Large quantities of Schistosoma mansoni schistosomules were cryopreserved in 35% ethanediol, and attempts were made to improve their morphology and infectivity after thawing. Several variables affected the appearance of the thawed organisms. These included: 1) the particular in vitro method used to transform cercariae to schistosomules; 2) addition of serotonin to the thawing medium; 3) changing the thawing temperature; and 4) culturing schistosomules for extended post-thaw times before assessing their condition. Varying either the postemergence age of the cercariae before transformation or the concentration of organisms in the freezing suspension had no effect on their survival. Although damage to many of the thawed schistosomules became apparent only upon extended in vitro cultivation, the addition of fetal calf serum to the thawing medium usually retarded this deterioration. When injected into mice, approximately 5% of cryopreserved schistosomules matured to adult worms, representing 71% of the value for maturation of unfrozen schistosomules. These studies define conditions of importance for the successful preservation of very large quantities of schistosomules (up to 500,000 in 1 ml volume) in liquid nitrogen.
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