66. Development of The Axial Skeleton

  1. Development of the vertebral column: during week 4, cells of the sclerotome migrate medially to surround the spinal cord and notochord to form a long mesenchymal column. The development takes place in 3 essential stages
    1. THE PRECARTILAGE STAGE: the cells of the sclerotome migrate in 3 directions
      1. Ventromedially to surround the notochor The caudal part of each sclerotome condenses into densely packed cells; the cranial part forms loosely packed cells
        1. Some of the densely packed cells migrate opposite the center of the myotome and give origin to the intervertebral disk
        2. The rest fuse with the loosely arranged cells of the underlying sclerotome to form the mesenchymal centrum (central mass of the body) of the vertebra
          1. Thus, each vertebra develops from 2 adjacent sclerotomes and is referred to as an intersegmental structure
        3. The spinal nerves lie near the intervertebral disks, thus leave the column via the intervertebral foramina
        4. The intersegmental (intercostal) arteries, which at first lie between sclerotomes, pass midway and come to lie on each side of the vertebral bodies
        5. The notochord regresses entirely in the region of the vertebral bodies, persists and enlarges in the region of the intervertebral disks, and undergoes mucoid degeneration to form the gelatinous disk center, the nucleus pulposus
          1. The nucleus is later surrounded by circular fibers, the anulus fibrosus. Thus, the intervertebral disk consists of the nucleus and anulus
      2. Dorsally the mesenchyme covers the neural tube and forms the vertebral arch
      3. Ventrolaterally the mesenchyme forms costal processes (in the thorax form ribs)
    2. CHONDRIFICATION STAGE: during week 6, centers of chondrification are seen in the mesenchymal vertebrae
      1. Two centers in each centrum fuse at the end of the embryonic period to form the cartilaginous centrum. In addition, centers are seen in the vertebral arches, which fuse with each other as well as with the centrum
        1. Extensions of chondrification centers in the vertebral arch give form to the spinous and transverse processes
    3. OSSIFICATION STAGE begins in the embryonic period and ends at about 25 years of age
      1. Prenatal period: 3 primary centers of ossification are seen at the end of the embryonic period: 1 in the centrum and 1 in each half of the vertebral arch. Thus, at birth, each vertebra has 3 bony parts connected by cartilage
      2. Postnatal period: the vertebral arch halves fuse by the first year
        1. The vertebral arch articulates with the centrum at the neurocentral joints, the latter disappearing when the arches finally fuse with the centrum at 3 to 6 years
        2. After puberty, 5 secondary centers are seen: 1 for the tip of each transverse process, 1 for the tip of the spinous process, and 2 annular epiphyses on the upper and lower surfaces of the body of the vertebra
        3. The vertebral body consists of the centrum, parts of the neural arch, and facets for the rib heads. Thus, the centrum and body are not synonymous
        4. All secondary centers unite with the rest of the vertebra by age 25 years
  2. Rib development: from the mesenchymal costal processes of the thoracic vertebrae. They became cartilaginous in embryonic life and ossify later. The vertebral and costal junctions are synovial joints
  3. Development of the sternum: a pair of mesenchymal sternal bands develops ventrolaterally in the body wall, independent of the ribs. Chondrification occurs in the bands and the costal cartilages attach to them. The bands fuse craniocaudally, in the median plane, to form a model for the manubrium, body segments, and xiphoid. Ossification centers appear before birth except for the xiphoid (occurs in childhood)

development of the axial skeleton: image #1