Development of floor plate, neurons and axonal outgrowth pattern in the early spinal cord of the notochord-deficient chick embryo

Abstract

The notochord is probably involved in the development of the neural tube. In this study, a fragment of caudal notochord was extirpated in ovo from chick embryos at 1.5 days of incubation. At 4.5 days a distinct notochord-deficient region at thoracolumbar level was found. Profound effects were seen, especially at the cranial site of this region. Somites were smaller than normal, or even not recognizable, and in some cases the myotomes were fused in the midline. The spinal cord appeared reduced in size and lacked a floor plate. The average amount of spinal cord neurons was 23% of the normal value, the cells being located circularly along the outer margin of the spinal cord, except for the roof plate. Axonal roots left the cord in the ventral midline only. Caudal to this site, neurons or floor plate cells were alternately present in the ventral spinal cord, and axonal roots left bilaterally. In a caudal direction, a normal morphology gradually reappeared. The possibility is discussed that reduction in spinal cord size and amount of neurons is a direct or indirect effect of the absence of the notochord, and that the sclerotome may be involved. The results further substantiate the idea that the notochord is a prerequisite for full development of the floor plate. They indicate that complete induction of the floor plate in this region has to occur between 1.5 and 2.0 days, which indicates that this process is critical both in time and duration. The results provide evidence that median neuroepithelial cells resemble lateral neural tube cells, in that both can develop into neurons, and both are sensitive to the influence of the notochord in differentiating into a floor plate.