114 T. KRENACS and M. ROSENDAAL projections toward B cells from the beginning of culture (Figure 3). Gap junctions were sometimes in close association with B cells and the enveloping FDC processes. This was most obvious in FDC-B-cell clusters in later cultures (Figure 4). The intimate relationship between FDC, B cells, and gap junctions was very similar to the tissue findings (see Figure 1A) and raised the possibility of heterologous communi- cation between these cells. We used dye-transfer studies and established that this was the case (Krenacs et al., 1997). We microinjected elongated FDC with a small-molecular-weight fluorescent dye Lucifer Yel- low, which passes through gap junctions. In a few minutes, one to three neighboring FDC lit up and sometimes the dye was also delivered to B cells through gap junctions. Besides FDC, some isolated germinal center B cells expressed the gap-junction protein too (Figure 5). Function for Gap Junctions For assessing the potential of direct cell-cell com- munication in the germinal center, it is useful to outline briefly what functions have been attributed to gap junctions elsewhere. That gap junctions serve important function(s) is expected from their ubiqui- tous presence in multicellular organisms (Kumar and Gilula, 1996) and because their channel-forming connexins are highly conserved throughout evolution (Warner, 1992). Functional data collected so far can be grouped along two main lines. On one hand, gap junctions provide a low-resistance pathway for the rapid propagation of local signals through coupled cell networks (Bruzzone et al, 1996). Depolarizing waves are held to spread in this way between excitable cells such as cardiac myocytes (Rook et al., 1990), smooth muscle cells (Miller et al., 1989), or neurones (Bruzzone et al., 1996), resulting in rapid and coordinated responses. In nonexcitable tissues, local stimuli may generate oscillating Ca+2 waves across coupled cells. In this way, gap-junctional communication is involved, for instance, in the integration of insulin secretion in the endocrine pancreas (Vozzi et al., 1995) or in the propagation of vasopressin action between hepatocytes (Nathanson et al., 1995). On the other hand, gap junctions are thought to provide a pathway for growth regulatory signals and FIGURE 3 FDC-B-cell culture simultaneously immunostained for (A) CD35 and (B) Cx43 after 4 hr of culture. Sheets and narrow processes of FDC decorated with immunolabeled gap junctions (B; arrow) are extending toward and enveloping B lymphocytes (A). Bar: 10/.tm.