Ontogeny of IgM-producing cells in the mandarin fish Siniperca chuatsi identified by in situ hybridisation.

The ontogeny of IgM-producing cells was studied in juvenile mandarin fish Siniperca chuatsi, an important fish in China's aquaculture sector. The IgM-producing cells were localised through in situ hybridisation with a probe complementary to the Ig mu-chain in lymphoid-related tissues, including head kidney, spleen, thymus, intestine and gills. In head kidney, transcripts of Ig mu were first detected at 20days post-hatching (dph) with a few positive signals, and the number of IgM-producing cells increased obviously from 39dph onwards. At 136dph, a large amount of positive cells were observed in the entire organ with clusters of these cells located around the blood vessels. In spleen, IgM-producing cells were found from 26dph onwards, followed by an increase until 67dph; clusters of positive cells were also detected around blood vessels at 102dph. In thymus, IgM-producing cells were first observed at 39dph; thereafter, no obvious increase was detected until 78dph. The positive cells in thymus were distributed mainly in the outer zone of thymus. A few IgM-producing cells were still observed in thymus of 1-year-old mandarin fish. IgM-producing cells were not detected in the intestine until 87dph, with several discrete positively stained cells distributed in the lamina propria. IgM-producing cells, scattered mainly in primary gill filaments around blood vessels, were detected in gills from 90dph. As in other teleosts, these results indicated that the head kidney appears to be the primary organ for IgM production in mandarin fish, and IgM-producing cells exist in all organs examined in the present study, implying their lymphoid role in fish. In addition, it is suggested that vaccination after 20dph may be much more effective in mandarin fish.