RB formation in L^−/− plasma cells gives rise to Mott cells. (A) Sorted CD138⁺ spleen cells from 3-month-old normal (NM) and L^−/− mice were stained with PAS, showing tightly packed glycoproteins in purple-magenta and hematoxylin-stained cell nuclei in blue. For L^−/− cells, PAS staining was found either in connection with the nucleus or in isolation as spherical particles. The arrow indicates a morula cell. (B) PAS stainings of spleen cell smears from a 14-month-old L^−/− and a normal aged mouse. RBs in smears, frequently surrounded by nonlymphoid cells, are larger than from sorted cells, prepared as single-cell suspensions devoid of aggregates. (C-E) Intracellular staining after fixation and permeabilization with FITC-labeled (C) anti-IgM (μ⁺), (D) anti-IgA (α⁺), and (E) anti-IgG (γ⁺). The DAPI counterstain shows nucleotides/dsDNA in blue. Frequencies of positive cells were < 1% for all stainings, consistent with the expected low number of plasma cells.