Sequential observations of exencephaly and subsequent morphological changes by mouse exo utero development system: analysis of the mechanism of transformation from exencephaly to anencephaly.

Anencephaly has been suggested to develop from exencephaly; however, there is little direct experimental evidence to support this, and the mechanism of transformation remains unclear. We examined this theory using the exo utero development system that allows direct and sequential observations of mid- to late-gestation mouse embryos. We observed the exencephaly induced by 5-azacytidine at embryonic day 13.5 (E13.5), let the embryos develop exo utero until E18.5, and re-observed the same embryos at E18.5. We confirmed several cases of transformation from exencephaly to anencephaly. However, in many cases, the exencephalic brain tissue was preserved with more or less reduction during this period. To analyze the transformation patterns, we classified the exencephaly by size and shape of the exencephalic tissue into several types at E13.5 and E18.5. It was found that the transformation of exencephalic tissue was not simply size-dependent, and all cases of anencephaly at E18.5 resulted from embryos with a large amount of exencephalic tissue at E13.5. Microscopic observation showed the configuration of exencephaly at E13.5, frequent hemorrhaging and detachment of the neural plate from surface ectoderm in the exencephalic head at E15.5, and multiple modes of reduction in the exencephalic tissue at E18.5. From observations of the vasculature, altered distribution patterns of vessels were identified in the exencephalic head. These findings suggest that overgrowth of the exencephalic neural tissue causes the altered distribution patterns of vessels, subsequent peripheral circulatory failure and/or hemorrhaging in various parts of the exencephalic head, leading to the multiple modes of tissue reduction during transformation from exencephaly to anencephaly.