HOMEOGENES EXPRESSION DURING MOUSE EMBRYOGENESIS

Murine Hox genes (homeogenes), which are grouped in four Hox complexes, display coordinately regulated expression during embryogenesis. Our in situ hybridization studies show that the Hox-1.4 gene is expressed in regions of the nervous system and in various mesodermal derivatives, with well-defined anterior expression boundaries. Generally, genes located in 5' positions in the complexes show more posteriorly restricted expression in the embryo. Hox genes expression domains are thus colinear to the order of the genes along the chromosome. The structural and functional organization of the Hox complexes is analogous to those of their Drosophila counter-parts, the homeotic genes. Genes at 5' positions in the Hox-4 complex display in addition, a coordinate expression during limb development: genes located 5' are expressed later and in more posterior and distal areas of the limb compared to genes located at 3' positions. This coordinate expression may rely on a stimulatory molecule released by the polarising region of the limb. In the chick wing bud, these Hox-4 genes can be activated in an ectopic position following polarising region grafts or local retinoic acid delivery. The same genes appear to be sequentially activated in posterior regions of the gastrulating embryo. On the other hand, a cluster of three adjacent genes in the 3' part of the Hox-4 complex exhibit additional expression specificity in the somatic cells of the fetal gonad and the epithelia of the excretory apparatus. Coordinate expression of Hox-4 genes is also detected in another axial structure, the genital bud. Region-specific combinations of homeoproteins may be a universal mechanism to provide positional information in all axial structures of the vertebrate embryo.