Prof Anthony Hollander, ARC Professor Of Rheumatology And Tissue Engineering University of Bristol, said:

"In the original tracheal tissue engineering approach we grew stem cells in a laboratory and seeded them back onto decellularised donor trachea before implanting. The advantage of this approach is that we had a lot of control over the type and number of cells being implanted. The new method used by Paolo Macchiarini involves the use of bone marrow cells that are minimally manipulated in the operating theatre before being seeded onto the scaffold material that is then immediately implanted.

"The advantage of the new approach is that it can be performed quickly and cheaply and so if successful it could be made available to large numbers of patients at relatively low cost.

"The disadvantage is that there is minimal control of the type of stem cells (bone marrow is a mixture of cells) and the number of cells. Also there is a very short time between seeding the cells onto the scaffold and the time of implantation (48 hours in the original method; a matter of minutes in the new method). For these reasons the new technique is inherently more unpredictable.

"The wider implications:

1. If it works and there is a reasonably consistent outcome in most patients then there is a real chance of saving lives at rather low cost for those patients with airways disease
2. However this method should not be extrapolated to other organs which are thicker/larger and more complex in terms of the different types of cells that have to be reseeded."