A new feature of intercellular transfer observed in cultured sheep bone marrow mesenchymal stem cells

Introduction: Intercellular communication governs the exchange of signalling molecules between cells. The different mechanisms employed in cell-to-cell communication include the secretion and receptor-mediated binding of diffusible messengers such as hormones and growth factors, the transport of small molecules through gap junctions, exosomes as well as tunnelling nanotubes. In this study, we report a new feature of intercellular transfer in cultured sheep bone marrow mesenchymal stem cells (BM-sMSCs). Methods: Bone marrow was harvested from iliac crest of healthy sheep. Mononuclear cells were isolated by gradient centrifugation, plated in culture flasks and incubated in the presence of Coon’s medium in a humidified atmosphere at 37 °C with 5% CO2. Non-adherent cells were discarded after 3 days and adherent cells were cultured until they reached near-confluence (10 days). The cells were trypsinized and the pellets were fixed with 3% glutaraldehyde and post-fixed in OsO4 1%. After dehydration and embedding in Epon 812, ultrathin sections were counterstained with lead citrate and uranyl acetate and observed at a transmission electron microscope. Results: The cell populations consisted of two undifferentiated cell types: electron-lucent and electron-dense cells. Both showed prominent RER, glycogen aggregates and filopodia. The electron-lucent cells were more numerous. The nucleus in electron-lucent cells was euchromatic and contained a prominent nucleolus, whereas in electron-dense cells it was irregular in shape and full of heterochromatin. No intercellular junctions were observed between either electron-lucent cells or electron-dense cells, whereas spot-like fusions of the plasma membrane occur between the electron-lucent cell surface and electron-dense cells. Across this distinct membranous connection, a flow of cytoplasm - but not of organules – occurs from electron-dense to electron-lucent cells. Conclusions: This finding shows a new cell-to-cell connection implicated in intercellular transfer and stimulates further studies to identify the cell types involved and the nature of the substances transferred.