Morphological and Ultrastructural Insights into the Goldfish () Spleen: Immune Organization and Cellular Composition.

The spleen plays a critical role in the immune and hematopoietic systems of teleost fish, functioning as a major secondary lymphoid organ. This study provides a detailed morphological and ultrastructural assessment of the spleen in goldfish (), focusing on its immunological organization and cellular diversity. Through light and transmission electron microscopy, we examined red and white pulps, identifying key features such as melanomacrophage centers (MMCs), ellipsoids, and various immune cell types. The red pulp was rich in sinusoidal capillaries and splenic cords, whereas the white pulp housed lymphocytes, dendritic cells, macrophages, and telocytes, all contributing to immune regulation. Notably, ellipsoids were surrounded by reticular and macrophage sheaths, forming a filtration barrier against pathogens. Ultrastructural analysis revealed diverse immune cells with active morphological traits, including macrophages with pseudopodia and pigment granules, dendritic cells with dendrite-like extensions, and epithelial reticular cells involved in forming the blood-spleen barrier. These findings highlight the complex immunological microarchitecture of the goldfish spleen and its functional relevance in teleost immune responses.