DISTRIBUTION OF HYALURONAN IN THE EPIPHYSEAL GROWTH-PLATE - TURNOVER BY CD44-EXPRESSING OSTEOPROGENITOR CELLS

In the present study, we have examined the distribution of both hyalur onan and its receptor, CD44, during the process of endochondral ossifi cation in the mouse tibia. Histochemical staining revealed that a larg e amount of hyaluronan was present in the lacunae located in the zone of hypertrophy, but it was greatly reduced or absent from the zone of erosion. In addition, hyaluronan was present in the cytoplasm of osteo progenitor cells located in the zone of erosion. These cells also expr essed CD44 on their surfaces, as revealed by double-label immunohistoc hemistry. These results suggested that the osteoprogenitor cells may u se CD44 to bind and internalize hyaluronan, and subsequently degrade i t with lysosomal enzymes. To test this possibility, we examined the hu man cell line, MG-63, which closely resembles osteoprogenitor cells. T hese cells produced several different forms of CD44, as determined by western blotting (85, 116 and 150 kDa). In addition, the binding of is otopically labeled hyaluronan to detergent extracts of these cells was blocked by a monoclonal antibody to CD44. Similarly, the degradation of hyaluronan by these cultured cells was also inhibited by a monoclon al antibody to CD44. To determine if these cells could remove hyaluron an from the growth plate, the cells were cultured directly on top of t hin sections of the epiphysial region of long bone. After 16 hours, th e sections were stained for hyaluronan. The MG-63 cells removed signif icant amounts of hyaluronan present in the zone of hypertrophy, and th is effect was blocked by an excess of soluble hyaluronan and by a mono clonal antibody to CD44. Taken together, these results suggest that, u nder physiological conditions, osteoprogenitor cells in the zone of er osion can use a CD44-dependent mechanism to remove hyaluronan.