Eggs, larvae, post-larvae and sexually immature juveniles of the teleost Dicentrarchus labrax (L.) were assayed for the expression of genes encoding the T cell receptor b and\ud CD8a. RT-PCR of RNA extracted from larvae revealed TCRb transcripts from day 25 posthatching (ph) and CD8a transcripts from 26 days later.\ud At day 51 ph, CD8a and TCRb mRNAs were localised by in situ hybridisation in thymocytes of the outer and lateral zones of the thymic paired glands. From day 75 ph onwards the signal was mainly detected in the outer region, drawing a cortex–medulla demarcation.\ud In 1-year-old fish, CD8a+ and TCRb+ thymocytes almost filled the cortex and extended in large cords in the medulla. A CD8a-TCRb+ subcapsular lymphoid zone was evident near the\ud septa coming from the inner connective capsule that delimited the thymus.\ud The localisation of CD8a and TCRb transcripts demonstrated a compartmentalisation of the juvenile thymus due to distinct localisation of thymocytes at different developmental stages
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