was not upregulated in stimulated granulosa cells from masu salmon, which suggested that DHP is synthesized by a different enzyme. Accordingly, the current study aimed to identify the specific 20β-hydroxysteroid dehydrogenase (20β-HSD) responsible for DHP production by granulosa cells during final oocyte maturation in masu salmon. RNA sequencing was performed on granulosa layers that were isolated from ovarian follicles at 1 month before ovulation and incubated with or without forskolin, which was used to mimic luteinizing hormone, and ∼12 million reads were obtained, which yielded 71,062 contigs of >100 bp. tBlastx analysis identified 1 contig (#f103496) as similar to 17β-hydroxysteroid dehydrogenase type 12 (hsd17β12); however, because the full-length #f103496 sequence was different from hsd17β12, it was termed hsd17β12-like (hsd17β12l). We found that mammalian cells transfected with full-length hsd17β12l exhibited considerable 20β-HSD activity, as indicated by efficient conversion of exogenous 17OHP to DHP. In addition, we found that hsd17β12l mRNA levels were consistently low in follicles during vitellogenic growth; however, the levels increased significantly during final oocyte maturation. The levels of hsd17β12l mRNA were also considerably increased in granulosa layers in which 20β-HSD activity was induced by salmon pituitary extract. Therefore, we suggest that hsd17β12l, not CR/20β-HSD, is the 20β-HSD responsible for DHP production by granulosa cells in masu salmon during final oocyte maturation.