Cardiovascular diseases are one of the leading causes of morbidity in the Western world, and of these, myocardial infarction is the main cause of mortality. In myocardial infarction, the blood supply to the myocardium is impaired and, as a result, a scar is formed in the affected area. Not only does this scar not participate actively in cardiac function, it also significantly burdens the healthy parts of the heart, which are now left to handle the required load by themselves. Current clinical treatment focuses on medicinal and/or surgical means designed mainly to facilitate cardiac activity and to prevent the recurrence of additional heart attacks, however it is unable to rehabilitate the damaged scar tissue. Thus, the only solutions for end-stage heart patients are heart transplantation or use of ventricular assist devices. These two options are highly limited, both in terms of supply and in terms of cost, and this creates a significant gap between the number of people who need treatment and those who actually receive it. This gap has led to extensive tissue engineering and cell therapy research, aiming to develop alternatives for rehabilitating and regenerating the damaged tissue.
In recent years, several engineered tissue substitutes were developed and characterized as a rehabilitative treatment following myocardial infarction, mainly in animal models. These substitutes are based on two principal components: cells and/or a supportive scaffold comprising biomaterials.
Such an ideal engineered system should have several major characteristics. At the cellular level, among the cell types present in the heart and required for rehabilitation, the cells occupying the vascular system that nourishes the heart tissue and the myocardial cells are the most critical. Unfortunately, despite substantial investment in cell research, mature, beating myocardial cells still cannot be reproduced in laboratory conditions, and the production of massive cell quantities is therefore limited.
Concurrent with cell research, substantial effort is being invested in developing scaffolds that would be used as supportive platforms for the transplanted cells but also for the transplantee's damaged tissue. These scaffolds should have biomechanical properties that are compatible with those of the myocardium, support the cells and the rehabilitating tissue while providing the required biochemical signals, and degrade as the natural extracellular matrix is secreted. In terms of their composition, the scaffold and its degradation products should be biologically compatible.
One of the materials considered most suitable for creating such a scaffold is the natural extracellular matrix. This is a protein matrix that maintains and supports the cells in the healthy tissue. According to recent publications, matrices such as these have been produced from several tissues; one such matrix was produced by Prof. Machluf from porcine heart tissue, which is physiologically similar to the human heart. In the matrix production process (called decellularization), cells are selectively removed from the tissue, while the biochemical composition and the mechanical properties of the original tissue are preserved. Because it is natural and has a protein composition which is 98% identical in pigs and humans, this matrix, which is produced from pig hearts, had substantial biological activity and the immune system tolerates it. Thus, the immune system does not reject transplanted organs based on it, as it does in the case of other foreign materials, and might even provoke a rehabilitation response in the transplanted heart.
Despite substantial progress in myocardial tissue engineering, clinical application is delayed, since the majority of the research is done in static growth conditions, on small animal models and with thin engineered tissue (1-2 mm) that does not meet the physiological requirements for clinical needs. One of the reasons for this is that due to a diffusion limitation, cells seeded on thin scaffolds and grown in standard conditions fail to penetrate more than one hundred micrometers into the scaffold. This diffusion limitation is also a barrier when trying to penetrate deeply into the tissue during production of the extracellular matrix. Since the left ventricle is about 7,000 times bigger than this diffusion barrier (about 10-15 mm in human and in pig hearts), the production itself of engineered tissue with a clinically relevant thickness, and the support for cells seeded in them during preparation of the implant in the lab and post-transplantation require a change in perception. This change requires the use of complex systems that are flexible and adaptable to multiple variables, and entails designing "smarter" scaffolds with a complex structure comprising an inherent feeding infrastructure. These smart scaffolds should be supplied through innovative bioreactor systems, and proper work procedures should be set that allow cultivating tissue cultures in dynamic conditions in a manner that mimics physiological conditions.
In an article published recently in the scientific journal Tissue Engineering, the research group headed by Prof. Marcelle Machluf of the Faculty of Biotechnology and Food Engineering focused on the development of optimal decellularization processes for porcine myocardial slabs with physiological thickness. This development places special emphasis on the isolation of a matrix that is as clean as possible from cellular components that could provoke immunological rejection, while retaining its inherent vasculature, for the purpose of dynamic feeding currently being studied in Prof. Machluf's lab.
This research was financed by the Office of the Chief Scientist and was conducted in cooperation with Singapore's research agency.
The characterization of the decellularization process developed and reported in this work has led to the creation of a thick, complex scaffold which supports human stem cells and comprises extracellular proteins. This product does not provoke a response from cells of the immune systems, and preserves the ultra-structural characteristics of the extracellular matrix in the natural tissue, as well as its inherent vasculature. Additional tests were carried out to demonstrate the ability of the produced scaffold to support the growth of human cells under static and dynamic conditions while relying on the function of the vasculature it retains. These scaffolds, when seeded in the appropriate cells, could serve as laboratory platforms that mimic the myocardium for research purposes, as models for the extracellular matrix in humans, and as future implants for treatment of myocardial infarction.