Modified cryopreservation and xenotransplantation of human parathyroid tissue

A modified cryopreservation technique for human parathyroid tissue was compared with the standard method using a programmed freezer.

Total parathyroidectomy was performed in three groups of 6-week-old Rowett nude rats. Group I (control) underwent no transplantation of parathyroid tissue (n=9). After 10 days, the rats of groups II (n=15) and III (n=15) underwent xenotransplantation of 20 mg cryopreserved human parathyroid tissue, which had been stored in liquid nitrogen at -196 degrees C for 1-22 months prior to xenotransplantation. The parathyroid tissue was derived from 15 parathyroidectomized patients with renal hyperparathyroidism. Two tissue samples were obtained from every patient. One sample from every patient was cryopreserved by means of the technique of programmed cryopreservation: programmed freezing of human parathyroid tissue at a controlled rate of -1 degrees C/min to -80 degrees C prior to transfer to liquid nitrogen (group II). The second sample from every patient was cryopreserved by means of a modified cryopreservation technique: immediate placement of human parathyroid tissue in a freezer at -20 degrees C and transfer to liquid nitrogen after 2 h (group III). Calcium and intact parathyroid hormone concentrations in serum were determined over a period of 60 days. Furthermore, individual differences in the calcium concentrations were assessed for each rat on the basis of the calcium levels recorded preoperatively and at day 60.

All animals in the control group developed hypocalcemia. Serum calcium concentrations returned to normal levels 60 days after xenotransplantation of parathyroid tissue, which had been cryopreserved using the modified technique (group III) in 12 of 15 rats (80%). At day 60, serum calcium had normalized in 10 of 15 rats (67%) after xenotransplantation of parathyroid tissue cryopreserved using programmed freezing (group II). After modified cryopreservation (group III), the median individual difference in the calcium concentrations was -0.16 mmol/l after programmed freezing (group II). At the end of the study, a median level of human intact parathyroid hormone of 3.5 pg/ml and 2.4 pg/ml was estimated for groups II and III, respectively. There was no statistical significant difference of the individual differences in the calcium concentrations or of the levels of human intact parathyroid hormone between groups II and III.

Human parathyroid tissue was successfully xenografted in the present experimental study. The results obtained after cryopreservation using the described modified technique were equivalent to those recorded after controlled freezing in a programmed freezer. The simplified cryopreservation technique therefore appears to be suitable for human parathyroid tissue.