Lymphocyte proliferation in germinal centers (GC's) is thought to be triggered by antigen retained extracellularly on the surface of special" dendritic" cells. The anatomy and function of these cells have not been studied directly or in detail. We therefore examined mouse spleen GC's developing in response to sheep erythrocyte stimulation.

We found that distinctive" follicular dendritic cells"(FDC's) were present in both the GC and adjacent mantle region of secondary follicles. The large, irregularly shaped nucleus, containing little heterochromatin, allowed for the light microscope (LM) identification of FDC's. By EM, the cell was stellate in shape sending out long, thin sheets of cytoplasm which could fold and coil into complex arrays. The processes were coated extracellularly by an amorphous electron-dense material of varying thickness, as well as particulates including variable numbers of virions. The FDC cytoplasm lacked organelles of active secretory and endocytic cells, such as well-developed rough endoplasmic reticulum (RER) and lysosomes. These anatomical features readily distinguished FDC's from other cell types, even those that were extended in shape.