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 (Carassius
auratus), 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