The Pax6 transcription factor plays a key role in ocular development of vertebrates and invertebrates. Homozygosity of the Pax6 null mutation in human and mice results in arrest of optic vesicle development and failure to initiate lens formation. This phenotype obscures the understanding of autonomous function of Pax6 in these tissue components and during later developmental stages. We employed the Cre/loxP approach to inactivate Pax6 specifically in the eye surface ectoderm concomitantly with lens induction. Although lens induction occurred in the mutant, as indicated by Sox2 up-regulation in the surface ectoderm, further development of the lens was arrested. Hence, Pax6 activity was found to be essential in the specified ectoderm for lens placode formation. Furthermore, this mutant model allowed us for the first time to address in vivo the development of a completely normal retina in the absence of early lens structures. Remarkably, several independent, fully differentiated neuroretinas developed in a single optic vesicle in the absence of a lens, demonstrating that the developing lens is not necessary to instruct the differentiation of the neuroretina but is, rather, required for the correct placement of a single retina in the eye.

During gastrulation, the three germ layers of the embryo are formed and organized along the anterior-posterior body axis. In the mouse, gastrulation involves the delamination of ectodermal cells through the primitive streak and their differentiation into mesoderm. These processes do not occur in embryos homozygous for a retrovirally induced recessive prenatal lethal mutation, the strain 413-d insertional mutation. Instead of giving rise to mesoderm, embryonic ectoderm in 413-d mutants overproliferates and then rapidly degenerates, although extraembryonic lineages remain viable. Here we isolate a candidate for the mutated gene which encodes a new member of the transforming growth factor-beta (TGF-beta) superfamily. Expression is first detected in primitive streak-stage embryos at about the time of mesoderm formation. It then becomes highly localized in the node at the anterior of the primitive streak. This region is analogous to chick Hensen's node and Xenopus dorsal lip (Spemann's organizer), which can induce secondary body axes when grafted into host embryos (reviewed in refs 5 and 6). Our findings suggest that this gene, named nodal, encodes a signalling molecule essential for mesoderm formation and subsequent organization of axial structures in early mouse development.

The hypothalamus is a region of the diencephalon with particularly complex patterning. Sonic hedgehog (Shh), encoding a protein with key developmental roles, shows a peculiar and dynamic diencephalic expression pattern. Here, we use transgenic strategies and in vitro experiments to test the hypothesis that Shh expressed in the diencephalic neuroepithelium (neural Shh) coordinates tissue growth and patterning in the hypothalamus. Our results show that neural Shh coordinates anteroposterior and dorsoventral patterning in the hypothalamus and in the diencephalon-telencephalon junction. Neural Shh also coordinates mediolateral hypothalamic patterning, since it is necessary for the lateral hypothalamus to attain proper size and is required for the specification of hypocretin/orexin cells. Finally, neural Shh is necessary to maintain expression of differentiation markers including survival factor Foxb1.