White pulp nodules of mouse spleen contain a minor population of cells with morphologic features that are identical to those of dendritic cells, a cell type recently described in vitro. They have characteristic large, irregularly shaped nuclei with distinctive chromatin patterns and small nucleoli. The cytoplasm is extended in processes that contain relatively few organelles. These presumptive dendritic cells can be distinguished from other cell types that are known to exist in spleen including those that have irregular or branching cell shapes. In particular, dendritic cells do not contain the large number of lysosomes seen in phagocytes, and do not actively interiorize i.v. administered colloidal thorium dioxide particles. They also lack the well developed secretory apparatus (rough endoplasmic reticulum and Golgi zone) and microfilament bundles that are noted in connective tissue cells. These morphologic observations, combined with previous in vitro work, substantiate the existence of a novel class of cells in mouse lymphoid organs
