The Early Development of the Spine

Abstract

During early embryonic life (day 20, stage 9) somites start to develop on each side of the neural tube; they will increase in number during the next 2 weeks. The somites will give rise to the development of skeletal muscle and vertebrae [1]. The part which will form the muscle is called the myotome. The part forming the vertebra is called the sclerotome which will grow towards the midline. Between sclerotomes an intersegmental artery is present. This gives rise to a primitive (first) segmentation which will become obvious at stage 12 when the embryo measures 3.5 mm in length. Each segment has been called a primitive vertebra [3], a structure quite different from the definitive vertebra. The most important landmarks in the distinction between primitive and definitive vertebrae are their respective locations, the position of the artery, and the location of vertebral pedicle and nerve root. Whereas in primitive vertebrae the artery appears to separate one vertebra from the other, in the definite vertebra it lies in the middle of the vertebral body. The sclerotomes lie lateral to the neural tube at stage 12. Therefore, the primitive vertebrae and their segmentation are only evident in paraxial sagittal sections. For this reason we prefer to call the primitive vertebra a primitive segment. The aspect of segmentation is heightened by conspicuous skin ridges (Fig. 4.1). At that stage, the cells of the primitive segment are uniform in shape and density. One stage later, cells of the cranial part of the primitive segment are less densely packed than those of the caudal half. Both halves are separated by an intrasclerotomic fissure, well seen at stage 14 [2]. In frontal sections one can appreciate that the alternation in density is only seen lateral to the midline. At the midline the cell density remains uniform (Fig. 4.2).