Subcellular location of IGF-I in chondrocytes from rat rib growth plate

Ultrathin cryosections of segments of growth plates from rat rib and isolated, gelatin-embedded chondrocytes from the same source were used to demonstrate the ultrastructural location of IGF-I in the different layers of the growth plate. Sections were incubated with a polyclonal rabbit antiserum to free IGF-I and colloidal gold coated with goat anti-rabbit IgGs. The gold label was present in all chondrocytes, localized almost exclusively to the cytoplasm. Particularly the cells in the proliferating zone displayed considerable amounts of gold label. In these cells, gold label was predominantly present over the rough endoplasmic reticulum and membrane bound vesicles, with only negligible gold label over the nuclei and cartilage matrix. The distribution of gold label in the chondrocytes from the proliferating zone, especially in the rough endoplasmic reticulum, strongly suggests that IGF-I can be synthesized by these cells. However, further studies are needed to verify whether there is a local synthesis of IGF-I in the chondrocytes and whether it is under control of growth hormone.