Tissue interactions in the developing chick diencephalon

Here, we describe tissue explant experiments in chick embryos aimed at characterising the roles of different forebrain areas in ZLI formation. We found that: the ZLI becomes specified unexpectedly early; flanking regions are required for its characteristic morphogenesis; ZLI induction can occur independently from ventral tissues; interaction between any prechordal and epichordal neuroepithelial tissue anterior to the midbrain-hindbrain boundary is able to generate a ZLI; and signals from the dorsal diencephalon antagonise ZLI formation. We further show that a localised source of retinoic acid in the dorsal diencephalon is a likely candidate to mediate this inhibitory signal.Our results are consistent with a model where planar, rather than vertical, signals position the ZLI at early stages of neural development and they implicate retinoic acid as a novel molecular cue that determines its dorsoventral extent.Global signalling gradients regionalise the emerging vertebrate brain at the earliest stages of neural development [1-3]. This crude initial pattern is subsequently refined by the activity of local signalling centres [4-6]. Dorsoventral (DV) neural patterning is regulated by two such signalling centres, the roof plate and the floor plate, which stretch along the dorsal and ventral midlines of the neural tube, respectively [7-9]. Anteroposterior (AP) regionalisation is regulated by several discrete signalling centres, such as the anterior neural border [10-13], the midbrain-hindbrain boundary (MHB) [14-16], rhombomere 4 in the hindbrain [17,18] and, subsequently, the boundaries between rhombomeres [19,20]. Recently, we and others have shown that the interface between the prethalamus (Pth) and the thalamus (Th), the zona limitans intrathalamica (ZLI), also acts as a signalling centre that is essential for the establishment of these major diencephalic subdivisions [21-23].For a considerable time during development, the ZLI is the only structure in the alar part of th