Cerebral dysraphia (future anencephaly) in a human twin embryo at stage 13.

Cerebral dysraphia was studied histologically and by graphic reconstruction in a twin at stage 13, and comparisons were made with the normal (discordant) twin. The normal, bidirectional closure of the rostral neuropore was investigated in several embryos, from which it was concluded that the situs neuroporicus is represented by the future commissural plate rather than by the (adult) lamina terminalis. In the abnormal twin the neural tube was open over part of the midbrain and forebrain, although the situs neuroporicus was closed. The experimental production of anencephaly by Giroud and co-workers was reviewed, and comparisons between embryonic staging systems in the rat, mouse, and human were made. Three corresponding phases are found in the human: 1) cerebral dysraphia, occurring before or during Carnegie stage 11 (approximately 23-25 days); 2) exposure of a highly developing and well-differentiated brain during the remainder of the embryonic period; and 3) degeneration of the exposed brain throughout the fetal period, resulting in anencephaly. Hence the abnormal twin described here is believed to represent a precursor of typical anencephaly, and is the earliest example of purely cerebral dysraphia so far recorded.