In mixed cultures, bone marrow-derived macrophages lined up with SV-LEC, incorporating into cord-like structures and expressed Podoplanin (Figure 6D/iii�Cvi). Interestingly, GFP+ macrophages were predominantly located at the tips and at branch points of growing cord-like structures selleck catalog (Figure 6D/iii�Cvi) and seemed to guide SV-LEC to form a new sprout as observed by time-lapse video microscopy (Figure S8, Supplemental Movie S3). The live visualization of GFP+ macrophages guiding LEC together with the observation that macrophages located at the tip of the lymphatic sprout exhibit filopodia-like structures (Figure 6D/iii�Cvi) strongly suggest that instead of capping the exposed ends, they actively instigate the new sprout.

These results demonstrate that bone marrow-derived macrophages have the ability to form lymphatic tube-like structures in vitro, a process requiring FGF signaling. Their preferred incorporation at tips and branchpoints of pre-existing lymphatic cord-like structures data suggest a role of macrophages in lymphatic endothelial cell sprouting. Discussion Research on BMDC in patho-physiological processes, such as atherosclerosis, limb/heart ischemia and cancer, has in the past mainly focused on the importance of hematopoietic cells in promoting or attenuating inflammation, in clearing cancer cells, or in inducing immunological tolerance to neoplastic lesions. However, recent findings indicate that the bone marrow is also a rich source of progenitor cells with mesenchymal and endothelial potential [39], [40].

In the case of endothelial progenitor cells, the lineage relationship to the hematopoietic system is not clear. While some experiments have recently revealed that during development hematopoietic cells arise from a specialized endothelium named the hemogenic endothelium [41]�C[43], other reports provide evidence that the reverse direction of cellular conversion is also possible, i.e. that myeloid cells can contribute to the formation of blood endothelial cells [9], [44]. Here, we have used bone marrow transplantation experiments in two different mouse models of carcinogenesis to demonstrate that BMDC significantly contribute to tumor lymphangiogenesis, but rarely integrate into tumor blood vessels. We have performed lineage-tracing experiments to obtain insights into the ontogeny of bone marrow-derived TLEC.

First, transplantations of FACS-sorted bone marrow fractions representing different hematopoietic lineages or of total bone marrow expressing GFP under a myeloid specific Batimastat promoter indicate that integrated BMDC are derived from the myeloid lineage. Second, genetic tagging of myeloid cells with GFP confirms this notion; cells that have passed through the myeloid lineage are found integrated into the lymphatic vasculature surrounding tumors.