CHAPTER I: LITERATURE REVIEW - Repository.lib.ncsu.edu

esulted in the development of this bone torsion (Lanyon and Bourn, 1979). Remodeling andcorrection of malalignments of bones that may have started to grow out of line is hindered bya rapid rate of growth (Whitehead et al., 2003). Birds with severe angulations will walk ontheir hock joints causing ulceration of the overlying skin and sometimes secondary infection.When the deviation becomes so severe that the bird is unable to rise, particularly in the caseof valgus deformity this is called „twisted legs‟ (Thorp, 1992).Tibial DyschondroplasiaTibial dyschondroplasia (TD) was first described by Leach and Nesheim (1965). TDlesion is an unmineralized, unvascularized plug of cartilage that extends from the growthplate into the metaphysis (Leach and Gay, 1987). Dyschondroplasias are most commonlyfound in the proximal tibiotarsus, but has been reported to occur in most long bones(Farquharson and Jefferies, 2000). Tibial dyschondroplasia is most specifically related torapid growth as it is common in meat-type poultry and rare or absent in other birds (Julian,1998). Genetic effects concerning TD were demonstrated after the divergent selection for TDprevalence during 7 years that showed low and high TD-line chickens had 0 and 0.3 TDprevalences, respectively (Leach and Nesheim, 1972). The prevalence of TD in commercialbroiler chicken flocks is greater than 30%, but most TD lesions are subclinical (Riddell,1992).The precise cellular defect that occurs in TD is not completely clear, but it is generallyaccepted that TD is a consequence of an inability of maturing chondrocytes to undergoterminal differentiation, which normally leads to vascularization and mineralization(Farquharson and Jefferies, 2000). Regulators of chondrocyte differentiation involved in thepathogenesis of TD are growth factors such as transforming growth factor-β, insulin-likegrowth factor, and fibroblast growth factor (Loveridge et al., 1993; Thorp et al., 1995;Farquharson and Jefferies, 2000), nuclear receptors for 1,25-dihydroxy-cholecalciferol5