A novel stromal cell type in the rat marrow recognizable by its preferential uptake of lanthanum

A novel stromal cell type is described in the rat bone marrow. It is distinguishable from other stromal cells (macrophages, reticular cells, etc.) by its preferential uptake of the electron dense tracer lanthanum nitrate, which can then serve as a marker for this cell type. In low concentration of lanthanum, this cell type is the only marrow cell that takes up the tracer. Other stromal cells do not take ip up even in high concentration. This novel stromal type is associated with both erythropoietic and granulopoietic areas of the marrow tissue. Its branching cytoplasm is very light in density and contains no characteristic cytoplasmic organelles. Its function is not yet known.