Variation in the response of chick embryos to incision of the roof plate of the neural tube at different developmental stages.

The effects of microsurgical reopening of the neural tube were examined in chick embryos of Stages 12-18. The roof plate of the thoracic neural tube was incised for a length equivalent to 7 somites. The site of incision was studied histologically and by SEM and TEM at intervals up to 48 hours. 48 hours after operation, persistent neural tube defects were more frequent and longer in embryos of more advanced stages at operation. Exposure of embryos to Streptomyces hyaluronidase, which inhibits neurulation in normal embryos, has no effect on the healing of the incised neural tube in the young embryos. Healing of the lesion in younger embryos appeared to occur in two stages: initially, by repair of the surface ectoderm, by a cephalo-caudal zipper-like mechanism, followed by a reconstitution of the roof plate by migration of neurectodermal cells on the deep surface of the ectoderm. Neural tubes of older embryos splay open more widely on incision of the roof plate, apparently making healing mechanically more difficult. This wider splaying may be related to the decline of forces which maintain occlusion of the neural canal in younger embryos.