Isolation of Neuroepithelium and Formation of Minispheres

The brain of the early chick embryo initially grows by expansion of its fluid-filled cavity much as a balloon expands (1 -3 ). Such expansion occurs rapidly over 3-4 d, beginning at around 43 h of development (1 ,3 ), and is under the direct control of internal fluid pressure, as shown from brain intubation experiments (1 ,4 -8 ). Intraluminal pressure is generated by the accumulation of fluid within a closed system and promotes expansion of the brain. The source of the fluid within the early neural tube still remains unknown, although Weiss (9 ) reported that the neuroepithelium is secretory as soon as it forms a tube. Weiss’s experiments are only suggestive, however, and need to be repeated using contemporary methods in order to establish for certain whether the neuroepithelium is secretory, and if it is, at what stage it first becomes so. In addition, no information exists about how fluid crosses the neuroepithelium during this early expansion period. Although several reports exist summarizing the secretory nature of the choroid plexus, the choroid plexus does not develop until much later in embryogenesis.