Background and Objective: Nowadays, although umbilical cord blood is a commonly used source of hematopoietic stem cell, its low frequency of these cells is the main factor limiting its clinical application. The transplantation of hematopoietic stem cells derived from placenta tissue along with umbilical cord blood cells of the same sample may be an appropriate approach to solve this problem. In this study, we tried to mimic the niche of placenta by nano fiber scaffold in order to expand the hematopoietic stem cells derived from placenta tissue. Materials and Methods: Different stromal cells along with hematopoietic stem cells derived from placenta tissue were seeded on nano fiber scaffold produced from PLLA coated with collagen. Then, the rate of proliferation in these niches was studied. Results: The expansion in the mimicked niche associated with 3.6 fold increase in the number of cells but the capacity of forming colonies decreased significantly (P<0.0001). Also, the percentage of hematopoietic stem cells increased. Nevertheless, the differences were not statistically significant (P>0.05). Conclusion: Due to a decreased capacity in forming colonies of hematopoietic stem cells derived from placenta, the expansion of stem cells from a part of placenta is not an appropriate solution and other approaches such as isolation of hematopoietic stem cells from the whole placenta tissue should be considered. References 1- Delaney C, Gutman JA, Appelbaum FR. Cord blood transplantation for hematological malignancies: conditioning regimens, double cord transplant and infectious complications. Br J Haematol. 2009 147: 207-16. 2- Escalon MP, Komanduri KV. Cord blood transplantation: evolving strategies to improve engraftment and immune reconstitution. Curr Opin Oncol. 2010 22: 122-9. 3- Hofmeister CC, Zhang J, Knight KL, Le P, Stiff PJ. Ex vivo expansion of umbilical cord blood stem cells for transplantation: growing knowledge from the hematopoietic niche. Bone Marrow Transplant. 2007 39: 11-23. 4- Tung SS, Parmar S, Robinson SN, De Lima M, Shpall Ej. Ex vivo expansion of umbilical cord blood for transplantation. Best Pract Res Clin Haematol. 2010 23: 245-57. 5- Devin SM, Lazarus HM, Emerson SG. Clinical application of hematopoietic stem cells expansion: current status and future prospects. Bone Marrow Transplant. 2003 31: 241-52. 6- Shpall EJ, Quinones R, Giller R, et al. Transplantation of ex vivo expanded cord blood. Biol Blood Marrow Transplant. 2002 8: 368-76. 7- Yao CL, Chu IM, Hsieh TB, Hwang SM. A systematic strategy to optimize ex vivo expansion medium for human hematopoietic stem cells derived from umbilical cord blood mononuclear cells. Exp Hematol. 2004 32: 720-7. 8- Schofield R. The relationship between the spleen colony-forming cell and the haemopoietic stem cell. Blood Cells. 1978: 4: 7-25. 9- Mercier FE, Ragu C, Scadden DT. The bone marrow at the cross roads of blood and immunity. Nat Rev Immunol. 2011 12: 49-60. 10- Ugarte F, Forsberg EC. Haematopoietic stem cell niches: new insights inspire new questions. EMBO J. 2013 32: 2535-47. 11- Frenette PS1, Pinho S, Lucas D, Scheiermann C. Mesenchymal stem cell: keystone of the hematopoietic stem cell niche and a stepping-stone for regenerative medicine. Annu Rev Immunol. 2013 31: 285-316. 12- Mendez-Ferrer S, Michurina TV, Ferraro F, et al. Mesenchymal and hematopoietic stem cells form a unique bone marrow niche. Nature. 2010 466: 829-34. 13- Sideri A, Neokleous N, Brunet De La Grange P, et al. An overview of the progress on double umbilical cord blood transplantation. Haematologica. 2011 96: 1213-20. 14- Barker JN, Weisdorf DJ, DeFor TE, et al. Transplantation of 2 partially HLA-matched umbilical cord blood units to enhance engraftment in adults with hematologic malignancy. Blood. 2005 105: 1343-7. 15- Gekas C, Dieterlen-Lievre F, Orkin SH, Mikkola HK. The placenta is a niche for hematopoietic stem cells. Dev Cell .2005 8: 365-75. 16- Barcena A, Kapidzic M, Muench MO, et al. The human placenta is a hematopoietic organ during the embryonic and fetal periods of development. Dev Biol. 2009 327: 24-33. 17- Serikov V, Hounshell C, Larkin S, et al. Human term placenta as a source of hematopoietic cells. Exp Biol Med. 2009 234: 813-23. 18- Omidkhoda A, Kaviani S, Soleimani M, et al. Isolation and characterization of hematopoietic and mesenchymal stem cells derived from human placenta tissue. Sci J Iran Blood Transfus Organ. 2014 11: 4-11. 19- Ehring B, Biber K, Upton TM, et al. Expansion of HPCs from cord blood in a novel 3D matrix. Cytotherapy. 2003 5: 490-9. 20- Di Maggio N1, Piccinini E, Jaworski M, Trumpp A, Wendt DJ, Martin I. Toward modeling the bone marrow niche using scaffold-based 3D culture systems. Biomaterials. 2011 32: 321-9. 21- Oswald J, Steudel C, Salchert K, et al. Gene expression profiling of CD34+ hematopoietic cells expanded in a collagen I matrix. Stem Cells. 2006 24: 494-500. 22- Leisten I, Kramann R, Ventura Ferreira MS, et al. 3D co-culture of hematopoietic stem and progenitor cells and mesenchymal stem cells in collagen scaffolds as a model of the hematopoietic niche. Biomaterials. 2012 33: 1736-47. 23- Mortera-Blanco T, Mantalaris A, Bismarck A, Aqel N, Panoskaltsis N. Long-term cytokine-free expansion of cord blood mononuclear cells in three-dimensional scaffolds. Biomaterials. 2011 32: 9263-70. 24- Ferreira MS, Schneider RK, Wagner W, et al. Two-dimensional polymer-based cultures expand cord blood-derived hematopoietic stem cells and support engraftment of NSG Mice. Tissue Eng Part C Methods. 2013 19: 25-38. 25- Xue C1, Kwek KY, Chan JK, Chen Q, Lim1 M. The hollow fiber bioreactor as a stroma-supported, serum-free ex vivo expansion platform for human umbilical cord blood cells. Biotechnol J. In press.