Arsenic-induced exencephaly in the mouse and associated lesions occurring during neurulation.

Early tissue damage following a teratogenic dose of arsenic to the dam was studied in mice with the objective of detecting the primary lesion associated with the development of exencephaly. Animals were killed 6 to 21 h after a single 45 mg/kg intraperitoneal injection of sodium arsenate on day 8 of pregnancy and neurulation-stage embryos were fixed for histological and ultrastructural examination. In the prospective hindbrain, the most consistent feature associated with arsenate treatment was the widely separated neural folds which were not positioned for closure. Intracytoplasmic inclusions, interpreted as necrotic debris, were most numerous in the apical portion of the neural folds, sometimes extending into the mesenchyme, but they were not extensive in most embryos. In the prospective forebrain, necrotic debris was found throughout the neuroepithelium, in contrast to the posterior portions of the developing brain. It is not clear that necrosis of the neuroepithelium or mesenchyme would in itself be the primary lesion associated with exencephaly, although death of specific cells such as those participating in the fusion process could be involved. The potential effect of arsenate on physiological and biochemical processes which could affect neural tube closure is discussed.