Embryology of the Vertebral Column

Abstract

The fertilized egg (or first embryonic cell) is described as totipotent because it is capable of giving birth to an entire embryo. Embryonic development is the consequence of the gene activity within the cells and the interactions received by this cell from its environment. The result is changes in shape, movement, proliferation and death, differentiation, and specialization of cells.

During the first 3 weeks after fertilization, different structures will take place into the embryonic disc, leading to a trilaminar embryo. The migration of the epiblastic cells through the primitive streak and the node leads to the formation of the three primordial germ layers: the definitive endoderm which takes the place of the hypoblast, the ectoderm that remains on the surface, and the mesoderm which lies in between. Then, the notochordal process will elongate and develop into the notochord, representing an early version of the future vertebrae and bony skeleton. Ossification begins at the lower thoracic and upper lumbar levels and progresses more rapidly at the level of the vertebral body in caudal vertebrae and at the level of the posterior arch for the cranial vertebrae, finally forming the spinal column. Neural elements will also form and evolve throughout fetal life. At 20 weeks gestation, the conus medullaris is located at L4/L5 or higher. At 40 weeks, it has reached L3, meaning that there is a gradual ascent throughout fetal life.

It is commonly accepted that the fetal spine forms only a single kyphotic curvature and that lumbar lordosis appears with the acquisition of erect posture. This adage was contradicted in the recent literature, revealing that lumbosacral lordosis existed during fetal life. This means that the existence of this curvature could be genetically determined rather than related to the acquisition of erect posture.