UREA IS NECESSARY FOR THE CULTURE OF EMBRYOS OF THE MARSUPIAL FROG GASTROTHECA-RIOBAMBAE, AND IS TOLERATED BY EMBRYOS OF THE AQUATIC FROG XENOPUS-LAEVIS

Urea was found in the capsular fluid that bathes Gastrotheca riobambae embryos during incubation in the maternal pouch. The urea concentrati on in this fluid is higher than in blood from the mother, indicating t hat urea is accumulated by the embryo during the period of maternal in cubation. Gastrotheca tadpoles tolerate up to 500 mM urea with 86% sur vival after 24 hours and die in solutions of 0.5 mM ammonia. These fin dings suggest that urea plays a role in the adaptation of G. riobambae embryos to the conditions of water stress within the maternal pouch. To improve the in vitro culture conditions of early embryos taken from the maternal pouch, a saline solution that contains urea was designed (GRS). GRS plus 30 mM urea was used for the culture of cleavage to th e neurula stage embryos of G. riobambae. During organogenesis, the ure a concentration was raised to 60 mM. Early embryos of Xenopus laevis t olerate urea, and in addition, no inducing effects of urea have been d etected in animal cap explants of Xenopus.