STEREOSPECIFIC IN-VITRO EMBRYOTOXICITY OF L-HOMOCYSTEINE IN PREIMPLANTATION AND POSTIMPLANTATION RODENT EMBRYOS

Recently a derangement of homocysteine metabolism has been suggested a s a possible risk factor for neural tube defects and recurrent spontan eous abortion. To investigate a possible role of homocysteine in the a etiology of neural tube defects we tested the in vitro embryotoxicity of L-homocysteine by culturing day 10 post coitum post-implantation ra t embryos in whole embryo culture (WEC) for 24 hr and day 2 post coitu m pre-implantation mouse embryos for 48 hr. With an area under curve ( AUC) of 6.3 mM/hr, L-homocysteine significantly reduced the percentage of mouse embryos that developed into blastocysts. In rat WEC, an AUC for L-homocysteine of 3.6 mM/hr reduced the mitotic index of the neura l epithelium of the rhombencephalon and the cell density of the mesenc hyme adjacent to it, while at an AUC of 7.2 mM/hr L-homocysteine reduc ed the total morphological score and the number of malformations was i ncreased. Malformations most often seen were transparent rhombencephal on, no or delayed formation of forelimb buds, dysmorphogenesis of the somites, and blister formation dorso-laterally of the place of forelim b bud formation. The embryotoxicity of L-homocysteine was stereospecif ic since D-homocysteine caused no embryotoxic effects. Also the oxidat ion product L-homocystine (AUC, 72 mM/hr) and the metabolite L-methion ine (AUC, 144 mM/hr) were not embryotoxic. Both stereoisomers of homoc ysteinethiolactone were embryotoxic at an AUC of 72mM/hr. The results are discussed in relation to the metabolism of homocysteine and methio nine and their possible role in the neurulation process.