Separation of neural and surface ectoderm after closure of the rostral neuropore.

Separation of neural and surface ectoderm after closure of the rostral neuropore in the head region has been described by investigating the integrity of the basement membranes of these epithelia in 11- to 27-somite rat embryos. The basement membranes were visualized with polyclonal antibodies against laminin. Furthermore, cell degeneration has been investigated in relation to neural crest activity, and discontinuities of the basement membrane in 9- to 30-somite mouse embryos. The separation of the basement membranes of neural and surface ectoderm in the midline is a final phase during the fusion of the neural folds, which takes place from the closure of the rostral neuropore, at the 19-somite stage, until the 27-somite stage (rat embryos), and which occurs focally with variation in the midsagittal and the transverse planes. In the prosencephalon, neural crest activity is absent during the separation phase of both epithelia, but cell degeneration may contribute to the separation of the initially connected basement membranes. A disturbance in the separation of the neural and surface ectoderm may be the pathogenetic basis of midline skull defects, and of the fronto-ethmoidal encephalocele in particular.