Hox gene expression in adult tissues with particular reference to the adrenal gland

In comparison to the embryo, very little work has been carried out on the expression and role of Hox genes in the adult animal. An expression profile of all 39 vertebrate Hox genes on a select panel of adult human tissues reveals that in fact these genes are widely expressed throughout the adult human and a colinear pattern of expression is displayed similar to that of the developing embryo. Of particular interest is the abundance of Hox genes that are expressed within the adult adrenal gland. Adrenal cortical cells are continuously renewed to sustain production of zonal steroids. Cell proliferation occurs at the periphery of the cortex and cells are then displaced centripetally, phenotypically switching as they migrate through the gland before undergoing apoptosis at the zona reticularis/medullary boundary. It is still unclear which mechanisms cause the cells to differentiate as they cross the zonal boundaries and we hypothesise that Hox genes may be involved in the phenotypic switching of the adrenocortical cells. In situ hybridisation experiments were carried out on adult rat adrenal gland sections and Hox gene expression was localized within the zonal borders, coinciding with the localization of cells that undergo phenotypic differentiation, and thus supporting our hypothesis that Hox genes may be involved in the phenotypic switching of the adrenocortical cells. As in the developing embryo, the genes display colinear expression with the 3' Hox genes being expressed within the outer gland and the 5' genes within the inner zones.