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 de velopment and explored its regulation by the neuroectoderm. Experiment s were carried out on chick embryos. During early semitic 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 down regulated 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 lm x1 on interactions with the neuroectoderm was explored by carrying out ablations of the neural tube in organotypic explants containing the o tic presumptive ectoderm. Both the formation of the otic vesicle and e xpression of lmx1 were dependent on the presence of the neural ectoder m during stages preceding placode formation (4-6 somites). Thereafter, the formation of the otic vesicle was progressively autonomous, and b y the stage of 10 somites the otic ectoderm developed into otic vesicl es and expressed lmx1 in foreign environments. Dorsal and ventral neur oectoderms displayed differential effects on lmx1 expression. Ablation of the dorsal neural tube resulted in a reduced expression of lmx1, w hich 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, an d its domain expanded. The formation of the otic vesicle, however, was supported by either the dorsal or ventral neuroectoderm The experimen ts suggest that lmx1 is involved in early patterning of the otic vesic le, and they provide evidence for the regional segregation of organizi ng activities within the neural tube. (C) 1998 Academic Press.