Mechanisms And Effects Of Corticosterone On Bone Development Of Broiler Chickens(Gallus Gallus Domesticus)

The breeding methods of meat-type chickens of modern strain emphsized increased body weight and greater muscle deposition which imposed effects on the physiological function of broilers and led to leg disorders. This not only impaired animal welfare but also caused economic losses. It was the mashroom that imposed internal stress on the bone system of commercial broilers, which was one of the most important etiologies of leg disorders in broliers. Under the stress, hypothalamo-pituitary-adrenal (HPA) axis was activated with increased secretion of corticosterone (CORT) in birds. CORT was a kind of glucocorticoids existed in many species including microbe, plants, lower life form and vertebrates, and was responsible for many progresses of allostasis. Excess glucocorticoid induced bone loss and the disorder of bone development. Growth plate cartilage was central to the process of the longitudinal bone growth. There existed multiple signaling pathways including CORT regulating the endochondral ossification in the epiphyseal growth plate. However, the literature provided little information on the effects of CORT on the bone growth in broilers. Hence, the present study was designed to investigate the effects of CORT administration on the properties of bones morphology, biomechanics and histochemistry in growing broilers, and further elaborated the molecular mechanism of CORT on the proliferation and differentiation of cultured chondrocytes at the cellular level.1. Effects of CORT on bone development in broilers.One hundred broiler chicks (Gallus gallus, Ross strain) of both sexes were used in our in vivo study. Chickens were randomly allocated to sham control and CORT-treated groups and each group had50birds. At7d of age, the experimental birds were injected daily with CORT (4mg/kg of body mass) or corn oil for1week. At14and21d of age, body weight, growth performance, haematology, relative organ weights and bone parameters were conducted. The result showed that CORT administration significantly decreased BW gain and feed intake, while CORT administration significantly increasing relative liver weight of the chickens and the bone parameterswere also decreased. Histology and immunohistochemistry of type X collagen revealed that CORT reduced the lengths of proliferative and prehypertrophic zone in growth plate and the number of positive chondrocytes in the prehypertrophic zone, In conclusion exposure to CORT depressed the growth performance and retarded the longitudinal growth of the long bones by inhibiting the proliferation and differentiation of chondrocytes in growth plate in broilers. Concurrently, CORT can impair the growth performance and immune system of broiler chickens.2. Effects of CORT on the proliferation and differentiation of chicken growth plate chondrocytesGrowth plate chondrocytes were isolated from6weeks old broiler chickens for primary culture and after48h, chondrocytes were given different doses (10-10-10"6M) of CORT for24h,48h or72h. Then, the cell viability was detected. After co-incubate with CORT for48h, the activity of alkaline phosphatase (ALP) was determined. Meanwhile, the expression of parathyroid hormone-related peptide (PTHrP), Indian hedgehog (Ihh) and type X collagen (Col X) mRNAs were detected by quantitative real-time RT-PCR while the Col X protein expression was determined using western blot analysis. At10-9-10-6M concentration, CORT significantly inhibited the cell viability and ALP activity of chondrocytes.10"6M and10-8M CORT significantly decreased the PTHrP, Runx2and Col X mRNA levels while obviously increased the mRNA level of Ihh. And the maximal inhibitory effect of CORT on chondrocytes performance was at10-6M. Conversely,10-10M CORT markedly stimulated the expression of these factors. The protein level of Col X was consistent with its mRNA level. It can be concluded that CORT exerted both positive and negative effects on chondrocytes performance depending on its concentration. High dose of CORT suppressed the viability, proliferation and hypertrophy of chondrocyte, whereas lower dose of CORT stimulated the process of endochondral bone. CORT signaling may be mediated through the Ihh/PTHrP signaling pathway.