Embryonic stem cells injected into the mouse knee joint form teratomas and subsequently destroy the joint

To determine whether the joint space is a suitable environment for embryonic stem (ES) cells to grow and form cartilage.

We transplanted ES cells into the knee joint and a subcutaneous space of mice with severe combined immunodeficiency.

Teratomas formed in both areas. Those in the joints grew and destroyed the joints. The incidence of cartilage formation was the same in the knee joint and subcutaneous space, but the ratio of cartilage to teratoma was higher in the knee joint than in the subcutaneous space. The teratomas were proved to have been derived from the transplanted ES cells by detection of the neomycin-resistance gene that had been transfected into the ES cells.

It is currently not possible to use ES cells to repair joint tissues. Further optimization of donor ES cells to differentiate as well as inhibit tumour growth may help to meet these challenges.