Development and functional anatomy of the spine

The vertebral column is composed of alternating vertebrae and intervertebral (IV) discs supported by robust spinal ligaments and muscles. All of these elements, bony, cartilaginous, ligamentous, and muscular, are essential to the structural integrity of the spine. The spine serves three vital functions: protecting the spinal cord and spinal nerves, transmitting the weight of the body, and providing a flexible axis for movements of the head and the torso. The vertebral column is capable of extension, flexion, lateral flexion (side to side), and rotation. However, the degree to which the spine is capable of these movements varies by region. These regions, including the cervical, the thoracic, the lumbar, and the sacrococcygeal spine, form four curvatures (Fig. 2.1). The thoracic and the sacrococcygeal curvatures are established in fetal development, while the cervical and the thoracic curvatures develop during infancy. The cervical curvature arises in response to holding the head upright, while the lumbar curvature develops as an infant begins to sit upright and walk. Congenital defects and degenerative diseases can result in exaggerated, abnormal curvatures. The most common of these include a thoracic kyphosis (or hunchback deformity), a lumbar lordosis (or swayback deformity), and scoliosis. Scoliosis involves a lateral curvature of greater than 10°, often accompanied by a rotational defect. To appreciate the potential underlying causes of scoliosis, we need to understand the cellular and genetic basis of vertebral column and skeletal muscle development from somites. In this chapter, we will review the embryonic development of the spine and associated muscles and link them to the functional anatomy of these structures in the adult.