Objective To explore the repair effect and mechanism of human amniotic mesenchymal stem cells(hAMSCs)on endometrial injury.Methods hAMSCs were isolated using a two-enzyme digestion and then cultured.The third-passage(P3)cells were harvested to detect the surface markers by flow cytometry and to identify their trilineage differentiation potentials.Eighteen nulliparous female SD rats(8～9 weeks old,weighing 250～280 g)were randomly divided into 3 groups(n=6):normal control group,model group,and hAMSCs group.A rat model of intrauterine adhesions(IUA)was established in SD rats by using curettage combined with lipopolysaccharide(LPS)infection.In 2 weeks after modeling,the hAMSCs group received a bilateral uterine horn transplantation of 0.2 mL hAMSCs(1.0×10? cells/mL),while the model group received a same volume of PBS into both uterine horns.All rats were sacrificed in 2 weeks after transplantation.HE and Masson staining was used to observe endometrial thickness and gland number as well as endometrial fibrosis area.RT-qPCR and Western blotting were performed to detect the mRNA and protein levels of TGF-β1,Smad3,Smad7,epithelial-mesenchymal transition(EMT)markers(E-cadherin,Vimentin),fibrosis factor α-SMA,and endometrial estrogen receptor(ER)and progesterone receptor(PR)in endometrial tissues.Results The obtained cells were identified as hAMSCs due to the characteristics of surface markers and differentiation potentials.Compared with the normal control group,the model group showed decreased endometrial thickness,reduced gland number,increased fibrosis area,and enhanced mRNA and protein levels of fibrosis-related factors TGF-β1,Smad3,Vimentin,and α-SMA(P<0.01),while down-regulation of fibrosis-inhibiting molecule Smad7,the EMT marker E-cadherin,and endometrial receptors ER and PR at both mRNA and protein levels(P<0.01).hAMSCs transplantation increased endometrial thickness and gland number,decreased fibrosis area,and down-regulated mRNA expression of the aforementioned fibrosis-related factors(P<0.01),and up-regulated the mRNA expression levels of Smad7,E-cadherin,ER,and PR(P<0.01).The hAMSCs group also exhibited obviously down-regulated protein levels of TGF-β1,Smad3,and α-SMA(P<0.05),while enhanced protein levels of Smad7 and PR(P<0.05).Conclusion Intrauterine transplantation of hAMSCs can promote the repair of endometrial injury,and inhibits endometrial EMT and fibrosis through the TGF-β1/Smad7 signaling pathway.