B. H. Thorp

Skeletal disorders in the fowl: a review

Selection pressure for production traits in modern lines of poultry has placed increasing demands on skeletal integrity. Disruption of the normal process of skeletal growth and homeostasis results in bone diseases that are manifest throughout the modern poultry industry. Bone conditions in poultry can be grouped under three headings based on the age and type of fowls affected, and are indicative of the genetic and production stresses applied to the skeleton. In broilers during growth it is primarily pathologies of the growth plate that lead to most skeletal disorders. In broiler and turkey breeding stock the progressive degeneration of the articular cartilage results in osteoarthrosis, lameness and a consequential loss of reproductive performance. In laying hens bone fragility is most frequently the result of osteoporosis. Before attempting to determine the aetiology of a skeletal disorder an accurate diagnosis must be made. Only then can short- and long-term strategies be developed for the prevention and control of skeletal disorders. Diagnosis requires gross and histological examination, and also dietary, environmental and management analyses. The pathology often reflects lesions initiated when the bird was considerably younger and analyses must extend to assessing the factors prevalent during the initiation of lesions. Current studies are furthering the understanding of the aetiopathogenesis of avian skeletal disorders. For example, structural bone loss at the onset of follicular activity before egg-laying is pivotal to the development of osteoporosis in layers and deficiencies in growth factor expression are integral to the development of tibial dyschondroplasia.

The effect of dietary 1,25-dihydroxycholecalciferol in preventing tibial dyschondroplasia in broilers fed on diets imbalanced in calcium and phosphorus

Three experiments were carried out to investigate the effects of supplemental dietary 1,25-dihydroxycholecalciferol (1,25(OH)2cholecalciferol) and a low dietary Ca:P ratio on the occurrence of tibial dyschondroplasia (TD) in 3-week-old broilers. Histopathology was used to diagnose TD. In the first experiment, feeding a diet containing 7.5 g Ca and 7.6 g P/kg gave a higher incidence of TD than a control diet containing normal amounts of Ca and P (12 and 6 g/kg respectively). Increasing the dietary supplement of cholecalciferol in the imbalanced diet prevented rickets but did not decrease the incidence of TD. In the second experiment, supplementing the imbalanced diet with 10 micrograms 1,25(OH)2cholecalciferol/kg prevented TD completely but also gave a slight growth depression. In the third experiment the imbalanced diet was supplemented with 0, 2.5, 5 or 10 micrograms 1,25(OH)2 cholecalciferol/kg. The supplement of 2.5 micrograms/kg depressed and the higher supplements prevented the occurrence of TD, this time without a growth depression. Feeding the 10 micrograms/kg supplement for the first week only did not prevent TD. Plasma total Ca, inorganic P and alkaline phosphatase (EC 3.1.3.1) were unaffected by diet but 1,25(OH)2cholecalciferol was higher on the imbalanced than on the control diet. Supplementation of the imbalanced diet with 1,25(OH)2cholecalciferol did not increase plasma levels. It is concluded that 1,25(OH)2cholecalciferol is exerting a powerful biological effect in this model of TD, but the mechanism is unclear.

Avian tibial dyschondroplasia: The interaction of genetic selection and dietary 1,25 ‐dihydroxycholecalciferol

The effects of dietary 1,25-dihydroxycholecalciferol on the incidence and severity of tibial dyschondroplasia (TD) were assessed in broiler fowl selected for TD (high-TD line) and against TD (low-TD line). Assessment was by examination with a portable Lixiscope to identify lesions in live birds, gross pathology and histopathology. Eighty broilers in four groups of 20 were examined. When fed on standard diets the low-TD line had a 15% incidence whereas the high-TD line had a 63% incidence. The chicks in the high-TD line, when fed on a diet supplemented with 5 mug or with 10 mug 1,25-dihydroxycholecalciferol/kg diet, showed a marked reduction in the incidence of TD, to 21% and 0% respectively. The growth plate in the proximal tibiotarsi of the high-TD line showed accumulations of transitional chondrocytes, supporting the hypothesis that a failure of chondrocyte differentiation is the cause of dyschondroplasia. This hypothesis is further supported by studies which indicate 1,25-dihydroxycholecalciferol has a major role in chondrocyte differentiation.

Avian tibial dyschondroplasia as a cause of bone deformity

The course of tibial dyschondroplasia (TD) was followed in 20 broiler fowl fed ad libitum. TD initially developed between 2 and 5 weeks of age in all the birds. Tibial plateau angles (TP degrees ) were measured in the radiographs and used as an assessment of tibial bowing. There was a significant correlation between abnormal TP degrees and the age of onset, severity and persistence of TD, indicating that TD can be associated with the development of bone deformity in the form of tibial bowing (abnormal TP degrees ). Lameness and abnormal tibiotarsal torsion were only seen in broilers with abnormal TP degrees ; suggesting that altered TP degrees , may through altered biomechanical loading lead to other bone deformities and can contribute to gait abnormalities.

Estrogen and Cancellous Bone Loss in the Fowl

Abstract. Female birds model a type of woven bone prior to egg laying which is known as medullary bone. Medullary bone modeling is estrogen dependent and in the female fowl coincides with a decrease in cancellous bone volume. Medullary bone modeling was induced in male laying-strain fowl by the administration of estrogen and prevented in females by the administration of tamoxifen. In estrogen-treated males, medullary bone modeling was accompanied by cancellous bone loss; cancellous bone volume was significantly lower than in control males (P < 0.001). In females, the prevention of medullary bone modeling by tamoxifen treatment resulted in significantly higher cancellous bone volumes than in control females (P < 0.001). Estrogen therefore appears to play a role in cancellous bone loss in the fowl.

Expression patterns of chondrocyte genes cloned by differential display in tibial dyschondroplasia

Tibial dyschondroplasia (TD) appears to involve a failure of the growth plate chondrocytes within growing long bones to differentiate fully to the hypertrophic stage, resulting in a mass of prehypertrophic chondrocytes which form the avascular TD lesion. Many biochemical and molecular markers of chondrocyte hypertrophy are absent from the lesion, or show reduced expression, but the cause of the disorder remains to be identified. As differentiation to the hypertrophic state is impaired in TD, we hypothesised that chondrocyte genes that are differentially expressed in the growth plate should show altered expression in TD. Using differential display, four genes, B-cadherin, EF2, HT7 and Ex-FABP were cloned from chondrocytes stimulated to differentiate to the hypertrophic stage in vitro, and their differential expression confirmed in vivo. Using semi-quantitative RT-PCR, the expression patterns of these genes were compared in chondrocytes from normal and TD growth plates. Surprisingly, none of these genes showed the pattern of expression that might be expected in TD lesion chondrocytes, and two of them, B-cadherin and Ex-FABP, were upregulated in the lesion. This indicates that the TD phenotype does not merely reflect the absence of hypertrophic marker genes, but may be influenced by more complex developmental mechanisms/defects than previously thought.

Avian dyschondroplasia: local deficiencies in growth factors are integral to the aetiopathogenesis.

Rabbit polyclonal antibodies to chicken transforming growth factor-beta 3 (TGF-/J3) and to insulin-like growth factor-I (IGF-I) were used to detect the two growth factors in normal and dyschondroplastic broiler growth plates (physes). Histo-morphometry was used to estimate the percentage of chondrocytes containing IGF-I and TGF-beta3 in the lower proliferative, transitional and hypertrophic zones of the growth plate. In the normal chick growth plates IGF-I was present in 63% of transitional chondrocytes and in 67% of hypertrophic chondrocytes and TGF-beta3 was present in 81% of transitional chondrocytes and in 93% of hypertrophic chondrocytes. Both growth factors were found to be deficient within transitional chondrocytes at sites of dyschondroplasia, a condition in which there is a local defect in chondrocyte differentiation and the subsequent replacement of the cartilage by bone. In addition, both growth factors were identified in chondrocytes within areas of repair, where chondrocyte differentiation and endochondral ossification have resumed. This supports the hypothesis that the reduction in TGF-beta3 and IGF-I in dyschondroplasia is integral to the aetiopathogenesis and indicative that both these growth factors are part of the cascade of events associated with chondrocyte differentiation during endochondral ossification.

Cloning differentially regulated genes from chondrocytes using agarose gel differential display

The technique of RNA differential display has been used extensively to clone differentially expressed genes from a wide variety of cells and tissues. Recently, a simplified method of cloning differential display products, separated on agarose gels, was described. Here we report an adaption of this method, using total RNA, to clone differentially expressed genes. The approach is simple and rapid, and requires only small quantities of total RNA. Utilising this approach, we have cloned three differentially regulated genes from chondrocytes stimulated to hypertrophy in vitro, and confirmed their pattern of expression by Northern blotting. These gene fragments were sequenced and found to correspond to known genes, although only one has previously been isolated from chondrocytes.

Porcine osteochondrosis: Deficiencies in transforming growth factor-? and insulin-like growth factor-I

Osteochondrosis and dyschondroplasia are common multifocal disturbances of endochondral ossification in many species of domestic animals, and are characterized by the retention of avascular cartilage. These cartilage disorders are characterized by a failure of chondrocyte differentiation, matrix mineralisation and its replacement by bone. Rabbit polyclonal antibodies to transforming growth factor-beta (TGF-β) and to insulin-like growth factor-I (IGF-I) were used to detect the two growth factors in normal and osteo-chondrotic porcine epiphyses. In the normal pig epiphyses IGF-I and TGF-β were present in the chondrocytes of the epiphyseal hyaline cartilage and IGF-I was readily localised to the hypertrophic chondrocytes in the growth cartilage adjacent to the epiphyseal ossification centre. Both growth factors were found to be deficient in chondrocytes at sites of osteochondrosis. Both these growth factors are thought to be involved in the cascade of events associated with chondrocyte function during endochondral ossification. Deficiencies in TGF-β and IGF-I demonstrated in porcine osteochondrosis and previously shown in avian dyschondroplasia suggest further similarities in the pathogenesis of these conditions.

The development of lameness and bone deformity in the broiler following experimental infection with Mycoplasma gallisepticum or Mycoplasma synoviae

Groups of eight 7-day-old Mycoplasma-free broiler chicks were inoculated with one of two strains of M. gallisepticum (MG/S6, MG/B31/85) or one of two strains of M. synoviae (MS/B31/88 and MS/B94/91) into the left hock joint. Controls were similarly inoculated with sterile mycoplasma broth. The birds were assessed at 2, 3, 4 and 5 weeks of age (1, 2, 3 and 4 weeks post-injection) for lameness and joint and limb lesions. At 5 weeks of age they were killed and examined for gross skeletal abnormalities and microscopic lesions of both hock joints. The in-contact controls showed no joint swelling or lameness. The mycoplasma did not apparently spread to cause gross pathology in the joints of in-contact control birds, or to other joints in a bird injected with mycoplasma. Each mycoplasma strain caused specific pathologies manifest as varying degrees of lameness, left hock joint swelling, bone deformity and differences in the severity of articular cartilage and chondrodystrophic growth plate pathology. Shortening and bowing of the right tibiotarsus in the birds injected with mycoplasma indicated unilateral increased limb loading contributing to bone deformity. Chondrodystrophy seen with three mycoplasmal strains may have contributed to bone deformity in these birds. Lameness appeared to be a consequence of either joint swelling and/or bone deformity resulting in mechanical dysfunction. The role of joint pain in the lameness was not determined.