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Vertebral Column Development

- Discussion:
    - at 2-4th week of development, sclerotome cells arising from paired somites migrate medially & meet in midline around notochord;
           - these sclerotome cells separate the notocord from neural tube dorsally and from gut ventrally;
    - paraxial mesoderm becomes segmented into four cranial and eight cervical somites at 2-3 weeks;
    - sclerotomes form cell masses which forms a condensed layer of cells near its caudal border w/ less condensed mesenchyme both
           cranial and caudal to it;
    - somites each differentiate into cranial and caudal halves, which then reunite with the caudal and cranial halves, respectively,
           of the adjacent somite, forming each provertebra;
    - condensed layer migrates cranially to middle of adjacent myotome & differentiates into the anulus fibrosus of intervertebral discs;
           - notochord eventually constitutes the apical and alar ligaments as well as the nucleus pulposus of each intervertebral disc;
    - caudal portion of each sclerotome fuses w/ superior portion of adjacent sclerotome which will go on to form primordium of vertebral body;;
           - as vertebral body develops, portion of notochord within it becomes thinner and ultimately disappears;
           - between vertebral bodies, notochord that persists goes on to form intervertebral discs;
    - at birth, vertebra consists of three bones joined by cartilage, & osseous union is achieved over a period of years;
    - during 16th year, secondary centers of ossification appear in cartilage that still exists over cranial and caudal
           ends of vertebral bodies;
           - centers of ossification form disc-like epiphyses that unite with bony bodies by the 25th year;

- Development of the Atlas:
    - during formation of atlas, body appears, but it is soon appropriated by axis to form the dens;
    - as consequence, atlas is essentially neural arch that is closed ventrally to form a ring