INITIAL CLOSURE OF THE MESENCEPHALIC NEURAL GROOVE IN THE CHICK-EMBRYO INVOLVES A RELEASING ZIPPING-UP MECHANISM

According to a traditional viewpoint, initial closure of the anterior neural groove involves bilateral elevation of the edges of the neural plate, flattening of the midline area, subsequent convergence of the d orsal neural folds, and finally adhesion and fusion of the medial fold edges, In a transverse view, the shape of the neural groove thereby c hanges from V > U > toppled C > O. This sequence implicates that the n eural groove is wide almost from its inception, In the present study, a new mechanism of initial closure is proposed, based on observations in living chick embryos and on light and scanning electron microscopic observations during neurulation in the presumptive mesencephalic regi on, The medial part of the neural plate invaginates in ventral directi on, The walls of the arising neural groove appose, beginning in the de pth, and make subsequent contact. During continued invagination the ne ural walls extend in ventral direction, the apposition/contact zone sh ifts in dorsal direction up to the neural folds and the neural walls s eparate ventrally, resulting in the incipient neural tube lumen. The m echanism is best compared with a zipping-up releasing model, In a tran sverse view, the shape of the neural groove changes from V > Y > I > O , While, according to the traditional view, the neural folds have to c onverge from a distance in order to contact each other, in the present mechanism the walls and folds are sequentially in contact by the vent ro-dorsal zipping-up mechanism, thereby avoiding the possibility of mi smatch of the neural folds. The above process is initiated over a cons iderable longitudinal distance along the neural plate, but only at the mesencephalic level does the dorsal shift of the contact zone become complete, At other levels of the neuraxis, the contact zone releases p rematurely and the neural walls become widely separated well before th eir dorsal neural folds are in contact. These folds have to converge, therefore, in order to close, but their matching is facilitated by the alignment of the previously contacted neural folds at the mesencephal ic level as well as by guidance underneath the vitelline membrane. (C) 1997 Wiley-Liss, Inc.