The splotch (Sp) mouse mutant displays de-

A comparison was made of early face embryology in the inbred A/J mouse strain with a spontaneous frequency of about 12% cleft lip and the C57BL/6J strain which virtually never has cleft lip. From the time of their appearance the medial nasal, lateral nasal and maxillary processes are connected by an isthmus. Subsequently there is fusion between the adjacent epithelia of medial and lateral nasal processes. Failure of this fusion, followed by breakdown of the isthmus, results in cleft lip. Just before and when the adjacent epithelia of medial nasal and lateral nasal processes meet and fuse at the posterior end of the nasal pit, the medial nasal processes of A/J embryos were more prominent, more medially placed, and diverged less than those of C57BL/6J. It is suggested that (a) cleft lip is a threshold character or quasi‐continuous variant such that a very slight change in the divergence of the medial nasals leads to their partial or complete lack of fusion with the lateral nasals. This is followed by a lack of fusion of medial and maxillary processes, and a complete or partial breakdown of the isthmus, resulting in a cleft lip; (b) the strain differences in type of aspirin‐induced cleft lip and in topography and growth of the facial processes between the two strains are causally related to the A/J predisposition to cleft lip.

No abstract

Splotch (Sp) and splotch-delayed (Spd) are allelic mutations on chromosome 1 of the mouse. Embryos homozygous for either allele have neural tube defects (NTDs) and deficiencies in neural crest cell (NCC) derived structures. The fact that Spd mouse mutants sometimes have deficiencies in NCC derivatives in the absence of an NTD led to the hypothesis that neurulation and the release of NCCs may depend on a regulatory event that is common to both processes. Therefore, it may be possible to understand the cause of NTDs in these mutants by examining the basis of aberrant NCC derivatives. Caudal neural tubes were excised from day 9 Sp and Spd embryos and placed into gelatin-coated tissue culture dishes, or 3-dimensional basement membrane matrigel, and cultured for 72 hours. A cytogenetic marker was used to genotype the embryos. In planar cultures, no morphological differences were observed between NCCs from neural tube explants of Spd mutants compared to those from heterozygous or wild-type embryos. However, there appeared to be a delay in the release of NCCs from the neural tube in both Sp and Spd mutants, which was particularly evident in Sp. After 24 hours in culture, the extent of NCC outgrowth, as well as the number of NCCs emigrating from explanted neural tubes, was significantly lower in Sp and Spd mutant cultures than in controls. No differences were observed in the mitotic indices among cells which had emigrated. By 72 hours, mutant cultures and their non-mutant counterparts were similar in terms of outgrowth, cell number, and migratory capability. After 24 hours in 3-dimensional basement membrane matrigel, cell outgrowth from Sp explants was also significantly less than controls. The pattern of NCC outgrowth in both types of culture conditions indicates a 24 hour delay in mutant cultures compared to controls. This stems from a delay in the release of NCCs from the neural tube, suggesting that the defect lies within the neuroepithelium with respect to the release of NCCs.

Splotch and splotch-delayed mutants have anomalies in certain neural crest cell derivatives as well as neural tube defects. A genetic marker was used to identify mutant, heterozygote, and wild-type embryos within a litter, which enabled us to make intergenotypic comparisons. Histological studies of the lumbosacral region of day 15 and day 16 embryos indicated that the splotch-delayed mutant had similar but less severe defects in spinal ganglion development than those reported for splotch (Auerbach: Journal of Experimental Zoology 127:305-329, 1954). The ganglia were extensively reduced in size, residual, or missing in the splotch-delayed mutant, whereas in the splotch mutant, they were virtually nonexistent. Paired comparison analyses showed that all mutant embryos had a significant reduction in their volume of lumbosacral spinal ganglia when compared to their heterozygous and/or wild-type littermates. Also, some heterozygotes were found to have spinal ganglia volumes that were significantly reduced when compared to wild-type embryos. The volume of spinal ganglia was not related to the severity of the neural tube defect. In fact, three mutant embryos, which did not exhibit a neural tube defect, had spinal ganglia volumes comparable to or less than those mutants with open neural tube lesions or curly tails. This shows that the formation of abnormal neural crest cell derivatives is not a result of the neural tube closure defect. We hypothesize that the two anomalies observed in these mutants have a common etiological basis.