OVOTRANSFERRIN AND OVOTRANSFERRIN RECEPTOR EXPRESSION DURING CHONDROGENESIS AND ENDOCHONDRAL BONE-FORMATION IN DEVELOPING CHICK-EMBRYO

Ovotransferrin expression during chick embryo tibia development has be en investigated in vivo by immunocytochemistry and in situ hybridizati on. Ovotransferrin was first observed in the 7 day cartilaginous rudim ent. At later stages, the factor was localized in the articular zone o f the bone epiphysis and in the bone diaphysis where it was concentrat ed in hypertrophic cartilage, in zones of cartilage erosion and in the osteoid at the chondro-bone junction. When the localization of the ov otransferrin receptors was investigated, it was observed that chondroc ytes at all stages of differentiation express a low level of the ovidu ct (tissue) specific receptor. Interestingly, high levels of the recep tor were detectable in the 13-d old tibia in the diaphysis collar of s tacked-osteoprogenitor cells and in the layer of derived osteoblasts. High levels of oviduct receptor were also observed in the primordia of the menisci. Metabolic labeling of proteins secreted by cultured chon drocytes and osteoblasts and Northern blot analysis of RNA extracted f rom the same cells confirmed and completed the above information. Ovot ransferrin was expressed by in vitro differentiating chondrocytes in t he early phase of the culture and, at least when culture conditions al lowed extracellular matrix assembly, also by hypertrophic chondrocytes and derived osteoblast-like cells. Osteoblasts directly obtained from bone chips produced ovotransferrin only at the time of culture minera lization. By Western blot analysis, oviduct receptor proteins were det ected at a very low level in extract from differentiating and hypertro phic chondrocytes and at a higher level in extract from hypertrophic c hondrocytes undergoing differentiation to osteoblast-like cells and fr om mineralizing osteoblasts. Based on these results, the existence of autocrine and paracrine loops involving ovotransferrin and its recepto r during chondrogenesis and endochondral bone formation is discussed.