Histological and transcriptome analysis provides new insights into the hematopoietic and immune functions of head kidney, trunk kidney and spleen of adult large yellow croaker, Larimichthys crocea

• The key hematopoiesis-related genes were identified in three organs. • C8γ was only observed in trunk kidney. • The MMCs and associated lymphocyte aggregates were also observed in three organs. Many teleosts possess a unique immune system because hematopoiesis occurs in multiple organs, such as the head kidney and trunk kidney. The head kidney (HK) and trunk kidney (TK) are considered immune organs in that they contain macrophages and melanomacrophage centers (MMCs). The large yellow croaker ( Larimichthys crocea ), a vital marine-cultured fish in Asia, lacks bone marrow and lymph nodes, relying instead on the head kidney (HK), trunk kidney (TK), and spleen for hematopoiesis and immune functions. This study provides a comprehensive histological and transcriptomic analysis of these organs in adult large yellow croaker, revealing their normal physiological roles in hematopoiesis and immunity. Histological examination identified hematopoietic tissues, lymphoid tissues, and melanomacrophage centers (MMCs) in all three organs, with the spleen exhibiting a higher abundance of MMCs. The MMCs and associated lymphocyte aggregates were observed in three organs, indicating the presence of the germinal center–like structures. Transcriptome analysis revealed organ-specific gene expression patterns, with 47, 1368 and 1529 genes only expressed in HK, TK and spleen, respectively. The key hematopoietic genes such as runx1, cmyb, epor , and csf3r were highly expressed in both HK and TK, indicating their primary roles in hematopoiesis. The spleen, a major lymphoid organ, also expressed the hematopoietic genes. Immune-related genes, including tlr3, tlr7 , and tlr8 , were highly expressed in the spleen, highlighting its role in antiviral responses. Additionally, complement genes such as c6 and c7 were highly expressed in HK and TK, while c8γ was uniquely expressed in TK. These findings enhance our understanding of the molecular mechanisms underlying hematopoiesis and immune defense in teleosts, providing valuable insights for disease management in aquaculture.