Timing and positioning of occlusion of the spinal neurocele in the chick embryo

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Abstract
The timing and positioning of occlusion of the spinal neurocele were studied in living and serially sectioned chick embryos at stages 8–14. Occlusion occurs in three phases: preocclusion, incipient occlusion, and definitive occlusion. Preocclusion occurs at stage 8. At this time, the neural groove of the spinal cord has not yet closed to form a neural tube. Incipient occlusion begins as early as stage 9 and lasts until stage 11. The neural groove of the cranial spinal cord closes during these stages and incipient occlusion occurs concomitantly with this closure. Seventy-eight percent of the embryos exhibit incipient occlusion. Incipient occlusion extends along the mid-somitic region of the neuroaxis, occupying about one-half the length of the spinal cord. Injection of the brains of living embryos with dyes often reopens incipient occluded areas. Definitive occlusion occurs at stages 11–14 and is present in 89% of the embryos. Definitive occlusion is restricted to mid-somitic regions, as was incipient occlusion, but it extends approximately two-thirds the length of the spinal cord. Injection of the brains of living embryos with dyes rarely reopens definitive occluded areas, even when injection pressures are maximal. Six morphological types of definitive occlusion can be identified on the basis of the relative proportions and locations of total and partial occlusion. Closure of the cranial neuropore and roof plate of the hindbrian occurs near the end of the incipient occlusion phase, whereas closure of the caudal neuropore occurs well after definitive occlusion is initiated. Physiological occlusion is due to both anatomical occlusion and back pressure, which prevent the flow of liquids into a blind, narrow tube. Further studies are required to document the reopening phases of the spinal neurocele, as well as the forces involved in causing and maintaining occlusion and reopening.