Influence of rat placental lactogen-I on the development of whole rat embryos in culture

Rat placental lactogen-I (rPL-I), the first prolactin-like hormone expresse d in the placenta during pregnancy in the rat, is known to influence matern al functions. In the present study, we have investigated the effects of rPL -I on the growth and development of cultured whole rat embryos. Rat embryos , with or without ectoplacental cone (EPC) attached, were explanted at day 9 of gestation. After 48 h of culture, the embryos, enclosed by the yolk sa cs, were assessed by the presence of visible heart contractions ('heart bea ts'), crown-rump length (CRL) and yolk sac diameter (YSD). When intact embr yos with EPC were cultured, the concentrations of rPL-I and rPL-II (product s of EPC) in the medium were 850 +/- 841 and 92 +/- 181 ng/ml respectively (means +/- S.E.M.). In embryo cultures with the EPC removed, rPL-I levels d ecreased to less than or equal to 10 ng/ml, and only 70% of the embryos wer e viable, with visible heart beats. In the viable embryos, both CRL and emb ryonic DNA synthesis were reduced compared with controls, and the addition of rPL-I (1 mu g/ml) did not prevent this reduction. YSD and yolk sac DNA s ynthesis were also reduced compared with control embryos, and the addition of rPL-I significantly prevented this decrease by 45%. In embryos cultured with out EPC in the presence of neutralizing rabbit anti-rat prolactin seru m (anti-rPRL), embryonic and yolk sac DNA synthesis were reduced by 35% com pared with embryos exposed to normal rabbit serum. Addition of rPL-I signif icantly increased (P<0.05) embryonic and yolk sac growth. Thus the effects of rPL-I on embryo growth could only be seen in the absence of prolactin. T he addition of human prolactin in the presence of anti-rPRL also resulted i n significant increases (P<0.05) in embryonic DNA synthesis and CRL. These results suggest that rPL-I may substitute for prolactin to influence the gr owth of the rat embryo.