Formation of Pancreatic Duct Cells from Bone Marrow during Neonatal Development.

A debate has raged in recent years over whether cells within the bone marrow have the capacity to generate non-hematopoietic, specialized tissue in other organs. Previous reports have studied bone marrow transplantation in adult recipients; most models have found that tissue injury was necessary for organ engraftment of bone marrow and at best low levels of engraftment are seen. In the present study, we show complex ductal epithelial structures in the pancreas completely derived from bone marrow transplanted during the neonatal period in the absence of tissue injury. BRDU labeling showed that epithelial cells lining the ducts and islet cells were cycling at significantly higher levels in the neonatal than in the adult pancreas (P<0.0001). The contribution of transplanted bone marrow to neonatal pancreatic development was thus explored. To exclude the effect of injury on engraftment patterns, non-conditioned, immune deficient neonatal mice were used as bone marrow recipients. 5-10x10⁶ bone marrow cells from male transgeneic GFP mice were injected into female neonatal Nod/Scid/b2M^null mice 24 hours after birth through the superficial temporal vein. Two months after bone marrow transplantation, hematopoietic engraftment of donor cells was obtained in all transplanted mice, with 22.3 ±4.9% GFP+ cells observed in the peripheral blood (n=9 mice) and pancreas was analyzed with immunohistochemistry, FISH and confocal microscopy. Co-staining of GFP, with anti-cytokeratin (CK) Ab for epithelium, and FISH for the presence of donor Y chromosome indicated that up to 40% of ducts (mean 8.4%) contained epithelium derived from donor bone marrow. In approximately half of these donor derived ducts, there were clusters of large and small ducts, all expressing GFP as if whole branching structures were derived from donor bone marrow. In addition, 2.5% of islets contained cells that co-expressed GFP and Insulin. Donor derived duct and islet cells did not co-express hematopoietic markers excluding donor blood cell contamination. Unlike pancreatic damage models, no donor derived vascular endothelial cells were found in the study. In contrast, bone marrow transplanted into adult mice rarely generated ductal epithelium (0.7% ducts) or islet cells. These findings indicate that the neonatal period may provide a unique window during which mechanisms exist to recruit transplanted bone marrow to proliferating pancreatic tissue without tissue damage. The data also support that existence in bone marrow of pluripotent stem cells or epithelial precursors that can home to pancreas and differentiate into complex organ specific structures.