Fig. 1.

Assessment of Cre recombinase activity. (A) The Prx1-cre mouse was crossed to the Rosa26-lacZ reporter (R26R) mouse. (i) Whole-embryo staining at E13.5 for lacZ demonstrates strong staining in the limbs. Some staining is also present in the cranium, which corresponds to the normal expression pattern for the Prx1 gene. (ii) Histological examination of the upper forelimb shows lacZ staining throughout the mesodermal tissues, including the muscles, connective tissues and bones (×10 magnification). (iii) There is absence of staining of the dermal and epidermal tissues (arrowhead), but strong staining in the germinal centers of bones (arrow). Magnification is ×40. (B) LacZ staining was performed on E15.5 embryos for Col1A1-cre mice that were crossed to R26R mice. Positive staining was noted especially in the facial bones (i) and ribs (ii), which became evident after removal of skin. LacZ staining is also visible in the skeletal tissues (arrowheads) of the lower extremity of a 3 week old mouse, which has been dissected free of muscle and soft tissue (iii).