The Development of the Lumbar Spine

Abstract

At the age of 6 weeks when the embryo measures 14 mm in length, vertebral body and pedicles are forming a single anlage (Fig. 7.1). A rather rounded shape of the vertebrae seen in the sagittal plane changes during the next few days into a squarer form, more obvious at 7 weeks (Fig. 7.2). The vertebral diameter in the sagittal plane (Fig. 7.3) is always smaller than that in the frontal plane. Progressing chondrocyte maturation of vertebrae and pedicles is seen at 8 weeks. At that stage the mucous streak inside the vertebral bodies is the only remnant of the notochord. Here, most notochordal cells have undergone degeneration. Vascular invasion of the vertebral bodies is seen at 10 weeks and seems to start from the posterior aspect. Chondrocytes in the center of the vertebral bodies and in the pedicles are now in Streeter phase 4. By 12 weeks the vessels have reached the center of the vertebrae where chondrocytes have now degenerated (Streeter phase 5). Thus, enchondral ossification of the lumbar spine starts later than that of the thoracic spine; this first phase of enchondral ossification is starting at three sites simultaneously, that is in the body and in both pedicles. By 14 weeks progression of ossification at all three sites is noted. Cartilage canals are present in all four corners of the vertebra, but no ossification takes place here (Fig. 7.15). Also there is no evidence of chondrocyte hypertrophy. Enchondral bone formation of the vertebral bodies progresses faster in the sagittal plane than in the frontal plane. The center of ossification increases in size with age (Fig. 7.15 and 7.16). By 20 weeks it occupies the entire depth of the body (Fig. 7.17), but both lateral walls still consist of cartilage. Columns of chondrocytes, cranial and caudal to the center of ossification, indicate the beginning of the second phase of enchondral ossification (see Chap. 2). Vascular ingrowth throughout the entire period is always more obvious from the posterior side.