## 3 Results
### Formation of Bioartificial Whole Rat Heart  
The bioartificial rat hearts (\(n=8\)) were macroscopically (Figure 7a-d) and microscopically characterized (Figure 8a-d). The two recellularization protocols involved a combined approach to recellularizing the whole heart, through the direct injection of hMSCs into the rat heart, in addition to infusion through the patent aorta for 24 h. From the macroscopic point of view, well-defined cardiac walls, both atria and ventricles were observed. The blood vessels were visible through the thickness of the heart. Fibrous bands, which could be the fibrous cytoskeleton of the heart, were concentrated at the base of the ventricular mass. H&E staining from different regions of the recellularized hearts confirmed the presence of an important number of cells distributed throughout the acellular scaffold. Histologically, a normal myocardium contains cardiomyocytes that are densely packed and form a syncytium mass. Here, the examined tissue contained loosely arranged cardiomyocytes-like cells that had visible ramifications that connected, evident cross-striations, and detectable nuclei. Although the histological architecture was not normal, a structural organization was still discernable. On the examined material, approximately 20% of the matrix contained cells with evident nuclei, with alternating acellular areas and zones of dense cellularity. The optical microscopy (OM) did not show any significant differences between the hearts recellularized with 5-aza using the two protocols (Figure 8a-d).  
Figure 6: Total DNA and protein concentrations measured in decellularization solution. Comparison of DNA and protein concentrations in washing fluid during the development of heart decellularization using the modified [PERSON] experimental device revealed a quasilinear increase in the parameters and a shorter average decellularization time needed for the process when using an electrical current. (**a**) The DNA concentrations for the control group reached the plateau phase at 840 min, whereas for the electric field group the plateau phase was reached at 480–540 min; (**b**) protein concentrations for the control group reached the plateau phase at 840 min, and for the electric field group the protein plateau phase was reached at 480–540 min.  
Figure 8: Histological analysis of recellularized rat heart scaffolds. H&E staining showed cells that resembled cardiomyocytes, arranged in fibers exhibiting cross-striations and eosinophilic cytoplasm, which centrally presented the formation of a nucleus. (**a**—Ob 40\(\times\); **b**—Ob 100\(\times\)) First protocol of ECM scaffold recellularization with hMSCs. (**c**—Ob 40\(\times\); **d**—Ob 100\(\times\)) Second protocol of ECM scaffold recellularization with hMSCs.  
Figure 7: (**a**–**d**) The macroscopic view of the recellularized scaffold using hMSCs. (**e**) Freshly isolated native rat heart. (**f**) Decellularized rat heart using 1% SDS in the modified Langendorff experimental device.  
Further experiments need to be conducted for complete cellular coverage in the recellularized rat heart as functional cardiac tissue requires viable cells for contractility and gap junction formation.