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The role of a notochord fragment on the origin of an additional floor plate area in the neural tube is investigated by quantitative morphological methods. In 1.5 to 2 day chick embryos a notochordal fragment was implanted in close apposition to the lateral wall of the neural groove in the region between prospective wing and leg bud. At 4 days, adjacent to the implant a distinct area of the neural wall was present, which resembled the natural floor plate with respect to its thickness, the abluminal location of elongated nuclei and the absence of neuroblasts. The mitotic density of this area was reduced. This "additional floor plate" was distinct when the experiment was performed at 1.5 days but was hardly recognizable when it was carried out at 2 days. From these results it is concluded that a) the notochord induces floor plate like structures and diminishes proliferation, and b) that the period of floor plate induction by the notochord is very restricted.

Cranial neural tube defects (NTDs) occur in mice carrying mutant alleles of many different genes, whereas isolated spinal NTDs (spina bifida) occur in fewer models, despite being common human birth defects. Spina bifida occurs at high frequency in the Axial defects (Axd) mouse mutant but the causative gene is not known. In the current study, the Axd mutation was mapped by linkage analysis. Within the critical genomic region, sequencing did not reveal a coding mutation whereas expression analysis demonstrated significant up-regulation of grainyhead-like 2 (Grhl2) in Axd mutant embryos. Expression of other candidate genes did not differ between genotypes. In order to test the hypothesis that over-expression of Grhl2 causes Axd NTDs, we performed a genetic cross to reduce Grhl2 function in Axd heterozygotes. Grhl2 loss of function mutant mice were generated and displayed both cranial and spinal NTDs. Compound heterozygotes carrying both loss (Grhl2 null) and putative gain of function (Axd) alleles exhibited normalization of spinal neural tube closure compared with Axd/+ littermates, which exhibit delayed closure. Grhl2 is expressed in the surface ectoderm and hindgut endoderm in the spinal region, overlapping with grainyhead-like 3 (Grhl3). Axd mutants display delayed eyelid closure, as reported in Grhl3 null embryos. Moreover, Axd mutant embryos exhibited increased ventral curvature of the spinal region and reduced proliferation in the hindgut, reminiscent of curly tail embryos, which carry a hypomorphic allele of Grhl3. Overall, our data suggest that defects in Axd mutant embryos result from over-expression of Grhl2.

Progression of neurulation in the chick embryo has not been well documented. To provide a detailed description, chick embryos were stained in ovo after the least manipulation possible to avoid distortion of the neural plate and folds. This allowed a morphological and morphometric description of the process of neurulation in relatively undisturbed chick embryos. Neurulation comprises several specific phases with distinct closure patterns and closure rates. The first closure event occurs, de novo, in the future mesencephalon at the 4–6 somite stage (sst 4–6). Soon afterwards, at sst 6–7, de novo closure is seen at the rhombocervical level in the form of multisite contacts of the neural folds. These contacts occur in register with the somites, suggesting that the somites may play a role in forcing elevation and apposition of the neural folds. The mesencephalic and rhombocervical closure events define an intervening rhombencephalic neuropore, which is present for a brief period before it closes. The remaining pear‐shaped posterior neuropore (PNP) narrows and displaces caudally, but its length remains constant in embryos with seven to ten somites, indicating that the caudal extension of the rhombocervical closure point and elongation of the caudal neural plate are keeping pace with each other. From sst 10 onward, the tapered cranial portion of the PNP closes fast in a zipper‐like manner, and, subsequently, the wide caudal portion of the PNP closes rapidly as a result of the parallel alignment of its folds, with numerous button‐like temporary contact points. A role for convergent extension in this closure event is suggested. The final remnant of the PNP closes at sst 18. Thus, as in mammals, chick neurulation involves multisite closure and probably results from several different development mechanisms at varying levels of the body axis. © 1996 Wiley‐Liss, Inc.

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.(ABSTRACT TRUNCATED AT 250 WORDS)