Histological examination of cryopreserved rat tracheal grafts

It has been reported that tracheal tissue treated by cryopreservation can be used for tracheal replacement in the absence of immunosuppressants. However, the mechanism of reduced antigenicity is unclear. We investigated this issue in cryopreserved rat trachea using detailed histologic evaluation. Rat tracheal segments were preserved in a cryopreservative solution at -85 degrees C. The epithelium of tracheal segments (n = 6 in each group) was subjected to light microscopic and scanning electron microscopic examination before freezing and after cryopreservation for 1 week, 1 month, 3 months, and 6 months. The expression of major histocompatibility complex II (MHC-II) was determined using frozen sections. Ultrastructure of dendritic cells (DCs) was observed by transmission electron microscopy. Tracheal epithelium was partially intact even after 6 months in cryopreservation, although cellularity decreased with time. MHC-II antigen expression was detected even at 6 months, although expression was lower than that measured on fresh tissue. Tracheal tissue DCs displayed dilatations of perinuclear cisterna and degeneration of vacuoles. Density of the mitochondrial matrix was increased. These results suggest that damage to the epithelium and DCs during cryopreservation and concomitant loss of MHC-II expression might explain the reduction of antigenicity.