Background In bone marrow transplant patients,the microenvironment in bone marrow is damaged after chemotherapy or radiotherapy.Subsequent to allogenic hematopoietic stem cell transplantation in patients with clinically successful engraftments,the source of mesenchymal stem cells (MSCs) remains controversial.To further verify the stimulatory effect of the simultaneous transplantation of cells from second donors on engraftment success for hematopoietic stem cell transplantation in support of donor MSCs engraftments,the aim of this study is to monitor the dynamics of the engraftment of bone marrow-derived MSCs in patients after transplantation with mismatched-sex hematopoietic stem and third-party cells.Methods In this study,the hematopoietic stem cells from 32 clinical donors of different sexes that resulted in successful engraftments were selected for transplantation and were classified into three groups for research purposes:group A consisted of 14 cases of transplantation with bone marrow and recruited peripheral hematopoietic stem cell transplantation,group B contained 8 cases of simultaneous re-transfusion of MSCs from the second donor,and group C contained 10 cases of simultaneous re-transfusion of umbilical blood from the second donor.The bone marrow from 32 patients with successful engraftments of hematopoietic transplantation were selected and sub-cultured with MSCs.Flow cytometry (FCM) was used to measure the expression of surface antigens on MSCs.Denaturing high-performance liquid chromatography (DHPLC) in combination with polymerase chain reaction amplification of short tandem repeats (STR-PCR) was used to measure the engraftment status of fifth-generation MSCs in patients.Fluorescence in situ hybridization (FISH) revealed the sex origin of the fifth-generation MSCs in 32 patients.Dynamic examinations were performed on patients receiving donor transplantations.Results The progenies of fifth-generation MSCs were successfully cultured in 32 cases.The results of FCM demonstrated that the expression levels of CD14+ and CD45+ cells were lower than 0.04％ in the fifth-generation MSCs.The analysis using DHPLC and FISH showed similar results.One patient from group B also received a temporary transplantation of MSCs from the donor.The MSCs in the remaining 31 patients all originated from the patients themselves.Conclusions After transplantation,the MSCs present in patients originated from the host.In patients transplanted with MSCs from a second donor,the phenomenon of temporary chimerization of MSCs was observed.