Background: Mesenchymal stem cells (MSCs) can differentiate into cardiomyocytes if an appropriate cellular environment is provided. Notch signals exchanged between neighboring cells through the Notch receptor can eventually dictate cell differentiation. In our Study, we show that MSC differentiation into cardiornyocytes is dependent on the Notch signal. Methods: We created a myocardial infarction model in rat by coronary ligation, administered direct intramyocardial injection of DAPI-labeled MSC immediately, and observed the differentiation of MSCs after 14 days by irnmunofluorescence staining against troponin T. We cultured MSCs and cardiornyocytes in four ways, respectively, in vitro. (1) MSCs cocultured with cardionlyocytes obtained from neonatal rat ventricles in a ratio of 1: 10. (2) The two types of cells were cultured in two chambers separated by a semipermeable membrane as indirect coculture group. (3) Notch receptor-soluble jagged I protein was added to indirect coculture group. (4) Both jagged1 protein and gamma-secretase inhibitor-DAPT were added to indirect coculture group. Two weeks later, we observed the differentiation percentage, respectively, by immunofluorescense staining. Results: We found the differentiation of MSCs which were close to cardiornyocytes in vivo. The differentiation percentage of the four cell Culture group was 30.13 +/- 2.16% 12.52 +/- 1.18%, 26.33 +/-2.20%, and 13.08 +/- 1.15%. Conclusions: MSCs can differentiate into cardiornyocytes in vitro and in vivo if a cardiomyocyte mcroenvironment is provided. 2. Cell-to-cell interaction is very important for the differentiation of MSCs into cardiornyocytes. 3. Jagged1 protein can activate Notch signal and enhance the differentiation or MSC into cardiomocyte, while the effect can be inhibited by DAPT. (c) 2005 Elsevier Inc. All rights reserved.