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Hi everyone, welcome to module 2. Today, we will discuss the general tissue engineering process and some of the important aspects to consider during tissue fabrication.

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In the fabrication process of tissue engineering, there are many factors and steps involved, such as cells, scaffolds, biomolecules, and stimuli. Here, we will explore the typical process for engineering a functional tissue unit.

First, the need for tissue engineering arises from tissue damage on a person. Tissue engineers will perform a biopsy on that person to retrieve usable cells. Afterwards, the cells are isolated. The cell source and type are very important to consider because of the several different possibilities and their effects on the final product.

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So where can tissue engineers obtain cells from? Autologous cells are extracted from the patient themselves. This leads to no harmful immune response or potential rejection, but can increase cell death at the donor site and has a limited quantity. This is because it would be worse for the patient if too many cells were removed and would lead to massive tissue damage in another area.

Allogeneic cells are extracted from donors of the same species. This approach reduces the amount of donor site morbidity, but can lead to a potential immune response, and also has limited cell quantity.

Xenogeneic cells are from donors of another species. There is not an issue of limited cell quantity, but it can lead to disease transmission and potential rejection of the tissue due to the patient’s immune response.

There are also different stages that cells can be in when they are sourced. Primary cell lines are directly sourced from the donor tissue. These include multiple cell types and require purification in order to achieve a functioning tissue. Secondary cell lines are either frozen or passaged primary cells. Passaging means moving the cells from one petri dish to another, and this increases the cell number and is an easier way to store cells. However, the freezing might alter the cell phenotype, which would affect the overall function of the tissue.

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Cells also go through different processes, such as cellular differentiation, during the fabrication process. Undifferentiated cells have yet to develop into cells with specialized structures and functions. Those specialized cells are known as differentiated cells, and have specific purposes. It is also possible for cells to go through dedifferentiation, which makes them less specialized than they were before. Finally, some mature cells can undergo transdifferentiation and become different specialized cells.

Here are some images that represent each type of cell during the differentiation process.

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What are stem cells? Stem cells are undifferentiated cells that are often used in tissue engineering because of their diverse lineages and possible functions.

There are several types of stem cells. Totipotent stem cells are at the beginning of the stem cell lineage, and they can develop into any type of cell. Next, there are pluripotent stem cells, which can be differentiated into any cell type except for extraembryonic tissue. Finally, there are adult stem cells which can be differentiated into any cell within the same class. For example, one type of adult stem cell could differentiate into all different types of heart muscle cells, but not any pancreatic cells.

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The process continues with the acquired cells being proliferated. This means having the cells reproduce and increase the total cell number. Afterwards, they go through differentiation into the specific cell type that is necessary for the tissue. Here, tissue engineers could choose to add scaffolds into the tissue unit to provide structure. However, there are multiple factors involved in choosing the type of scaffold and whether a scaffold is necessary.

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Scaffolds can be synthetic, which means that they are made of polymers, and are not biologically active, meaning they will not grow with the tissue. Natural scaffolds are biologically active, and are great for cells to adhere to and grow on. There are also different types of materials that can promote organization, growth, and differentiation in the cells. These can provide physical and chemical cues for the cells and make them function the way that the engineers would like them to. Here we see an image of scaffolds Dr. Lisa Larkin’s lab reviewed.

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Afterwards, the tissue starts to develop, and the tissue engineers can add different biomolecules to the tissue. The types and amounts of biomolecules are important to consider.

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Biomolecules include growth factors, which are vitamins or hormones that stimulate growth in cells. They lead the cells to duplicate and grow, which is important to achieve a usable size of tissue.

There are also ligands, which are molecules that send signals to cells and cause responses such as cell growth and death.

Finally, there are also cell adhesion molecules, which help the cells stick to each other and the surroundings, which would be the cell culture dish. These molecules are necessary for the cells to properly grow on the dish and form a whole tissue, not just separate cells.

The biomolecules we mentioned can alter a number of characteristics of the cells. They can change the phenotype of the cells, which is the cell’s appearance and function.

They can also affect the proliferation or reproduction of the cells, which is important for the growth and overall size of the tissue.

The biomolecules also play a role in cell differentiation, and can help the cells become specialized to fit the function of the tissue engineered unit.

The adhesion between the cells and the dish can also be affected by the biomolecules, and it is important for the tissue to develop a proper structure.

Finally, the cells can migrate, or move, as a result of different biomolecules. This is important for the distribution of cells and the structure of the tissue.

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Finally, the tissue engineers can add stimuli to the tissue to affect its growth. The type of stimulus, such as mechanical, electrical, or chemical, and the amount of stimulus are important factors in the development of the tissue.

When the tissue is finished developing and functional, the unit can be implanted into the patient. The unit should help improve the person’s bodily function in whatever area they were damaged in, and improve the quality of life of that patient. We’ve reached the end of this module, and I hope it was informative and helpful for you. You can now proceed to module 3. Thank you for watching!!