Ultrastructural analysis of normal and immunized spleen of the snapping turtle, Chelydra serpentina

In the ultrastructural comparison of normal, unimmunized spleens with immunized spleens at key intervals after antigenic stimulation with keyhole limpet hemocyanin (KLH), we noted cellular and cytological features which reflect the cellular kinetics of the primary immune response, particularly with respect to plasma cell production. Although lymphoblasts and mature plasma cells are present in the white and red pulp, respectively, intermediate stages of the plasma cell line are rarely found in normal spleen. Following antigenic challenge, we found a marked increase in lymphoblasts in the white pulp, most of them containing short segments of rough endoplasmic reticulum suggesting initial differentiation toward plasma cells. Following an apparent migration of cells from the white to the red pulp, we found plasma cells in various stages of maturation in the red pulp cords and sinuses. The ultrastructural features of these cells reflect the differentiation of lymphoblasts into mature plasma cells. Both immature and mature plasma cells usually possess dilated cisternae of rough endoplasmic reticulum, suggesting that they are capable of producing and storing a secretory product, presumably antibody. We also noted a large number of immature macrophages and monocytes in immunized spleens. These cellular events and their cytological characteristics are compared to those described in other vertebrate classes.