Figure 1.

Expression of homeobox transgenes and Hoxc-8 in the developing skeleton. Our binary transgenic mouse system (A) is based on the potent viral transactivator VP16. The combination of two transgenes, namely the transactivator (TA) and a transresponder (TR) transgene, in the same individual leads to activation of the TR transgene. (B) Sites of IE-LacZ transgene activation indicated by β-galactosidase activity. VP16 expression under control of the Hoxc-8 promoter leads to TR gene expression in growth zones and cartilage of the skeleton at 17.5 days of development. (C) Section through a rib of the embryo in B showing staining for β-galactosidase in proliferating (p), prehypertrophic (ph), and some hypertrophic (h) chondrocytes. (D and E) Sections through two ribs at different planes of the respective growth zones with prehypertrophic cells (D) and hypertrophic cells (E) exhibiting β-galactosidase activity. (F) β-Galactosidase expression in proliferating cells in the prospective neural arch of a vertebra. Notably, there is no detectable transgene activation in perichondrial cells (arrows). (G–J) In situ hybridizations with a Hoxc-8 antisense probe to parasagittal sections of a mouse embryo at 15.5 days (G and H) and 18.5 days (I and J). Cells of the developing rib cartilage (bright-field exposure in G and I) express Hoxc-8, whereas the surrounding tissue is negative (H and J). Sense probe for Hoxc-8 gave no appreciable signal (not shown).