Changes in the vertebral column are often noted in skeletal material. Descriptions of these anomalies are often lacking, and their developmental origins are not often discussed. The skeleton of a young woman from the medieval cemetery of Tirup, in Denmark, has multiple defects of the axial skeleton, including extra thoracic and lumbar vertebrae, border shifting, extra ribs, block vertebra, and deformed sternum. This case study is particularly interesting because of the number and diversity of anomalies seen; the rarity of these defects, even in living populations; and her survival to adult age. Careful analysis of the bones and use of the morphogenetic method of determining development stages has led to the conclusion that the initial defect probably occurred very early in development, during blastogenesis, with the initial development of at least two extra somitomeres in the paraxial mesoderm. These extra elements in turn led to problems in union and differentiation, and later chondrification and ossification of the vertebra. The malformations of the vertebrae also induced changes in the ribs and sternum. The initial error of segmentation is identified as a developmental field defect, and the cascade of anomalies seen is a developmental sequence caused by the initial field defect. The genetic and environmental causes of developmental field defects are reviewed.
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