Effects of IGF-I, CNP, SCF, BDNF, Gas6, and SDF-1 on chondrocyte proliferation. A, Isolated bovine RZ cells were incubated with each of the factors and [3H]thymidine uptake measured after 24 h. Proliferation is expressed as counts per minute. B, RZ cells were incubated for 24 h with either IGF-I alone or IGF-I in combination with each of the factors. *, P < 0.01.