The effect of human GH (hGH) and insulin-like growth factor-I (IGF-I) on colony formation of rabbit epiphyseal tibial chondrocytes at different stages of maturation was studied in suspension culture. The epiphyseal growth plate from the proximal tibia of 8-week-old male rabbits was dissected and divided into three different (proximal, intermediate and distal) zones, each zone representing an enrichment of chondrocytes from the germinative, proliferative and hypertrophic cell layers respectively. Chondrocytes from these three zones were isolated by collagenase digestion and cultured in the presence of 10% (v/v) newborn calf serum in suspension stabilized with 0·5% (w/v) agarose for 14 days.

The colonies were classified according to diameter and the number of colonies was estimated as a function of colony size (distribution of cloning efficiency). Cell clusters with a diameter of 56 μm or more were classified as colonies. The cloning efficiency (number of colonies formed per 1000 seeded cells) was 10·1 ± 0·7 and 6·0 ± 0·8 for chondrocytes isolated from the proximal and intermediate zones respectively.

Insulin-like growth factor-I (25–200 ng/ml) increased the number of colonies in chondrocytes isolated from the proximal zone (122 ± 9·0–156 ± 8·4%; control value, 100%) and from the intermediate zone (136 ± 14·0–191 ± 29%). Low concentrations of hGH (10–40 ng/ml) stimulated colony formation in chondrocytes isolated from the proximal zone (125 ± 6·4– 137 ± 7·9%) whereas a high concentration of hGH (160 ng/ml) was ineffective. Chondrocytes isolated from the intermediate zone showed no response to low concentrations of hGH (10–20 ng/ml). High concentrations of hGH (40–160 ng/ml) reduced colony formation of chondrocytes from this zone (81 ± 9·1–53 ± 4·2% of control value). Chondrocytes isolated from the distal zone formed either none or an insignificant number of colonies in the absence or presence of hGH or IGF-I.

Both IGF-I and hGH caused a significant change in distribution of cloning efficiency in chondrocytes isolated from the proximal zone. IGF-I potentiated the formation of small colonies (colony diameter 64–176 μm) while hGH increased the number of large colonies (colony diameter 112–320 μm). In fact, very large colonies (colony diameter > 272 μm) were only seen in cultures containing hGH. Among chondrocytes isolated from the intermediate zone, IGF-I stimulated colonies of all sizes and no change was seen in the distribution of cloning efficiency. Human GH at 160 ng/ml significantly altered the distribution of cloning efficiency in chondrocytes from the intermediate zone due to an inhibitory effect on the formation of colonies of small size.

These results show that the cloning efficiency of epiphyseal chondrocytes in suspension culture is dependent upon their previous spatial location in the intact growth plate.

The finding that GH stimulated the formation of colonies of large size cells in from the proximal part of the growth plate indicates that GH interacts with a limited number of cells which exhibit a high proliferative capacity, e.g. with stem cell chondrocytes or early proliferative chondrocytes.

J. Endocr. (1987) 115, 263–271