Similarly, for the cephalochordates, the notochord is essential for locomotion and persists throughout life ( Holland et al.,2004). For the ascidian (tunicate) invertebrate chordates, the notochord exists during embryonic and larval free-swimming stages, providing the axial structural support necessary for locomotion( Satoh, 2003). In higher vertebrates,however, the notochord becomes ossified in regions of forming vertebrae and contributes to the centre of the intervertebral discs in a structure called the nucleus pulposis ( Linsenmayer et al.,1986 Smits and Lefebvre,2003 Swiderski and Solursh,1992). In some vertebrate clades, such as the agnathans (lampreys), and in primitive fish, such as sturgeons, the notochord persists throughout life. Accordingly, the notochord serves as the axial skeleton of the embryo until other elements, such as the vertebrae, form. As a tissue, it is most closely related to cartilage and is likely to represent a primitive form of cartilage. Second,the notochord plays an important structural role. In this role, the notochord is important for specifying ventral fates in the central nervous system,controlling aspects of LR asymmetry, inducing pancreatic fates, controlling the arterial versus venous identity of the major axial blood vessels and specifying a variety of cell types in forming somites( Christ et al., 2004 Danos and Yost, 1995 Fouquet et al., 1997 Goldstein and Fishman, 1998 Lohr et al., 1997 Munsterberg and Lassar, 1995 Pourquie et al., 1993 Yamada et al., 1993 Yamada et al., 1991). Here, it produces secreted factors that signal to all surrounding tissues,providing position and fate information. First, the notochord is positioned centrally in the embryo with respect to both the dorsal-ventral (DV) and left-right (LR) axes. ![]() In higher vertebrates, the notochord exists transiently and has at least two important functions. ![]() The notochord is an embryonic midline structure common to all members of the phylum Chordata ( Fig.
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