Identical embryopathogenesis for exencephaly and myeloschisis: an experimental study.

Extensive histological and immunohistochemical studies were performed to elucidate the histopathogenesis of exencephaly induced in chick embryo as an experimental model. The findings were compared with those identified in a chick myeloschisis experimental model and in human autopsy cases. The experimental model of exencephaly in chick embryos was developed by induction with various teratogens including ethylnitrosourea, salicylate, and phenytoin. None of the cases of exencephaly was exposed to a teratogen prior to or within Hamburger and Hamilton stage 12 (45 to 49 hours postincubation), when the anterior neuropore closes. The process of overgrowth in development of exencephaly was identical to that of myeloschisis, and the results suggested neuronal overmaturation in the histological and immunohistochemical studies. Although the late-stage degenerative change with neovascularization over the exposed neural tissue (placode) was more severe in human exencephaly, the present experimental study may suggest a possible common embryopathogenesis of dysraphism. Exencephaly should be regarded as the most severe form of cranium bifidum, as myeloschisis is in spina bifida.