Dark and light zones of germinal centres of the human tonsil: an ultrastructural study with emphasis on heterogeneity of follicular dendritic cells

Abstract

The cellular composition of the dark and light zones of germinal centres in human tonsils was quantitatively determined by electron microscopy. In addition to the well known germinal-centre B-cells, we defined the cleaved blast as a new distinct cell type in the germinal centre. The dark and the light zones clearly differed in their content of lymphoid and non-lymphoid germinal-centre cells. The dark zone was characterized by higher frequencies of cleaved blasts and small centroblasts, a higher blast-centrocyte ratio and a higher incidence of mitotic figures. In contrast, the light zone had higher frequencies of centrocytes, centroplasmacytoid cells, lymphocytes and follicular dendritic cells (FDC) and an inverted blast-centrocyte ratio. Seven distinct appearances of FDC (FDC.1-FDC.7) could be recognized on the basis of their ultrastructure. The distribution pattern of these subtypes differed between the dark and light zone. The undifferentiated subtypes FDC.2 and FDC.3 predominated in the dark zone. In contrast, the highly differentiated subtypes FDC.4 and FDC.5 were present at a much higher density in the light zone. These findings suggest that the dark and light zones are different compartments with regard to proliferation and differentiation of germinal-centre B-cells and that both compartments have a specific microenvironment influenced by FDC.

Ultrastructure of the cortical epithelium of the rat thymus after in vivo exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

“…In this life cycle, the pale cells represent a highly active stage, whereas the dark cells are degenerating elements (Van de Wijngaert et al 1984). Such a structure-function relationship is supported by ultrastructural observations of accessory cells in other lymphoid microenvironments, likewise indicating gradual differentiation (Rademakers 1992;Rademakers et al 1992). Therefore, we interpret the formation of a more electron-dense epithelium after exposure to TCDD as an effect on epithelial cell differentiation.…”

Identification of human germinal center light and dark zone cells and their relationship to human B-cell lymphomas

“…12 Earlier histologic observations, many of which were carried out using human tonsil samples, classified GC B-cells into 2 cell types (centroblasts and centrocytes) based on morphologic criteria, such as size and nuclear contour. [13][14][15] Within a fully developed GC, centroblasts and centrocytes would distribute preferentially to opposite poles of this structure: a centrocyte-rich light zone (LZ), proximal to the lymph node capsule or spleen red pulp; and a centroblast-rich dark zone (DZ), proximal to the T-cell area. 2 In contrast, recent studies done in mice using techniques such as 2-photon microscopy and in situ photoactivation have shown that B cells in the LZ and DZ of mouse lymph node GCs are much more similar than traditional models suggest.…”

B cell attracting chemokine 1 (CXCL13) and its receptor CXCR5 are expressed in normal and aberrant gut associated lymphoid tissue

“…21 The observed partial overlapping of our immunostaining for the chemokine with several traditional FDC markers might result from BCA-1 secreted from another cell type and deposited on peripherally located FDCs. However, electron microscopy has suggested considerable heterogeneity of FDCs in tonsillar germinal centres 22 ; possibly FDC related, peripherally located so-called germinal centre bordering cells with long extensions have been described in rat lymph nodes. 23 Therefore, an FDC related subset of dendritic cells remains a potential but yet not proved source of BCA-1 in lymphoid tissue.…”