Tissue transplantation: procedure, prognosis

Skin allografts are used in patients with extensive burns and other conditions involving massive loss of skin. Allografts are used to cover large areas of damage, thereby reducing fluid and protein loss and preventing invasive infections. All allografts are eventually rejected, but the exposed areas produce well-vascularized granulation tissues that accept autografts from the patient's own healed skin. Skin cells can be grown in culture and then returned to the burned patient to cover large burns, or artificial skin created from cell culture on a synthetic scaffold can be used. Split-thickness skin grafts are used to promote healing of small wounds. A small piece of tissue a few millimeters thick is specially treated and the donor skin is positioned at the transplant site.

Cartilage transplantation is used in children with congenital defects of the nose or ears and in adults with severe joint damage or destruction (eg, severe osteoarthritis). Chondrocytes are more resistant to rejection, possibly because the small population of cells in hyaline cartilage is protected from immune cell attack by the cartilaginous matrix.

Bone grafting is used to reconstruct large bone defects (eg, after massive resection for bone tumors). Viable donor bone cells do not survive in the recipient, but the dead matrix of the allograft can stimulate recipient osteoblasts to recolonize the matrix and form new bone. The matrix acts as a scaffold for connecting and stabilizing the defects until new bone is formed. Cadaveric allografts are frozen to reduce the immunogenicity of the bone (which is dead by the time of implantation) and glycerolized to maintain chondrocyte viability. No immunosuppressive therapy is used after implantation. Although patients develop anti-HLA antibodies, early observations do not reveal cartilage degradation.

Autografts of adrenal medulla are stereotactically placed within the CNS, which has been reported to alleviate symptoms of Parkinson's disease. Allografts of adrenal tissue, especially from fetal donors, have also been proposed. Fetal ventral midbrain (mesencephalon) tissue has been reported to be stereotactically implanted into the putamen of patients with Parkinson's disease to reduce rigidity and bradykinesia. However, due to the ethical and political debate regarding the use of human fetal tissue, it seems unlikely that sufficiently large controlled trials will be conducted to adequately evaluate fetal neural tissue transplantation. Xenografts of endocrinologically active cells from pig donors are currently being tested.

Fetal thymus implants from stillborn infants can restore immunological responsiveness in children with thymic aplasia and the consequences of abnormal lymphoid development. Since recipients are immunologically non-responsive, immunosuppressive therapy is not required, but severe graft-versus-host disease may develop.