Reticulo‐endothelial stroma of the head‐kidney from the seawater teleost gilthead seabream (Sparus aurata L.): An ultrastructural and cytochemical study

Abstract

Background: Head‐kidney, considered the major fish lympho‐haemopoietic tissue, consists of cells of the different haemopoietic series supported by a network of stromal cells whose morphofunctional properties have not been established. We report the ultrastructure and cytochemical features of the reticulo‐endothelial stroma of the head‐kidney from the seawater teleost gilthead seabream (Sparus aurata L.). Methods: Samples of head‐kidney were processed for electron microscopic study. Some of the samples were incubated for acid and alkaline phosphatase, peroxidase, glucose‐6‐phosphatase, or ATPase. Results: The reticulo‐endothelial stroma of gilthead seabream head‐kidney consists of sinusoidal cells (endothelial and adventitial cells) and reticular cells (macrophage‐type reticulum and fibroblast‐like reticular cells). Transcytosis vesicles and rounded medium electron‐dense granules were observed in the cytoplasm of the endothelial cells. The adventitial cells partially covered the outside surface of the endothelial cells and were joined by desmosomes. The macrophage‐type reticulum cells were characterized by their cytoplasmic processes and acid phosphatase positive lysosomes. The fibroblast‐like reticular cells were joined by desmosomes and formed an extensive network between the haemopoietic parenchyma. They were peroxidase negative and acid and alkaline phosphatase, glucose‐6‐phosphatase, β‐glucuronidase, and ATPase positive. Conclusions: The ultrastructural and cytochemical features of the reticulo‐endothelial stroma of the gilthead seabream head‐kidney are similar to those of mammalian bone marrow, suggesting phylogenetic analogies between both tissues.