The Development of the Thoracic Spine
In general the development of the thoracic spine, and in particular its lower part, is ahead of that of either the cervical or the lumbar spine. At 6 weeks the vertebral body with its pedicles is easily identifiable as one cartilaginous anlage (Fig. 6.1). The pedicles grow in a posterolateral direction. All ribs are present as cartilaginous structures in close proximity with, but separated from, body and neural arches by blastemic tissue of uniform density. This interzone becomes more distinct at 7 weeks. By then vertebral bodies have a square shape in the sagittal plane. The relation of ribs to the vertebral bodies can be nicely seen at 8 weeks (Fig. 6.3). The diameter in the sagittal plane of vertebrae is always smaller than that in the frontal plane, as is the case in the lumbar spine. When by 10 weeks vascular invasion of the vertebrae starts, and it does so mostly from the posterior aspect, enchondral ossification of ribs is well on its way (Fig. 6.6). The three-layered interzone of the costotransverse joints is now well seen (Fig. 6.6). At 10 weeks the fusion of the right and left laminae posteriorly has taken place (Fig. 6.7), a fact observed by Bardeen as early as 1905 . Enchondral ossification starts first in the ribs (Fig. 6.6).Shortly after, it takes place in the middle of the body and in both pedicles. Usually ossification is more advanced in the pedicles than in the body. There is only one ossification center in each body. The ossific nucleus increases in size at 12 weeks (Fig. 6.9) and cavitation of the costovertebral and costotransverse joints takes place. At 14 weeks the ossific nucleus nearly occupies the entire vertebra in the sagittal plane (Fig. 6.10). The size of vessels streaming in from the posterior aspect is impressive. The lateral expansion of the ossific nucleus is much slower and far from being complete at 20 weeks (Fig. 6.14) when compared with its anteroposterior extension (Fig. 6.16).
KeywordsCavitation Agglomeration Toluidine Azan
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