The two sites of fusion of the neural folds and the two neuropores in the human embryo.

Background: Since reports on a pattern of multiple sites of fusion of the neural folds in the mouse appeared, it has been widely assumed that a similar pattern must be valid for the human. In the absence of embryological evidence, claims have been made that such a pattern can be discerned by classifying neural tube defects.

Methods: The neural folds and tube, as well as the neuropores, were reassessed in 98 human embryos of Stages 8-13; 61 were controlled by precise graphic reconstructions.

Results: Careful study of an extensive series of staged human embryos shows that two de novo sites of fusion of the neural folds appear in succession: alpha in the rhombencephalic region and beta in the prosencephalic region, adjacent to the chiasmatic plate. Fusion from Site alpha proceeds bidirectionally (rostrad and caudad), whereas that from beta is unidirectional (caudad only). The fusions terminate in neuropores, of which there are two: rostral and caudal. Highly variable accessory loci of fusion, without positional stability and of unknown frequency, may be encountered in Stage 10 but seemingly not later, and their existence has been known for more than half a century.

Conclusions: Two sites of fusion (a term preferred to closure) of the neural folds and two neuropores are found in the human embryo. No convincing embryological evidence of a pattern of multiple sites of fusion, such as has been described in the mouse, is available for the human. The construction of embryological details from information derived from other species or from the examination of later anomalies is liable to error. Neural tube defects are reviewed and although they have been considered on the basis of five, four, or three sites of fusion, interpretations based on two sites can as readily be envisaged.