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

BONE DEVELOPMENTMorphogenesis of the avian skeleton involves the developmental cascade of patternformation, establishment of mirror-image bilateral symmetry, initiation and promotion ofendochondral bone differentiation, and growth culminating in functional weight bearinggoverned by the bone morphogenetic proteins in the limb bud in the chick embryo (Reddiand Anderson, 1976; Reddi, 1981). The key steps in the bone morphogenesis are chemotaxis,mitosis of progenitor stem cells, differentiation of cartilage, and replacement by bone (Reddi,2000). After chemotaxis, progenitor cells adher to the matrix as mediated by fibronectin(Weiss and Reddi, 1980). On day 3 of embryo development, the attached cells proliferate,and the daughter cells emerge as chondrocytes around 7 d of incubation. On day 9 thehyperthophic chondrocytes and associated matrix mineralize (Johnston and Comar, 1955).Bone formation is maximal around 11 days of incubation, as indicated by a major increase inalkaline phosphatase that is intimately associated with bone formation and osteoblastdifferentiation. The newly formed ossicle is the site of hematopoietic marrow differentiationat 21 d of incubation (Reddi and Anderson, 1976).Then, phosvitin and lipovitellin, important nutrients for bone development, aggregate inthe yolk spheres (Moran, 2007). Phosvitin has a large amount of phosphate, containing about90% of the yolk protein phosphorus (Burley & Cook, 1961; Perlmann, 1973). Calcium istransferred through yolk sac villi from the vascular system to adhere to the surface of yolkspheres. Calcified granules are not absorbed but accumulate within the yolk sac until piping(Moran, 2007). Dissolution of mammillary knobs adjacent to the chorioallantois-shellmembrane interface represents the dominant source of blood calcium (Abdel-Salam et al.,2006). Thus, endochondral ossification begins in-ovo but primarily occurs inmediately afterhatching (Bain and Watkins, 1993). In fact, Everaert et al. (2008) demonstrated that the shellbecomes weaker during embryonic development, due to calcium mobilization (Vanderstoepand Richards, 1970), which results in the loosening of the shell membranes from the eggshellaround day 18 of incubation (Vanderstoep and Richards, 1970; Johansson et al., 1996).9