Closure of the neural tube in the cephalic region of the mouse embryo

In mouse embryos varying in age from 9 to 20 somites the first closure of the neural groove was found to occur in the cervical region. The fusion process gradually proceedéd in rhombencephalic direction until it reached a level just caudal to the otic pits. Shortly afterwards the prosencephalic walls fused together independent of the rhombencephalic closure. This prosencephalic fusion process proceeded caudally in the direction of the mesencephalon until it reached the rostral portion of the rhombencephalon. In this region the two independent fusion processes met each other. In addition the prosencephalic fusion proceeded in rostral direction toward the anterior neuropore, which was the last part of the brain vesicles to close. Hence, the closure of the brain vesicles is not a zipper‐like process proceeding from the rhombencephalon to the anterior neuropore, but occurs at several places at the same time and proceeds in a rostral as well as in a caudal direction. At the cellular level considerable differences in the fusion process were found to exist between the various brain vesicles. In the rhombencephalon the first bridge between the two opposing walls was formed by surface ectoderm and neural crest cells. In the mesencephalon single squamous ectoderm and a few neuroepithelial cells established the first contact, whereas in the prosencephalon the apical ends of several neuroepithelial cells fused together to overbridge the gap between the opposing walls. The surface ectoderm cells subsequently covered the neuroepithelial bridge. In the region of the anterior neuropore the fusion was similar to that between the prosencephalic walls, the only difference being that in the anterior neuropore area many more darkly stained particles indicating cell degeneration, were present than in the prosencephalon. It is thus concluded that considerable differences exist in the fusion of the neural walls between the various brain vesicles.