Brain expansion in the chick embryo initiated by experimentally produced occlusion of the spinal neurocoel

The brain expands in the early chick embryo from pressure generated by accumulation of cerebrospinal fluid (CSF) in a closed neural tube. The sealing of the neural tube occurs as the result of occlusion of the spinal neurocoel rostral to and before closure of the posterior neuropore. We have previously demonstrated the dependence of normal brain expansion upon intraluminal pressure. We had yet to demonstrate, however, that brain expansion actually depends upon natural occlusion of the spinal neurocoel. To demonstrate such dependence, we experimentally occluded the spinal neurocoels of embryos 5 hr younger than stage 11 embryos (in which occlusion of the neurocoel occurs naturally). The stage 10 chick embryos were cultured ex ovo and critically staged, and their spinal neurocoels were occluded using microcautery. All embryos were photographed immediately and at 5, 12, and 24 hr after cautery. Serial sections were made of selected embryos, in which the areas of both the brain and the head were measured. Wilcoxon-Mann-Whitney rank-sum nonparametric tests, Hodges-Lehmann estimators, bootstrapping techniques, and resampling randomization tests were used to determine whether the increases in the brain and head areas for the experimental embryos were significantly different from those of the control embryos during three distinct intervals of expansion: 0-5, 5-12, and 0-12 hr. From 0 to 5 hr, the brains of the precociously occluded embryos expanded significantly more than the brains of the non-occluded controls. From 5 to 12 hr, the brains of the embryos with naturally occluded neurocoels grew significantly larger than the brains of the embryos with precociously occluded neurocoels. At 12 hr, there appeared to be no difference in brain size for these two groups. We conclude that the data support the hypothesis that brain expansion is directly dependent upon occlusion of the spinal neurocoel.