CD44 expression in fetal rat bone: in vivo and in vitro analysis.

Knowledge of the expression pattern of the cell surface glycoprotein CD44 in the development and differentiation of bone is limited. We investigated CD44 expression (a) in bone sections of 21-day-old fetal rat calvaria (RC), metatarsals, and tibiae, (b) in primary cultures of RC cells undergoing differentiation in vitro, and (c) in three rat osteosarcoma cell lines: ROS 17/2.8, UMR 106.01, and UMR 106.06. By immunocytochemistry, Western, Northern and reverse transcription polymerase chain reaction analyses, we found that osteoblastic cells express the 'hematopoietic' or 'standard' CD44 (CD44s) isoform. Osteoblastic cells in vivo and in vitro stained at all detectable stages of differentiation, but intercellular heterogeneity of CD44s staining was evident, with lesser staining in preosteoblastic cells and greater staining in mature osteoblasts and osteocytes. As cells in RC cultures differentiated and formed bone in vitro, CD44s mRNA and protein levels as measured on immunoblots were invariant. All three osteosarcoma cell lines expressed CD44s mRNA and protein. The synthetic glucocorticoid dexamethasone, which stimulates osteogenesis in RC cells in vitro and regulates a number of osteoblast-associated genes, had no apparent effect on either CD44s protein or mRNA levels. The widespread presence of CD44s in osteoblastic cells at various maturational stages suggests that further analyses will be required to determine what role CD44s may play in osteogenesis and in bone tissue organization.