Up-regulation of the connexin43+ gap junction network in haemopoietic tissue before the growth of stem cells.

The early developmental stages of haemopoiesis are thought to be regulated by paracrine growth factors and by the haemopoietic environment. Are gap junctions involved here? Gap junctions are structures in cell membranes allowing the direct transfer of ions and small molecules between adjacent cells and are known to be involved in development. We have found that although connexin43 gap junctions are rare (0.00016 +/- 0.0002/microns2 tissue) in normal adult mouse marrow their expression is 80-fold higher (0.0292 +/- 0.0147/microns2) in neonatal marrow. One difference between neonatal and adult haemopoietic tissue is that in the latter more haemopoietic cells are dividing. To test if more gap junctions were due to increased division we altered adult blood-formation by mobilizing or destroying end cells--granulocytes and red cells--or by forcing stem cells to divide by making them regenerate an ablated blood-forming system. Mobilizing end cells had no effect on the number or distribution of gap junctions in marrow but forced stem cell division caused a 100-fold increase in gap junction expression and did so before any recognizable haemopoietic cells formed. There were greater than normal numbers of gap junctions in radio-protected adult mouse marrow. The cells coupled by gap junctions are TE-7+ mesodermally derived fibroblasts, STRO-1+ stromal cells, and CD45+ and CD34+ haemopoietic cells. We propose that there is a latent network of Cx43+ gap junctions in normal quiescent marrow. In response to events that call for active division of stem cells this network is amplified and coupled to haemopoietic stem cells, perhaps enabling them to divide.