Stem cells are undifferentiated cells that are capable of repeated mitotic divisions for long periods of time and given the right conditions, can differentiate into specialised cells, and these characteristics make them ideal for producing replacement tissues (cell replacement therapy).

Any disorder involving loss of, or injury to, normal cells is a potential candidate for stem cell replacement therapy. Cell replacement therapy for the nervous system has generated the most interest, due to the debilitating nature and widespread occurrence of neurodegenerative disorders such as Parkinson’s and Alzheimer’s.

The most attractive method for restoring brain function in, e.g., embryonic stem cells have been carried out in humans with some success – the transplanted cells not only survived but grew and established connections with adjacent neurons. However, the use of human embryonic stem cells is controversial and raises a number of ethical questions – researchers into Parkinson’s disease are currently exploring other sources of cells to help restore patients’ brain function.

Stem cells are increasingly being used for tissue engineering, and the primary objective of tissue engineering is to restore healthy tissues or organs for patients and thus eliminate the need for tissue or organ transplants, or artificial implants

Early research in this area used cells from the intended recipient, but in many cases such as genetic diseases, this was not practicable. In other situations, the organ from which cells were to be harvested was diseased, and so not enough normal cells were present to enable a successful culture – the use of stem cells overcomes both problems.

Tissue engineering requires an abundant supply of disease-free cells of specific types, and these cells then need to be induced to grow on a scaffold of natural or synthetic material to produce a three-dimensional tissue.

Tissue engineering scaffolds serve as a template for tissue growth, and need to have high pore sizes that enable the cells to grow while at the same time allowing the diffusion of nutrients by the surrounding tissues without having to be removed surgically.

This needs to be established carefully, as the rate at which the scaffold degrades needs to match, as far as possible, the rate of tissue formation – that is, while the new cells are manufacturing their own natural matrix structure around themselves, the scaffold is providing a support structure that will eventually break down, leaving newly formed tissue

Once a scaffold has been devised, suitable stem cells need to be cultured, and these cells are seeded onto the scaffold, which then enables further cell growth and proliferation. This cell-covered scaffold is then implanted into the patient at the site where new tissue is required, and as the new cells continue to grow and divide, the material making up the scaffold begins to degrade, or in some cases, to be absorbed. Such tissue engineering techniques are being used to develop a wide range of tissues, including bone, skin, cartilage and adipose tissue