Natural Scaffolds

Using cells in a dish to create a matrix and heal injuries

Our bodies are not just living cells stuck between other living cells. Tissue and organs are made not just of cells, but of a matrix of fibers, proteins, and small molecules that living cells secrete and live in. This material is called the extracellular matrix, which gives structure to our bodies and a pathway for cells to communicate with each other.

When the cells are washed away, the extracellular matrix, or ECM, remains, and can be used as a material to help repair injuries. Most ECM used for regenerative medicine or tissue engineering is derived from the tissue of either the patient being treated or an animal, like a pig or cow. However, this source is ultimately limited by the amount of donor material available and the amount that can be proven disease free.

ECM can also be made by cells grown in a lab. When living cells are put in a plastic dish, they start growing in number and making their own new matrix. This cell-derived ECM can be made from a patient’s own cells, eliminating any worries about availability or disease transfer. Weixiang Zhang, et al., put together a review article discussing cell-derived ECM and it’s orthopedic applications.

“…cell-derived ECM can be obtained from autologous cells cultured under sterile conditions in vitro, thereby avoiding the shortcoming of decellularized ECM derived from tissue. In addition, cell-derived ECM scaffolds are more readily customizable through the use of different types of cells…” Weixiang Zhang, et al.1

Why use ECM?

Extracellular matrix is the natural environment for cell growth. All other biomaterials merely attempt to approximate ECM in one way or another. Immune rejection and the inability to get cells out of the matrix hindered use of this material in the past, but there are now several protocols that prepare this material so that it can be used.

ECM inherently has the right biochemistry to regulate cell growth and behavior. It can securely anchor cells so they stay in the area of injury, promote relevant gene activation, and keep cells from changing themselves into less helpful cell types.

The preparation techniques do degrade the mechanical structure and strength of the material, some techniques more than others, but you chose the technique for the application, so this is less of a problem than a specification for the intended application.

How to make cell-derived ECM?

There are several methods to make cell-derived ECM, but two are the most common. In the first method, living cells are put on a plastic dish and allowed to grow until they fill the dish. When cells start touching each other, they stop expanding and start making matrix. This flat sheet is then washed with mild chemicals to remove all living cells, and the resulting decellularized sheet of ECM is lifted out of the dish.

The other common method creates pellets of ECM rather than sheets. After the cells create the sheet of ECM in the dish, the sheet is detached using cycles of freezing and thawing, killing the cells, which are then washed away and the ECM is centrifuged, compacting the matrix into a pellet.

An interesting method that can’t be reproduced with tissue-derived matrix puts cells onto a three-dimensional scaffold. Using a polymer that can easily be dissolved, beads or grill shapes are laid down in the cell culture dish. Instead of making flat sheet, the cells make ECM with a three-dimensional pattern guided by the polymer. The polymer and cells are then washed away, leaving a patterned matrix scaffold rather than flat sheets or pellets.

How is cell-derived ECM used?

Cell-derived extracellular matrix methods have not been studied as long as tissue-derived ECM method, and many of the applications are not unique. Most of the cartilage repair applications are reproductions of what has already been done with tissue-derived ECM.

The bone tissue engineering applications have not all been successful. ECM was created with cells from bone marrow, but the applications so far seem to be limited to controlling the development of stem cells. Some researchers are experimenting with combining the cell-derived ECM with other materials, like calcium and phosphate minerals found in bone, with conflicting results. Until the methods to create the cell-derived ECM are more standardized, it will be hard to compare experimental results.

Conclusions and Comments

Cell-derived ECM technologies have the potential to address limitations of tissue-derived methods. Using a patient’s own cells to create the ECM means a more gentle decellularization process can be used because immune rejection is not an issue. This also means there is no problem with material availability. Eliminating disease transmission will continue to be mentioned as a benefit, but given modern screening protocols, this danger is already extremely low with tissue-derived methods.

The opportunity to create ECM sheets with interesting shapes and structures, using other polymers as guides, will lead to new applications. Layers of ECM created by different types of cells will also be an interesting lead to follow.

The biggest hurdle cell-derived technologies will have to overcome is the non-native structure and composition of ECM created in a plastic dish as opposed to tissue-derived ECM created in a functional, living organ. There will be no benefit to an ECM product that still needs addition of more therapeutic proteins to be effective.

Regardless, cell-derived ECM methods and applications will be interesting to study as they are compared to the more common applications using tissue-derived ECM.

  1. Weixiang Zhang, Yun Zhu, Jia Li, Quanyi Guo, Jiang Peng, Sichen Liu, Jianhua Yang, Yu Wang. “Cell-derived extracellular matrix: Basic characteristics and current applications in orthopedic tissue engineering.” Tissue Engineering Part B 22(3): 193-207.
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