The Pilot Study on the Histological and Ultrastructural Characteristics of Major Immune Organs (Spleen, Head Kidney, and Trunk Kidney) and Analysis of Pathological Features in Sichuan Taimen (Hucho bleekeri).

Sichuan taimen (Hucho bleekeri) is a glacial relict and critically endangered fish species restricted to the Yangtze River drainage in China. The lack of knowledge regarding the morphology of immune organs may limit the timely detection of health threats, thereby hindering effective conservation efforts. To address this gap, the pilot study provides the first histological and ultrastructural characterisation of the main immune organs (spleen, head kidney, and trunk kidney) of H. bleekeri. The spleen comprises red and white pulp; the red pulp is abundant in erythrocytes, while the white pulp contains leukocytes within a reticular network. Melano-macrophage centers, involved in immune and phagocytic activities, are prominent in the spleen and kidneys. The head kidney functions as a primary haematopoietic organ, exhibiting distinct lymphoid and non-lymphoid zones supported by a reticular framework. The trunk kidney serves an excretory role, and its tissue contains dispersed renal corpuscles. Ultrastructural analyses revealed typical cellular features, including well-developed rough endoplasmic reticulum and melano-macrophage-associated lysosomes. Notably, structural abnormalities-including mitochondrial swelling, cristae fragmentation, and mild dilation of the rough endoplasmic reticulum-were observed in the head kidney; these may potentially reflect early immune activation or sublethal environmental stress. These baseline findings would provide critical morphological references and lay a foundation for future studies on immune function, disease pathology, and conservation strategies in H. bleekeri.