Abstract: OBJECTIVE： To evaluate developmental toxicity of cerium nitrate with in vitro postimplantation whole embryo cultures (representing early，middle and late embryonic developments) using a micromass test. METHODS： Embryos of GD 9.5 SD rats were cultured at 37 ℃ for 48 h in immediately centrifuged serum which contained different doses of cerium nitrate (0.00， 0.50， 0.75 and 1.00 mmol/L). Growth，development and total morphological scores (TMS) of the embryos were documented according to the Brown's Method. Results of BALB/c 3T3 cytotoxicity and of the early embryonic development toxicity of cerium nitrate were evaluated using the whole embryo prediction model by ECVAM. In addition， limb bud cells of embryos were isolated from GD13 SD rats and cultured in micro-mass culture for 5 days with 0.03，0.06， 0.13， 0.25， 0.50， 1.00， 2.00 and 3.00 mmol/L cerium nitrate. The 50% inhibitions of cell viability and growth (IC₅₀) of limb bud cells were determined by neutral red staining and the 50% inhibition of cells differentiation (ID₅₀) was measured by alcian blue staining. The embryonic toxicities of cerium nitrate in the middle and advanced stages of embryos were evaluated with the prediction model of the micro-mass test (ECVAM). RESULTS： In the in vitro post-implantation whole embryo culture test，the no-observed-adverseeffect level (NOAEL) of cerium nitrate on embryo growth was 0.50 mmol/L. Cerium nitrate above 0.75 mmol/L inhibited embryonic growth， and reduced the yolk sac diameter (YSD) and crown-rump length (CRL) significantly (P < 0.05). Cerium nitrate at 0.75 mmol/L also reduced the number of somites (P < 0.05) which indicated developmental malformation of embryos. In the micro- mass test， the NOAEL of cerium nitrate on proliferation of limb buds was 0.25 mmol/L，the NOAEL on differentiation from limb bud cells to chondrocytes was 0.12 mmol/L， the IC50 and ID50 for limb bud cells were 1.23 and 0.76 mmol/L， respectively. CONCLUSION： Based on the prediction model of whole embryo cultures and the micro- mass test， our results indicate that cerium nitrate was a weak embryotoxic chemical.

Key words: cerium nitrate, whole embryo culture, micromass test, developmental toxicity