Ultrastructural Heterogeneity of Follicular Dendritic Cells in the Human Tonsil

Abstract

Follicular dendritic cells (FDC) form the framework of the germinal centre (GC). They act as accessory cells, by providing a favourable microenvironment for the germinal centre reaction.^1-2 Secondary GCs consist of a basal dark zone, mainly composed of immature blast cells and showing extensive mitotic activity, and an apical light zone containing a heterogeneous population of lymphoid cells. FDC are present both in the dark zone and in the light zone of the GC. Only few properties of the FDC present in these zones are known. The FDC density framework shows only minor differences between the dark and the light zone. Cytochemical localization of immune-complexes and the ectoenzyme 5’-nucleotidase indicate differences in the expression of these markers in FDC in these zones.³ These staining patterns suggest differences in the functional stage of FDC. To date, the ultrastructural morphology of FDC in relation to these functionally different GC compartments is not well established. In human GCs the different types of lymphoid and non-lymphoid cells can be distinguished on the basis of their ultrastructure.⁴ We performed a qualitative and quantitative ultrastructural study of the FDC population in the dark and the light zones of GCs. This approach was aimed to define the light and the dark zones of GCs, and to study - on a structure-function basis -the contribution of FDC to these distinct microenvironments.