Chapter 18: Development of the Back

The complex formation of the vertebral column, a manifestation of the body's segmentation or metamerism, begins with the somites, which develop from paraxial mesenchymal populations situated laterally to the notochord and neural tube. Somite formation, termed somitogenesis, is an autonomous process involving a mesenchymal to epithelial transition driven by a conserved segmentation clock. This process relies on the rhythmic expression of messenger RNAs for signaling pathways, including Notch, Wnt, and fibroblast growth factor (FGF), occurring in a sequential craniocaudal progression. Somite derivatives include the dermomyotome (forming muscle and the dermis of the back) and the sclerotome (forming the axial skeleton). Dorsoventral patterning of the somite is critically regulated by signaling molecules such as Sonic Hedgehog (SHH) from the notochord/floor plate and Wnt factors from the neural tube and overlying ectoderm. Sclerotomal cells undergo a pivotal process called resegmentation, where a single definitive vertebra is constructed from the union of the caudal half of one bilateral sclerotome pair and the cranial half of the next caudal pair, ensuring that spinal nerves exit at the level of the intervertebral disc. The intervertebral discs possess a dual origin: the anulus fibrosus develops from the sclerotomal mesenchyme (perichordal disc), while the nucleus pulposus originates from the localized aggregation and expansion of notochordal tissue. Sclerotomal subcompartments specify different bony fates, contributing to the centrum, neural arches, and ribs. The dermomyotome proliferates cells from its edges to form the myotomes; the epaxial myotome produces the intrinsic dorsal back muscles, innervated by dorsal spinal nerve rami, while the hypaxial myotome gives rise to ventrolateral trunk muscles (intercostal and abdominal) and muscles of the limbs, tongue, and diaphragm. Vertebral development proceeds through blastemal and cartilaginous stages, with primary ossification centers appearing in the centrum and neural arch halves, followed by secondary centers at puberty. The occipitocervical junction, forming from occipital sclerotomes 1 to 4 (contributing to the basiocciput and foramen magnum rim) and cervical sclerotome 1 (forming occipital condyles), is a complex region prone to anomalies such as basilar invagination or Chiari malformation type I (CM-I). Failure of midline fusion of the vertebral arch elements results in spinal dysraphism, commonly known as spina bifida, which includes subtypes like spina bifida occulta and spina bifida cystica (e.g., meningomyelocele), often leading to a complication known as tethering of the spinal cord.