Regionalized organizing activity of the neural tube revealed by the regulation of lmx1 in the otic vesicle.

LIM homeodomain genes have been involved in patterning in a variety of organisms. I have analyzed the expression of lmx1 during early ear development and explored its regulation by the neuroectoderm. Experiments were carried out on chick embryos. During early somitic stages (4-6 somites), lmx1 was expressed in the neural tube and in a stripe of the dorsal ectoderm. The ectodermal expression domain was then restricted to the otic placode (7-10 somites). At otic cup stages, lmx1 was downregulated in ventral and medial aspects of the otic epithelium facing the neural tube. This resulted in a dorsal and lateral restriction of lmx1 that persisted until the otic vesicle stage. The dependence of lmx1 on interactions with the neuroectoderm was explored by carrying out ablations of the neural tube in organotypic explants containing the otic presumptive ectoderm. Both the formation of the otic vesicle and expression of lmx1 were dependent on the presence of the neural ectoderm during stages preceding placode formation (4-6 somites). Thereafter, the formation of the otic vesicle was progressively autonomous, and by the stage of 10 somites the otic ectoderm developed into otic vesicles and expressed lmx1 in foreign environments. Dorsal and ventral neuroectoderms displayed differential effects on lmx1 expression. Ablation of the dorsal neural tube resulted in a reduced expression of lmx1, which was more dramatic during early placode and preplacode stages (5-7 somites). Removal of the ventral aspect of the neural tube (including the notochord) had opposite effects, expression of lmx1 increased, and its domain expanded. The formation of the otic vesicle, however, was supported by either the dorsal or ventral neuroectoderm. The experiments suggest that lmx1 is involved in early patterning of the otic vesicle, and they provide evidence for the regional segregation of organizing activities within the neural tube.