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EC number: 200-315-5
CAS number: 57-13-6
Effects observed in Xenopus laevis were
considered to be primarily caused by the high osmolarity of the
Fertilized Xenopus eggs, whose jelly
layers and vitelline envelope have been removed, were cultured in
Steinberg's and GRS with and without the addition of urea. Up to stage
8, embryos were cultured with the addition of 30 mM urea. Afterwards,
the concentration of urea was raised to 50 mM and embryos were cultured
until stage37. Xenopus embryos acquired a characteristic U shape when
cultured in GRS with urea (Fig.2a). Similar abnormalities were observed
when embryos were cultured within the vitelline envelope in this
solution (data not shown). In contrast, embryos cultured in GRS without
urea developed normally (Fig.2b). Embryos cultured in Steinberg's
solution with or without urea developed normally (Figs.2c,d). The
altered morphology observed when Xenopus embryos were cultured in GRS
plus urea (Fig.2a) may be due to the high osmolarity of this solution.
It has been reported that urea may
influence the biological activity of proteinaceous inducing factors
(16). In addition, there exists the possibility that urea may interact
with extracellular matrix proteins or other proteins, including
receptors for inducing factors, integrated into the plasma membrane. By
change of the conformation or liberation of masked endogenous factors,
mesodermal and neural structures maybe induced. However, we could show
in the animal cap assay with Xenopus, that such an effect of urea can be
excluded under our experimental conditions. We used not only 50mM urea,
but raised the concentration to 250 mM urea in Steinberg's solution for
the treatment of animal cap explants. Treatments with urea, including
250 mM urea for 3.25hours did not have mesodermal or neural inducing
activity in animal explants of Xenopus.
The segmenting eggs and early embryos
of X.laevis are routinely cultured in solutions of low ionic strength
suchas0.1MR.WhenembryosofXeno-pusareplacedin full strength physiological
saline solutions, the epithelial layer is destabilized, resulting in
abnormal ingression of surface cells during gastrulation and in
The publication of del Pinto et al.
1994 state that "Until recently, embryos of Gastrotheca could be
cultured in vitro only within the embryonic capsule (9). Based on the
findings that urea occurs in the capsular fluid during incubation in the
maternal pouch, and that free-living tadpoles of Gastrotheca tolerate
urea, we designed a saline solution, that contains urea, which allows
successful culture of early embryos of this frog. In addition, we found
that urea is tolerated by embryos of X.laevis."
The 120 LC0 for Gastrotheca riobambae
was 18000 mg urea/L.
The NOEC for X. laevis (stage 8 to 37)
was >= 3000 mg urea/L.
The results are considered relevant
and reliable for the risk assessment.
In the study from Schuytema and Nebeker
(1999) the effect of urea on tadpoles of the Pacific treefrog,
Pseudacris regilla and the African clawed frog, Xenopus laevis was
investigated according to ASTM 1997.
In its pure form urea affects tadpoles only
at extremely high concentrations with LC50 for P. regilla of 17999 mg
urea/L and 19526 mg urea/L for X. laevis. Growth effects were observed
at 12,800 mg urea /L, the NOEC is 5145 mg urea/L for both species.
The results from this study are considered
relevant and reliable for the risk assessment.
Urea with concentrations up to 1200 mg urea/L had no effect on the mortality and the development of tadpoles of the Asian Toad Bufo gargarizans during a 7 day exposure (Zhao et al 2018). Some effects on e.g. the position of the eyes might be present (even if there might be a statistical significance, the results from the highest test concentration are within the range of the control animals.
Aquatic amphibians are not sensitive to urea
Aquatic amphibians are not sensitive to
Zhao et al. (2018) reported the effect of
urea on Asiatic toad (Bufo gargarizans) tadpoles as 7 -day LC0 tadpoles
>= 1200 mg/L (mortality) and the 7-day NOEC tadpoles >= 1200 mg/L
del Pinto et al 1994 provided an 120 h LC0
of 18000 mg urea/L for Gastrotheca riobambae and a NOEC of >= 3000 mg
urea/L Xenopus laevis (stage 8 to 37).
Schuytema and Nebeker 1999 determined the 10
day-LC50 for Pseudacris regilla 17999 mg urea/L and the 10-day LC50 for
Xenopus laevis as 19526 mg urea/L The NOEC is 5145 mg urea/L for both
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