Decrease in the basal levels of cytosolic free calcium in chondrocytes during aging in culture: Possible role as differentiation‐signal

Cell‐ and matrix‐related parameters, which characterize the aging and differentiation process of cartilage in vivo, were measured in cultured chick epiphyseal chondrocytes during maintenance in a suspension culture for 34 days. A gradual decrease in the rates of proliferation and an increase in the size of the cells were observed. Ultrastructural examination revealed increased vacuolization and appearance of glycogen‐storing pools. The rate of proteoglycan synthesis gradually increased. Age‐related changes in the composition of the proteoglycan consisted of an increase in the ratio of keratan sulfate/chondroitin sulfate. The results indicate that the process of aging in culture resembles maturation and differentiation of cartilage tissue in vivo. The levels of cytosolic free calcium ions ([Ca²⁺]_i) were measured in fura‐2‐loaded cells during the course of aging in culture. A gradual decrease in [Ca²⁺]_i was observed. In 5‐day cultures, a value of 184 nM [Ca²⁺]_i was measured; this value decreased to 61 nM in 34‐day cultures. On the basis of the present data and the previous results, which showed that cartilage‐derived growth factors caused a decrease in [Ca²⁺]_i, concomitantly with enhancing differentiation, whereas factors which elevated [Ca²⁺]_i, caused an increase in proliferation and a decrease in proteoglycan synthesis, we suggest a model for control of chondrocyte differentiation and aging. The model suggests that the rate of differentiation may be paced by changes in steady‐state levels of [Ca²⁺]_i.