Adverse reproductive and developmental effects in Xenopus from insufficient boron

Frog embryo teratogenesis assay--Xenopus (FETAX) was utilized as a model system to evaluate the effects on embryo-larval development at various low boron (B) exposure levels in the culture media. Concentrations tested ranged from < 1 to 5000 microg B/L. A statistically significant (P < 0.05) increase in malformations was observed at < or = 3 microg B/L, but not at the greater concentrations. Abnormal development of the gut, craniofacial region and eye, visceral edema, and kinking of the tail musculature (abnormal myotome development) and notochord were observed. In subsequent studies, adult frogs were maintained for 28 d on two diets: (1) low B (LB, 62 microg B/kg) or (2) boric acid supplemented (BA, 1851 microg B/kg); the frogs were subsequently mated, and their offspring were cultured in media containing various levels of B. Results of the 28-d depletion studies indicated that frogs maintained under LB conditions produced a greater proportion of (1) necrotic eggs and (2) fertilized embryos, which abnormally gastrulated at a greater rate and were substantially less viable than embryos from frogs fed the BA diet. Malformations similar to those seen in the initial study were observed in embryos from the B-depleted adults maintained in an LB environment; 28 d on the LB diet enhanced the incidence of malformations associated with the LB culture media. These abnormalities were not observed in embryos cultured in > or = 4 microg B/L from adults cultured on the BA diet. These studies showed that insufficient B reproducibly interfered with normal Xenopus laevis development during organogenesis, substantially impaired normal reproductive function in adult frogs, and thus represent the first studies demonstrating the nutritional essentiality of B in an amphibian species.