Objective To investigate the isolation, incubation and identification of murine yolk sac mesenchymal stem cells(YS_MSCs). Methods Murine yolk sacs were harvested on the 10th day of postcoitus microisolation.Yolk sac cells were obtained after the yolk sacs were digested by type Ⅰ collagenase of 0.1% for 1 hour. Adherent cells were cultured in DMEM containing 10% FBS and 1% SP,and passaged when they became subconfluent.The ultrastructure of YS_MSCs was observed under transmission electron microscope; the surface markers of YS_MSCs were detected by flow cytometry; the alkaline phosphatase(AKP) expression of YS_MSCs was tested; and cytochemical changes of YS_MSCs were observed with cytochemical detection. Adipogenic differentiation of YS_MSCs was induced by 1μmol/L dexamethasone and 10mg/L insulin, and oil red O was used for fat staining. Results YS_MSCs were obtained in vitro, and most of them were of spindle shape;under transmission electron microscope,there were microvilli on the surface of YS_MSCs,there were abundant mitochondria, rough endoplasmic reticulum and golgi complex in the endochylema. The cell nuclei of YS_MSCs were big and the shape of cell nucleus was irregular; the analysis of flow cytometry suggested that the primary and first generation of YS_MSCs were positive for CD44,CD105, and the expression of CD34 was small;Cytochemistry showed that YS_MSCs were positive for glycogen,and negative for SB and AKP; Adipogenic differentiation of YS_MSCs was induced by 1μmol/L dexamethasone and 10mg/L insulin;accumulation of lipid_rich vacuoles appeared in the cells, positive for oil red O staining.Conclusion The biological characteristics of murine yolk sac mesenchymal stem cells are similar to, and more primitive than, those of adult mesenchymal stem cells.It suggests that YS_MSCs may be used as an