Is Far a Hox mutation?

The mouse First arch mutation, Far, causes a severe syndrome of craniofacial defects described previously. All of the known defects are derived from the anterior first arch, and to a very small extent, the dorsal second arch. Recently Far has been shown to be closely linked to Ulnaless on chromosome 2, and therefore in the vicinity of the Hox-4 gene cluster. This paper reports the results of several studies focused on the development origin of the most consistently expressed dominant effect caused by Far, an abnormal major bifurcation of the maxillary nerve. Nerve-stained whole-mount preparations of day 12 embryos showed that in Far mutants the maxillary nerve appears to have a central wedge missing from the normal single-stalked fan shape, and that the nerve defect in Far/Far and +/Far may be equally severe. The effect of retinoic acid on the development of the maxillary nerve was tested. Maternal treatment with 5 mg/kg retinoic acid on day 9 of gestation had no detectable effect on the maxillary nerve of +/Far embryos, and similar treatment with a teratogenic dosage (20 mg/kg) on day 8 or 9 produced no Far-like maxillary nerve defects in genetically normal embryos. The neural crest cells that give rise to nerves and mesenchyme of the first arch originate from specific rhombomeres, discrete segments of the developing head. The rhombomeres of 15 embryos at the 14-23 somite stages, of which 75% are expected to be +/Far or Far/Far, were examined. There was no detectable defect in segmentation or morphology of the rhombomeres compared with controls. The significance of ectopic cartilage in the palate of Far/Far mutants in relation to nerve bifurcation was explored. In histological studies, five out of six Far/Far day-15 fetuses had a rod of ectopic cartilage lateral to the posterior palate, running parallel to, and morphologically similar to, Meckel's cartilage, and lying between the two trunks of the abnormally bifurcated maxillary nerve. None of six +/Far day-15 fetuses examined had detectable ectopic cartilage in this region. We hypothesize that the maxillary nerve defects in Far mutants may be explained by the presence of an ectopic precartilaginous blastema that does not always further develop into detectable cartilage. The ectopic cartilage found in Far/Far resembles the epibranchial cartilage expressed in more posterior branchial arches and in the first arch of lower organisms, and therefore may represent an atavistic posteriorization of the anterior first arch in Far mutants.(ABSTRACT TRUNCATED AT 400 WORDS)