Region-specific expression of mouse homeobox genes in the embryonic mesoderm and central nervous system.

The homeobox is a 180-base-pair sequence characteristically found in homeotic and segmentation genes in Drosophila. Several copies of homeoboxes are also found in the mammalian genome, but it is not known whether these are components of morphogenetic loci in mammals as well. As a step toward understanding the function of mammalian homeoboxes, we have used in situ hybridization to define the spatial pattern of expression of two mouse homeobox genes in the midgestational mouse embryo. The two mouse homeoboxes studied here, Hox 1.2 and Hox 1.4, are located 20 kilobases apart on mouse chromosome 6. Our results demonstrate the following: (i) Hox 1.2 transcripts are localized mainly in the posterior myelencephalon, in the cervical central nervous system (CNS), and in several thoracic prevertebrae; (ii) Hox 1.4 transcripts are localized mainly in the posterior myelencephalon and in the cervical CNS; (iii) within the CNS region expressing Hox 1.4, the level of Hox 1.4 transcripts is higher in the mantle layer than in the ependymal layer and higher in the dorsal than in the ventral area. The specific localization of Hox 1.2 and Hox 1.4 transcripts in the embryonic CNS and the restricted pattern of expression along the rostrocaudal axis are strikingly reminiscent of the expression pattern of Drosophila homeoboxes in the fly embryo and larvae. Despite the different developmental strategies adopted by Drosophila and mammals, functional similarities may exist between Drosophila and mammalian homeobox genes.