NORMAL DEVELOPMENT OF THE VERTEBRAL COLUMN AND SPINAL CORD
During gestation week 3, pluripotent ectodermal cells proliferate along the surface of the embryo to form a plate that is termed the primitive streak. The cranial end of the primitive streak develops a nodular "mass" of rapidly proliferating cells; this is the primitive node (Hensen node). The primitive node secretes molecules that control the early stages of neural induction. A small depression (the primitive pit or the neuropore) that is contiguous with a groove in the primitive streak develops in the primitive node. At approximately the sixth embryonic day, cells migrate from the primitive node in a cephalad direction to form the notochordal process, which eventually develops into the notochord. The notochord is involved in the formation of the vertebral column.
The notochord induces a thickening of the dorsal ectoderm that results in the formation of the neural plate. By the end of the third week of gestation, the lateral portions of the neural plate are thickened and elevated, thereby forming the neural folds. The thinner midline portion of the plate forms the neural groove. The neural folds progressively bend dorsally along the entire length of the neural groove, eventually fusing in the midline to form the neural tube. The Pax3 gene has a role in neural tube fusion.
Neurulation is the process of spinal cord development in which the neural plate forms from neural ectoderm and then folds into a neural tube. Fusion of the neural folds begins in the upper cervical region and proceeds in both cephalad and caudal directions. As fusion occurs, a continuous ectodermal covering forms over the neural structures. The anterior neuropore is the most cephalad segment of the neural tube; this closes at approximately gestation day 24 at the lamina terminalis. The posterior neuropore is the caudal end of the neural tube; this closes in the region of the future lumbar spine at approximately day 26 or 27.
Separation of the neural tube from the overlying ectoderm is termed disjunction. As this separation is occurring, the overlying ectoderm fuses at the midline, dorsal to the closed neural tube. Perineural mesenchyme migrates into this space between the neural tube and the ectoderm and surrounds the neural tube; this tissue (paraxial mesoderm) eventually forms the meninges, the vertebrae, and the paraspinal muscles.
The distal portion of the spinal cord forms by the processes of canalization and retrogressive differentiation (secondary neurulation). The remnant of the primitive streak at the caudal pole of the embryo gives rise to an aggregate of undifferentiated cells, termed the caudal cell mass. This tissue extends between the posterior neuropore and the cloacal membrane in the tail fold of the embryo. The caudal cell mass gives rise to neural tissue and vertebrae caudal to the posterior neuropore. The resulting solid cord of cells in the lumbosacral ...