METHANOL-INDUCED NEURAL-TUBE DEFECTS IN MICE - PATHOGENESIS DURING NEURULATION

A spectrum of cephalic neural tube defects was observed in near-term ( gestation day [GD] 17) mouse fetuses following maternal inhalation of methanol at a high concentration (15,000 ppm) for 6 hr/day during neur ulation (GD 7-9). Dysraphism, chiefly exencephaly, occurred in 15% of fetuses, usually in association with reduction or absence of multiple bones in the craniofacial skeleton and ocular anomalies (prematurely o pen eyelids, cataracts, retinal folds). Measurements of cerebrocortica l width in grossly normal, methanol-exposed fetuses revealed significa nt semiquantitative differences in the thicknesses of the frontal cort ex and its constituent layers (neuroepithelium, intermediate cortex/su bventricular plate, and cortical layer 1) as well as apparent increase s in subventricular plate cellularity relative to controls. Subsequent ly, the early morphogenesis of these neural changes was investigated i n neurulating mouse embryos to define tissue-specific patterns of meth anol-induced damage that lead to cephalic axial dysraphism. Following daily 6-hr maternal inhalations of 15,000 ppm methanol during GD 7-8, the cephalic neural fold margins were swollen, blunted, and poorly ele vated on GD 8.5 and 9 relative to controls. Histopathology of exposed GD 8.5 embryos revealed microcephaly in association with reductions in the cell density and mitotic index of at least 47% in the cranial mes oderm. The mitotic index in the embryonic neuroepithelium was also red uced by 55%, and groups of neural crest cells were displaced to the ne ural folds dorsal to the foregut (relative to the more ventral locatio n in the facial regions of control embryos). When examined on GD 9.5 a nd 10.5, maternal methanol exposure (15,000 ppm for 6 hr/day) during G D 7-9 resulted in stunting, delayed rotation, and microcephaly in over 90% of the affected embryos. Persistent patency of the anterior neuro pore and prosencephalic hypoplasia were seen in >40% and vp to 90% of embryos, respectively. Shallow optic vesicles, stunted branchial arche s, scoliosis, and hydropericardium were also observed. Many 10.5-day-o ld embryos were edematous. Occult dysraphism, recognized grossly by ab normally narrow cephalic conformation and histopathologically by the a bsence of mesoderm in the mesencephalon, was present in at least 21% o f methanol-exposed embryos on GD 9.5 and 10.5. Nile blue vital dye sta ining of methanol-exposed embryos revealed no difference in dye accumu lation between control and treated embryos on GD 8.5, 9.0, or 9.5. The re were no apparent dysmorphogenic effects in control embryos at any s tage of development. These data suggest 1) that exposure to a high con centration of methanol injures multiple stem cell populations in the n eurulating mouse embryo and 2) that significant neural pathology may r emain in older conceptuses even in the absence of gross lesions. (C) 1 994 Wiley-Liss, Inc.