Histology and ultrastructure observations of major immune organs of Acipenser dabryanus

【Objective】This study aimed to clarify the histological characteristics and cellular composition of major immune organs of Acipenser dabryanus， a rare and endangered fish species endemic to China， laying theoretical foundation for disease control and health management of this species.【Method】Artificially bred F₂ generation of Acipenser dabryanus were used as experimental subjects. Gross anatomy was performed to determine anatomical locations of major immune organs （spleen， head kidney， and thymus）. Histological sectioning techniques hematoxylin-eosin （HE） staining， Masson staining， and modified Gomori ammoniacal silver staining， combined with transmission electron microscope， were used to observe histological characteristics and cellular ultrastructure of spleen， head kidney， and thymus.【Result】The spleen surface of Acipenser dabryanus was covered by an uneven smooth muscle layer， whereas no trabecular structures extending from capsule into the spleen parenchyma were observed. The mesothelial cells in outer capsule layer exhi-bited two morphological forms （squamous and nearly round）. The spleen parenchyma exhibited an interwoven distribution of white pulp and red pulp， with a highly developed vascular system and numerous lymphocytes， nucleated erythrocytes， and melanomacrophages distributed throughout. Transmission electron microscope observation revealed abundant immune effector cells， including lymphocytes， macrophages， and granulocytes in the spleen tissue. Phagosomes were observed in macrophages， indicating that the spleen was involved in blood filtration and non-specific immune functions. The head kidney was located at the anterior end of the kidney and symmetrically distributed as left and right lobes， with absent nephrons and loss of excretory function. Masson staining and modified Gomori ammoniacal silver staining showed that reticular cells， endothelial cells， interwoven reticular fibers， and collagen fibers formed a three-dimensional supporting framework in head kidney tissue， providing an essential microenvironment for the development and differentiation of various immune cells. Lymphocytes， macrophages， and eosinophils with densely distributed elongated or rod-shaped specific granules in cytoplasm were observed in head kidney tissue that functioned as a major lymphoid organ of Acipenser dabryanus with both hematopoietic and immune functions. The thymus of juvenile Acipenser dabryanus was located within the connective tissue in posterior dorsal region of the branchial cavity， adjacent to the gill tissue， making routine non-destructive isolation difficult. In 30-day-old juveniles， the thymic parenchyma showed a relatively uniform internal structure， with no obvious division between the cortex and medulla or between the inner and outer regions. The free margin of thymus was covered by a continuous epithelial cell layer composed of squamous epithelial cells and some columnar epithelial cells， which mainly served as a physical barrier. Many deeply stained thymocytes were densely distributed within the thymus parenchyma. Irregular reticuloendothelial cells were scattered among thymocytes， and together they formed a stromal network supporting the proliferation and maturation of T lymphocytes.【Conclusion】The spleen， head kidney， and thymus of Acipenser dabryanus are distinctive in tissue structures， with clear functional divisions. The organs constitute a well-developed immune defense network， providing a cell biological basis for resistance to pathogen invasion and maintenance of homeostasis in Acipenser dabryanus.