Figure 1. Transgenic Mice with Inducible CreERt Expression in Osteoblast Lineage Cells.

(A) Simplified representation (top) of the line of progression of osteoblast (OB) differentiation and the typical gene product by which each stage is characterized (orange, below). The Osx-CreERt and Col1(3.2 kb)-CreERt transgenes employed in this study start to be expressed in osteoblast precursors and mature oste-oblasts, respectively. Schematic outline (bottom) of the initiation of bone formation during development, occurring between E14 and E16 in the stylopod and zeugopod bones of mice. The initial invasion of the cartilaginous bone model is associated with its transformation into the primary ossification center (POC) that contains bone trabeculae and is surrounded by cortical bone. HC, hypertrophic cartilage.

(B) Whole-mount X-gal staining of E14.5 Osx-CreERt embryos carrying a Rosa26R reporter transgene. (left to right) Embryos unexposed to 4OHTam or not carrying the Osx-CreERt transgene, Alizarin red mineralization staining and corresponding pattern of LacZ activity in Osx-CreERt(Tg/−) embryos exposed to 4OHTam at E13.5. The mandible became disconnected from the embryo proper. Magnifications (right) highlight X-gal staining in the bony regions of the ribs, calvaria and hind limb (HL).

(C) X-gal staining of Col1-CreERt(Tg/−) embryos harvested similarly at E14.5 after 4OHTam exposure at E13.5. Right, magnified ribs and HL.

(D) E15.5 Osx- and Col1-CreERt(Tg/−) embryos injected with 4OHTam at E13.5, and magnified HL and forelimb (FL). Red arrows, reduced staining in the ribs and HL of Col1-CreERt embryos, compared with Osx-CreERt embryos.

See also Figure S1 on the generation and characterization of the osteoblastic CreERt mice.