No abstract

Children exposed to systemic glucocorticoids often exhibit growth retardation and after the cessation of therapy catch-up growth occurs in many, but not all patients. The developmental regulation and underlying cellular mechanisms of catch-up growth are not fully understood. To clarify this issue, we established an in vitro model of catch-up growth. Here we present a protocol for the long-term culture (up to 160 days) of fetal (E20) as well as postnatal (P8) rat metatarsal bones which allowed us to characterize ex vivo the phenomenon of catch-up growth without any influence by systemic factors. The relevance of the model was confirmed by the demonstration that the growth of fetal and postnatal bones were stimulated by IGF1 (100 ng/ml) and inhibited by dexamethasone (Dexa; 1 mM). We found that the capacity to undergo catch-up growth was restricted to postnatal bones. Catch-up growth occurred after postnatal bones had been exposed to Dexa for 7 or 12 days but not after a more prolonged exposure (19 days). Incomplete catch-up growth resulted in compromised bone length when assessed at the end of the 4-month period of culture. While exposure to Dexa was associated with decreased chondrocyte proliferation and differentiation, catchup growth was only associated with increased cell proliferation. We conclude that the phenomenon of catch-up growth after Dexa treatment is intrinsic to the growth plate and primarily mediated by an upregulation of chondrocyte proliferation.

The epiphyseal growth plate consists of a layer of cartilage present only during the growth period and vanishes soon after puberty in long bones. It is divided to three well-defined zones, from epiphyses; resting, proliferative, and hypertrophic zones. Chondrocyte proliferation and differentiation and subsequent bone formation in this cartilage are controlled by various endocrine, autocrine, and paracrine factors which finally results into elimination of the cartilaginous tissue and promotion of the epiphyseal fusion. As chondrocytes differentiate from round, quiescent, and single structure to flatten and proliferative and then large and terminally differentiated, they experience changes in their gene expression pattern which allow them to transform from cartilaginous tissue to bone. This review summarizes the literature in this area and shortly describes different factors that affect growth plate cartilage both at the local and systemic levels. This may eventually help us to develop new treatment strategies of different growth disorders.

Tamoxifen (Tam) has been used experimentally to treat boys with gynecomastia and girls with McCune-Albright syndrome. This drug was recently shown to inhibit the growth of cultured fetal rat metatarsal bones and thus might also affect bone growth in vivo. Four-week-old Sprague-Dawley rats were gavaged daily with vehicle alone (peanut oil), Tam (40 mg/kg/d; 1 or 4 wk), or estradiol (40 g/kg/d; 4 wk). Five of the 10 rats in each group were killed after 4 wk and the other five after 14 wk of recovery. Bone growth was followed by repeat DXA scans, whereas other bone parameters and spine length were evaluated by pQCT and X-ray at the time of death. Four-week Tam treatment significantly decreased body weight, nose-anus distance, spinal and tibial bone lengths, trabecular BMD, cortical periosteal circumference, and bone strength and also reduced serum IGF-I levels (424 ± 54 versus 606 ± 53 ng/ml in control; p < 0.05). Analysis of the tibial growth plate of treated rats showed elevated chondrocyte proliferation (BrdU) and apoptosis (TUNEL), as well as decreases in the number of hypertrophic chondrocytes and in the size of terminal hypertrophic chondrocytes. Despite a complete catch-up of body weight after 14 wk of recovery, the tibia was still shorter (p < 0.001) and its cortical region was smaller. We conclude that, when administered at a clinically relevant dose, Tam causes persistent retardation of longitudinal and cortical radial bone growth in young male rats. Our findings suggest that this inhibition results from local effects on the growth plate cartilage and systemic suppression of IGF-I production. Based on these rat data, we believe that Tam, if given to growing individuals, might compromise cortical bone growth, bone strength, and adult height.

Börjesson AE, Windahl SH, Karimian E, Eriksson EE, Lagerquist MK, Engdahl C, Antal MC, Krust A, Chambon P, Säven-dahl L, Ohlsson C. The role of estrogen receptor-␣ and its activation function-1 for growth plate closure in female mice.