Friday, December 18, 2009


Retinol-binding protein 4 is expressed in chondrocytes of developing mouse long bones: implications for a local role in formation of the secondary ossification center.

"Retinol-binding protein 4 (Rbp4) is the major carrier of retinol in the bloodstream, a retinoid whose metabolites influence osteogenesis, chondrogenesis and adipogenesis. Rbp4 is mainly produced in the liver where it mobilizes hepatic retinol stores to supply other tissues. Rbp4 was present in a variety of locations in developing embryonic and postnatal mouse hindlimbs. Rbp4 was present in a restricted population of epiphyseal chondrocytes and perichondral cells correlating to the future region of secondary ossification. With the onset of secondary ossification, Rbp4 was detected in chondrocytes of the resting zone and in chondrocytes that bordered invading cartilage canals and the expanding front of ossification. Rbp4 was less abundant in proliferating chondrocytes involved in primary ossification."

"Serum retinol, complexed to Rbp4, is taken up into cells via the Rbp4 receptor, Stra6. In the cell, retinol metabolites (collectively known as retinoids) can interact with retinoic acid/rexinoid receptor (RAR/RXR) heterodimers to activate gene transcription. Abnormal retinoid levels impact bone growth"

"Rbp4 was detected in chondrocytes toward the middle of the epiphysis, in the region where the secondary ossification center will form"

"ucleated cells with small amounts of cytoplasm did not contain Rbp4 whereas Rbp4 was abundant in the cytoplasm where the nucleus had begun to deteriorate and was still present in cells without nuclei. With complete breakdown of the cell, Rbp4 was no longer detected in empty lacunae"

"Rbp4 is localized to chondrocytic cells in three regions of embryonic tibia; two relating to secondary ossification and the third relating to primary ossification. Primary ossification contributes to elongation of the limbs, whereas secondary ossification mediates expansion and mineralization of the epiphyses. Formation of the secondary ossification center is preceded by invagination of the perichondrium and invasion of the epiphyseal cartilage by vascularised canals"

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