Evolution and Characteristics of Schistosoma mansoni Eggs Laid In vitro

Adult pairs of Schistosoma mansoni were maintained in vitro in media consisting either of 90 parts of Medium 199 and 10 parts of calf serum, or 80% Eagle's Minimal Essential Medium and 20% calf serum. In both media, some pairs oviposited on the day of in vitro isolation, and 70% were laying eggs after 48 hr cultivation. When worms were cultured continuously in Medium No. 199, the average number of eggs laid per female per day during the first 5 days of in vitro maintenance, was 24.3 ? 4.0 (standard error of the mean) which was only 8% of the rate of the same strain in mice. Oviposition occurred only during the first 10 days of in vitro life and 85% of the eggs were laid over the first 5 days. Changing Medium 199 daily did not significantly alter these parameters, nor did maintenance in Eagle's medium, when this medium was changed daily. However, if Eagle's medium was not changed, the egg-laying rate was reduced to 5.7 eggs per female per day on days 1 through 5 of in vitro life. Very few eggs matured in Eagle's medium under either of the conditions used. The eggs which did mature required 7 days to reach the miracidial stage. A much higher per cent of eggs matured in Medium 199 and the maturation period was 5 days, one day shorter than the in vivo rate. From differences in rates of oviposition and per cents of egg maturation under the three conditions used, it was concluded that the nutritional requirements of oviposition are different from those required for embryonic development. Eggs which matured in vitro sometimes hatched abnormally. The range in the per cents of eggs maturing in vitro in Medium 199: 16.5 to 36.6%, was about the same as reported in vivo. Some immature eggs which died in vitro did not immediately exhibit signs of death which are commonly found in vivo. It was therefore concluded that these differences in appearance of dead eggs are due to the environment of the host. Spine to spine clumping of eggs was noted in these cultures. Examination of the spine by electron microscopy showed no recognizable structure for holding at the tip of the spine. Schistosoma mansoni has been cultured in vitro in different media (Clegg, 1965; Lee and Chu, 1935; Newsome, 1962; Robinson, 1960; Ross and Bueding, 1950; Senft and Senft, 1962), and sometimes eggs have been laid (Newsome, 1962; Robinson, 1960; Senft and Senft, 1962) in the in vitro environment. Although eggs have matured in vitro to the miracidial stage (Newsome, 1962), their rate of maturation has not been reported nor have the types of eggs laid been described. These experiments were designed to study in vitro the rates of oviposition and embryonic development, the types of eggs found, and the effect of different media on these parameters. MATERIALS AND METHODS