Fetal development of the transverse atlantis and alar ligaments at the craniovertebral junction

Although the fetal development of the craniovertebral junction has long been of major interest to embryologists from the viewpoint of segmentation, development of the associated ligaments has received scant attention. Using semiserial horizontal sections from 18 embryos and fetuses (six embryos with a crown-rump length (CRL) of 20-26 mm or ~6-7 weeks of gestation; five fetuses with a CRL of 32-58 mm or 8-9 weeks; seven fetuses with a CRL of 90-115 mm or 14-15 weeks) without any abnormalities of cartilage configuration such as atlas assimilation, we studied the ligamentous structures along and around the odontoid process of the axis. The transverse atlantis and alar ligaments originated from a common mesenchymal condensation possibly corresponding to the proatlas segment: the former started to develop slightly earlier than the latter, and the morphologies of both were established at 7 weeks of gestation. Development of the joint cavitation around the odontoid process began in the mid-anterior area at 6 weeks, but was not fully completed even at 15 weeks (115 mm CRL). The presumptive joint cavity expressed vimentin and CD34 and contained abundant CD68-positive macrophages. We always found a mid-anterior joint cavitation facing the basi-occipital, but the embryological meaning remained unclear. The apical ligament appeared most likely to originate from the notochord sheath. The notochord was exposed from the tip of the odontoid process toward the loose epidural tissue and entered the occipital bone, but was difficult to trace to the anterior surface of the basi-occipital.