Observations on closure of the neuropores in the chick embryo.

Neuropore closure was studied in chick embryos by light and electron microscopy. Surface ectoderm reflects over the crests of the neural folds at all craniocaudal levels, merging with the neural ectoderm lining the neural groove. Apices of surface ectodermal cells have an essentially identical morphology prior to approximation of folds, both within the presumptive fusion sites and more laterally. Cells of these areas have slightly convex profiles exhibiting few cellular protrusions. Each neural fold contains a superficial half, composed of neural ectoderm covered by surface ectoderm, and a deep half consisting entirely of neural ectoderm. Initial contact between folds usually occurs near the junction between these halves in cranial regions, but is restricted primarily to surface ectoderm at caudal levels. Subsequent fusion of folds at all levels involves both ectodermal layers. Cellular protrusions and small, morphologically unspecialized intercellular junctions often interconnect cells of apposed folds in areas undergoing fusion. The anterior neuropore closes at stages 10-11, but fusion of folds in this region is not completed until stages 13-14. Fusion occurs dorsoventrally in this area and is more advanced internally than externally. Numerous pleomorphic inclusions and a few apparently necrotic cells are present in areas bordering the anterior neuropore. The posterior neuropore closes at stages 12-13 and fusion is completed in this region during stages 13-14. The caudal end of the posterior neuropore closes dorsal to the developing tail bud. Several morphological features of this closure may at least partially account for the high susceptibility to myeloschisis localized specifically at caudal spinal cord levels.