The structural and ultrastructural organization of the cellular constituents of the trunk kidney of grass carp (Ctenopharyngodon idella).

The trunk kidney of grass carp mainly consisted of renal tubules with a few interstitial hematopoietic tissues. The component structure of fish nephron markedly varies between different species of fish. The nephron of grass carp consisted of morphologically distinct segments; renal corpuscles, neck segment, proximal, intermediate, distal, and collecting tubules. The glomerulus of renal corpuscles mainly composed of mesangial cells and well-developed podocytes that extended their processes to the endothelium of glomerular capillaries forming the filtration barrier. The podocytes expressed both α-SMA and the transforming growth factor gene, TGF-β. The proximal convoluted tubules (PCTs) expressed α-SMA iNOS2, and TGF-β. The cytoplasm of PCT was rich in mitochondria and rER, in addition to the presence of well-developed basolateral tubular system and apical brush borders. Collecting tubules distributed throughout the kidney and lined by principal and flask cells. The interstitial hemopoietic tissues contained iNOS2 -positive polymorphic granulocytes, CD3-positive T lymphocytes, rodlet cells, dendritic cells, macrophages, melanomacrophage centers, and telocytes. This study described for the first time the cellular components of the nephron and its associated hemopoietic tissues that can act as a basis for studying the structural changes that may occur in the kidney of grass carp during water salinitiy, environmental, or experimental conditions.