Which of the following is not a cell source in tissue engineering?

In tissue engineering, a variety of cell sources are essential for creating functional tissues. Stem cells are a common choice because of their ability to differentiate into various cell types, which makes them versatile for regenerating tissues. Primary cells, which are directly isolated from living tissues, maintain the characteristics of their tissue of origin and provide a natural environment for the engineered tissue. Passaged cells, derived from primary cells that have been cultured and expanded in vitro, also serve as an important source, providing sufficient quantities of cells for tissue engineering applications.

The choice indicating "None of the above" signifies that all of the provided options—stem cells, primary cells, and passaged cells—are indeed recognized sources used in tissue engineering. Therefore, this choice accurately reflects the fact that all listed options are valid cell sources, reinforcing the understanding of the diverse cell types utilized in the field of tissue engineering for successfully developing and maintaining engineered tissues.