Neural fold fusion in the cranial region of the chick embryo.

Cranial neural fold fusion in the chick embryo is known to commence in the midbrain region before progressing cranially and caudally to involve the fore- and hindbrain regions, respectively. The two epithelial layers at the tips of the neural folds that participate in fusion are the surface ectoderm and the neuroepithelium. We have examined and compared cranial neural fold fusion in both layers, and our results show that fusion of the neuroepithelial component of the neural folds, unlike that of the surface ectoderm, starts in the caudal portion of the forebrain. Second, contrary to the widely accepted opinion, we have demonstrated that in the hindbrain region, fusion of the neuroepithelial component of the neural folds does not occur. Soon after neural fold apposition, a neuroepithelial eminence appears in rhombomeres 1 and 2, and this, together with other neuroepithelial cells in the dorsal midline of the hindbrain, undergoes massive apoptosis. The absence of neuroepithelial fusion in the hindbrain may be due to the presence of massive apoptosis among neuroepithelial cells that should have participated in the fusion process. The events described above may predispose the hindbrain to the development of neural tube defects. The appearance of cranial neural crest cells in the midline during their migration may enhance the fusion of the surface ectodermal portion of the neural folds.