Neural tube closure in Xenopus

Neurulation is the process by which progenitors of the central nervous system are shaped, separated from and brought beneath the epidermis. The cellular basis of these movements have been investigated in amphibians (Jacobson, 1981; Jacobson and Gordon, 1976; Keller et al., 1992b), chicken (Schoenwolf and Smith, 1990b; Smith and Schoenwolf, 1997) and mice (Bush et al., 1990; Sausedo and Schoenwolf, 1994; Smith et al., 1994) but remains poorly understood. In the chick, which has been used extensively as a paradigm for vertebrate neurulation, a broad neural plate folds and the margins of the plate are raised and brought into apposition at the neural folds as cells take on stereotypical shapes. Analysis of serial sections and electron microscopy have identified cell movements and shape changes accompanying neural tube formation in chick (Schoenwolf and Alvarez, 1989; Schoenwolf and Smith, 1990a) where these cell behaviors (apical contraction and interkinetic nuclear migration) result in the formation of a ‘medial hinge’ overlying the notochord and a ‘dorsal lateral hinge’ near the prospective sulcus limitans, that together bring the neural folds into opposition. The epidermis then fuses, the neural ectoderm fuses and the neural crest is released from neural epithelium of the newly formed neural tube. In this manner, it is thought that the flat neural plate rolls into a tube with the lumenal face of the neural tube forming from the apical face of the neural plate. However, additional processes appear to be involved in chick neurulation. By following the rapid events at the start of neurulation, van Straaten and coworkers (1996) found that the lateral face of the two neural folds ‘zip’ into apposition, starting near the floorplate and proceeding dorsally. They found that the lumen of the neural tube nearly disappears after apposition and then re-opens to form the lumen after the neural 4547 Development 126, 4547-4556 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV6419