Segment-specific expression of a homoeobox-containing gene in the mouse hindbrain

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Abstract
THE process of segmentation, in which the developing embryo is divided into repetitive structures along its antero–posterior (A–P) axis, as a means of organizing and coordinating the body plan is found in a wide range of organisms. In Drosophila, homoeotic genes are involved in all levels of segmental organization and in determining segment identity1. The roles of these genes in seg-mentation have been found mainly by mutational studies2,3, but also by in situ hybridization, which has shown their domains of expression. In contrast to Drosophila, however, embryonic expression of homoeobox-containing genes in vertebrate organisms has not been found to follow a segmental pattern4. Vertebrate segmentation can be clearly seen in the mesodermal somites, but repetitive morphological structures in the central nervous system (neuromeres)5–13 have only recently been shown to have develop-mental significance. Neuromeres in the hindbrain (rhombomeres) have been defined as segmental units by their pattern of nerve formation in the developing chick14 and by the alternating expression of Krox-20, a gene encoding a zinc-finger DNA-binding protein, in the 9.5-day-old mouse15. Here we report that a mouse homoeobox-containing gene, Hox-2.9, is expressed in a segment-specific manner in the developing mouse hindbrain. This expression is in a region which is flanked by the regions of expression of Krox-20, and is precisely contained within a single neuromere, rhombomere 4.