AbstractSynovial hyperplasia is a feature of the chronic synovitis associated with basic calcium phosphate crystals hydroxyapatite (HA), octacalcium phosphate, tri‐calcium phosphate and calcium pyrophosphate. Each of these crystals stimulated mitosis of cultured human skin fibroblasts or canine synovial fibroblasts in a concentration‐dependent fashion. We examined the effect of pure somatomedin C (Sm‐C) on HA crystal induced mitogenesis. Confluent cultures of human fibroblasts were rendered quiescent by incubation in the presence of 1 platelet‐poor‐Sm‐C free plasma (PPSCFP) for 24 hours. HA crystals stimulated thymidine incorporation 2.3‐fold over control value. Addition of Sm‐C significantly augmented the effect of HA crystals (P<0.01). Nearly identical effects were observed in the presence of 100 μg/ml HA crystals or 15 ng/ml PDGF. Monoclonal antibodies against Sm‐C had little effect on the basal3H thymidine uptake by control cells incubated in 1 PPSCFP but blocked over 50 of the HA crystal of PDGF‐induced3H thymidine incorporation both in the presence or absence of Sm‐C. The incomplete blocking suggested either the presence of other “progression” factors, such as insulin‐like growth factor II in the conditioned media or the possibility that HA or PDGF in high enough dosage enabled cells to escape their dependen
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