Ultrastructure of chondrogenetic interactions between bone matrix gelatin and mesenchymal cells

The ultrastructure of post fetal chondrogenesis was investigated in chondrogenetic implants and explants of bone matrix gelatin: (A) in direct contact with living cells; (B) separated from living cells by cellulose acetate membranes; (C) in serum or in a culture medium with no contact with living cells. Ruthenium red (RR) staining coarse granules free of interconnecting filaments aggregated in interstitial fluid and inside the surface of the bone matrix gelatin within 72 hours in systems with either direct or indirect contact with living connective tissue cells. No RR granules formed when control denatured or autolysed bone matrix gelatin were substituted for undenatured bone matrix gelatin. No RR granules formed in bone matrix gelatin incubated in serums or media in the absence of living cells in the system. These observations suggest the possibility that 4 days before overt transmembrane cartilage cell differentiation, covert (presumptive) chondroblasts synthesize and secrete precursor substance for aggregation of filament-free RR granules. The relationship between chondromorphogenesis and filament-free RR granules requires further investigation.