A VERTEBRATE GENE-RELATED TO ORTHODENTICLE CONTAINS A HOMEODOMAIN OF THE BICOID CLASS AND DEMARCATES ANTERIOR NEUROECTODERM IN THE GASTRULATING MOUSE EMBRYO

We studied the expression of two vertebrate homeobox genes, Otx1 and O tx2, related to orthodenticle, a gene expressed in the developing head of Drosophila. Both genes are expressed in restricted regions of the developing rostral brain including the presumptive cerebral cortex and olfactory bulbs. The expression patterns of the two genes in dienceph alon suggest that they both have a role in establishing the boundary b etween presumptive dorsal and ventral thalamus. They are also expresse d in regions of the developing olfactory, auricolar and ocular system, including the covering of the optic nerve. Otx1 expression is detecta ble from day 8 of gestation in telencephalic, diencephalic and mesence phalic regions. From day 10.5 of gestation its expression extends to s ome metencephalic areas. Otx2 appears to be already expressed in the e piblast of prestreak embryos. It persists in the entire embryonic ecto derm for some time after the onset of gastrulation. In midstreak embry os its expression appears progressively restricted to the anterior emb ryonic ectoderm corresponding to presumptive fore- and midbrain. In ea rly midgestation embryos it is expressed in telencephalic, diencephali c and mesencephalic regions but from day 11.75 of gestation its expres sion disappears from dorsal telencephalon and is confined to diencepha lic and mesencephalic regions. Otx2 is one of the earliest genes expre ssed in the epiblast and immediately afterwards is expressed in anteri or neuroectoderm, demarcating rostral brain regions even before headfo ld formation. Its gene product contains a homeodomain of the bicoid cl ass and is able to recognize and transactivate a bicoid target sequenc e.