Osteocrin (Ostn), a bone-active molecule, has been shown in animals to be highly 
expressed in cells of the osteoblast lineage. We have characterized this protein 
in human cultured primary human osteoblasts, in developing human neonatal bone, 
and in iliac crest bone biopsies from adult women. In vivo, Ostn expression was 
localized in developing human neonatal rib bone, with intense immunoreactivity 
in osteoblasts on bone-forming surfaces, in newly incorporated osteocytes, and 
in some late hypertrophic chondrocytes. In adult bone, Ostn expression was 
specifically localized to osteoblasts and young osteocytes at bone-forming 
sites. In vitro, Ostn expression decreased time dependently (p<0.02) in 
osteoblasts cultured for 2, 3, and 6 days. Expression was further decreased in 
cultures containing 200 nM hydrocortisone by 1.5-, 2.3-, and 3.1-fold (p<0.05) 
at the same time points. In contrast, alkaline phosphatase expression increased 
with osteoblast differentiation (p<0.05). Low-dose estradiol decreased Ostn 
expression time dependently (p<0.05), whereas Ostn expression in cultures 
treated with high-dose estradiol was not significantly changed. These results 
demonstrate that Ostn is expressed in human skeletal tissue, particularly in 
osteoblasts in developing bone and at sites of bone remodeling, suggesting a 
role in bone formation. Thus, Ostn provides a marker of osteoblast lineage cells 
and appears to correlate with osteoblast activity.