Stereospecific in vitro embryotoxicity of l-homocysteine in pre- and post-implantation rodent embryos.

Recently a derangement of homocysteine metabolism has been suggested as a possible risk factor for neural tube defects and recurrent spontaneous abortion. To investigate a possible role of homocysteine in the aetiology of neural tube defects we tested the in vitro embryotoxicity of l-homocysteine by culturing day 10 post coitum post-implantation rat embryos in whole embryo culture (WEC) for 24 hr and day 2 post coitum 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 neural epithelium of the rhombencephalon and the cell density of the mesenchyme adjacent to it, while at an AUC of 7.2 mm/hr l-homocysteine reduced the total morphological score and the number of malformations was increased. Malformations most often seen were transparent rhombencephalon, no or delayed formation of forelimb buds, dysmorphogenesis of the somites, and blister formation dorso-laterally of the place of forelimb bud formation. The embryotoxicity of l-homocysteine was stereospecific since d-homocysteine caused no embryotoxic effects. Also the oxidation product l-homocystine (AUC, 72 mm/hr) and the metabolite l-methionine (AUC, 144 mm/hr) were not embryotoxic. Both stereoisomers of homocysteinethiolactone were embryotoxic at an AUC of 72 mm/hr. The results are discussed in relation to the metabolism of homocysteine and methionine and their possible role in the neurulation process.