Abstract
Efficacy of adult (bone marrow, BM) versus fetal (amniotic fluid, AF) mesenchymal stem cells (MSCs) to replenish damaged rat heart tissues with new cardiovascular cells has not yet been established. We investigated on the differentiation potential of these two rat MSC populations in vitro and in a model of acute necrotizing injury (ANI) induced by cryoinjury. Isolated BM-MSCs and AF-MSCs were characterized by flow cytometry and cytocentrifugation and their potential for osteogenic, adipogenic, and cardiovascular differentiation assayed in vitro using specific induction media. The left anterior ventricular wall of syngeneic Fisher 344 (n = 48) and athymic nude (rNu) rats (n = 6) was subjected to a limited, nontransmural epicardial ANI in the approximately one third of wall thickness without significant hemodynamic effects. The time window for in situ stem cell transplantation was established at day 7 postinjury. Fluorochrome (CMTMR)-labeled BM-MSCs (2 × 106) or AF-MSCs (2 × 106) were injected in syngeneic animals (n = 26) around the myocardial lesion via echocardiographic guidance. Reliability of CMTMR cell tracking in this context was ascertained by transplanting genetically labeled BM-MSCs or AF-MSCs, expressing the green fluorescent protein (GFP), in rNu rats with ANI. Comparison between the two methods of cell tracking 30 days after cell transplantation gave slightly different values (1420,58 ± 129,65 cells/mm2 for CMTMR labeling and 1613.18 ± 643.84 cells/mm2 for genetic labeling; p = NS). One day after transplantation about one half CMTMR-labeled AF-MSCs engrafted to the injured heart (778.61 ± 156.28 cells/mm2) in comparison with BM-MSCs (1434.50± 173.80 cells/mm2, p < 0.01). Conversely, 30 days after cell transplantation survived MSCs were similar: 1275.26 ± 74.51/mm2 (AF-MSCs) versus 1420.58 ± 129.65/mm2 for BM-MSCs (p = NS). Apparent survival gain of AF-MSCs between the two time periods was motivated by the cell proliferation rate calculated at day 30, which was lower for BM-MSCs (6.79 ± 0.48) than AF-MSCs (10.83 ± 3.50; p < 0.01), in the face of a similar apoptotic index (4.68 ± 0.20 for BM-MSCs and 4.16 ± 0.58 for AF-MSCs; p = NS). These cells were also studied for their expression of markers specific for endothelial cells (ECs), smooth muscle cells (SMCs), and cardiomyocytes (CMs) using von Willebrand factor (vWf), smooth muscle (SM) α-actin, and cardiac troponin T, respectively. Grafted BM-MSCs or AF-MSCs were found as single cell/small cell clusters or incorporated in the wall of microvessels. A larger number of ECs (227.27 ± 18.91 vs. 150.36 ± 24.08 cells/mm2, p < 0.01) and CMs (417.91 ± 100.95 vs. 237.43 ± 79.99 cells/mm2, p < 0.01) originated from AF-MSCs than from BM-MSCs. Almost no SMCs were seen with AF-MSCs, in comparison to BM-MSCs (98.03 ± 40.84 cells/mm2), in concordance with lacking of arterioles, which, instead, were well expressed with BM-MSCs (71.30 ± 55.66 blood vessels/mm2). The number of structurally organized capillaries was slightly different with the two MSCs (122.49± 17.37/mm2 for AF-MSCs vs. 148.69 ± 54.41/mm2 for BM-MSCs; p = NS). Collectively, these results suggest that, in the presence of the same postinjury microenvironment, the two MSC populations from different sources are able to activate distinct differentiation programs that potentially can bring about a myocardial-capillary or myocardial-capillary-arteriole re-constitution.