This is a long-term preservation technique that ensures that most of the cells'       functions remain intact. After processing, the tissue is plunged into a cryopreservant solution. To avoid hyperosmolarity shock, which would damage cells and jeopardise their vitality, tissues are first placed in polyethylene bags.     The nitrogen vapour storage tanks are kept at a constant temperature. Tissue   preserved in liquid nitrogen subsequently presents lower immunological responses. The presence of histocompatibility receptors of thawed tissue previously stored at -160⁰C is considerably reduced.