![]() In Osx-null embryos, cartilage elements are largely normal but osteoblast differentiation fails to complete, resulting in a complete lack of bone tissue. Genetic studies have revealed the essential role of Osx in osteoblast differentiation. ![]() In addition, Osx is also detected in early hypertrophic chondrocytes at a relatively weak level. Later during development, the perichondrial Osx-expressing osteoprogenitors co-migrate with the blood vessels that invade the hypertrophic cartilage, and to generate osteoblasts responsible for depositing the trabecular bone. During embryonic skeletal development, Osx is initially expressed in the perichondrium flanking the hypertrophic cartilage, where osteoblasts first arise to produce the bone collar (cortical bone). Osterix (Osx or Sp7) is a zinc finger family transcriptional factor critical for osteoblast differentiation. ![]() Thus, potential contributions from the non-osteoblast-lineage cells should be considered when Osx-Cre is used to study gene functions in postnatal mice. Beyond the skeleton, Osx-Cre also targets the olfactory glomerular cells, and a subset of the gastric and intestinal epithelium. The targeting of adipocytes and perivascular cells appears to be specific to those residing within the bone marrow, as the same cell types elsewhere are not targeted. By crossing the Osx-Cre mouse with the R26-mT/mG reporter line and analyzing the progenies at two months of age, we find that Osx-Cre targets not only osteoblasts, osteocytes and hypertrophic chondrocytes as expected, but also stromal cells, adipocytes and perivascular cells in the bone marrow. Because the strain has been increasingly used in postnatal studies, it is important to evaluate its targeting specificity in mice after birth. The mouse strain was initially characterized during embryogenesis, and found to target mainly osteoblast-lineage cells. The Osx-Cre mouse line (also known as Osx1-GFP::Cre) expresses GFP::Cre fusion protein from a BAC transgene containing the Osx regulatory sequence. Osterix (Osx or Sp7) is a zinc-finger-family transcriptional factor essential for osteoblast differentiation in mammals.
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