Natural wound formation: endodermal responses in experimental primary neural induction in the chick embryo.

Natural wound formation in experimental primary neural induction has been studied by SEM and in paraffin wax sections in embryos from 0 minutes to 10 hours of re-incubation. Stage 4 host and graft embryos were removed from hen's eggs and mounted as for New culture. Graft Hensen's nodes were transplanted into "pockets" created in the host area pellucida and re-incubated for up to 10 hours. Initially the cut edges of the graft establish contact with the host ectoderm layer. After 4 hours the cut edges of the graft move from the host ectoderm to the host endoderm layer. Several small openings form in the host endoderm over the graft tissue. By 6 hours, these openings join to form a single natural wound through which the underlying graft is exposed to the external environment. At 8 hours the graft forms a head-fold and neural folds are evident. During 8 to 10 hours of re-incubation the edges of the graft which attach to the edges of the host endoderm meet in the midline and close the opening in the host endoderm; simultaneously, the graft forms a neural tube. The endodermal wounds form by cell re-arrangement and by a minor contribution from cell loss.