The sinus endothelial cell architecture in the mouse lymph node. Structural peculiarities and close correlation with the fibroblastic reticular cells

The sinus endothelial cell architecture has been studied in the mouse lymph node by light and electron microscopy after tissue fixation with an osmium-zinc iodide mixture. This histochemical method allowed visualization of sinus endothelial cells in the medullary and, partly in the intermediate sinuses. The osmium-zinc iodide reaction also demonstrated fibroblastic reticular cells, which represent the main stromal cell population in the lymph node and form the skeletal framework of the organ as well as the adventitia of sinuses. In the mouse lymph nodes, sinuses were lined by a simple layer of sinus endothelial cells whose integrity and continuity showed interruptions of various extension. In areas devoid of complete sinus endothelial cell lining, fibroblastic reticular cells located in the nearly parenchyma were able to reach the border of the sinus, being thus in direct contact with the lumen content. Dendritic sinus endothelial cells as well as intermediate forms between sinus endothelial cells and fibroblastic reticular cells could be observed. The close structural characteristics that sinus endothelial cells have in common with fibroblastic reticular cells and the finding of transitional cells with intermediate morphology between sinus endothelial cells and fibroblastic reticular cells, suggest a possible origin of sinus endothelial cells by migration and differentiation of fibroblastic reticular cells located in the sinus adventitia.