Motor System

Abstract

The development of the spine and spinal cord starts at the third week of gestation. At this stage, the embryo consists of two layers of cells called the blastoderm. On about 15th day of gestation, a groove is formed in the midline of the blastoderm and gradually lengthens. This groove is called the primitive groove. The primitive groove gradually deepens at the rostral end of the embryo and extends to the caudal end. The resulting depression is called the primitive pit, and the cells surrounding the primitive pit are called the primitive knot. The rostral end of the embryo eventually forms at the primitive pit and primitive knot. The entire structure (primitive pit, primitive knot and primitive groove) is called primitive streak. The primitive streak forms the longitudinal axis of the embryo, distinguishing the left and right sides of the embryo. Thus, at the third week of gestation, the embryo develops the rostral/caudal, left/right, and ventral/dorsal direction. The epiblast cells proliferate and migrate through the primitive streak to form a three-germ layer embryo. Epiblast cells migrate to replace the hypoblast cells, forming the endoderm. While the epiblast cells continue to migrate to the area between the epiblast and endoderm to form the mesoderm. At this point, the epiblast cells are also renamed as ectoderm. Two structures are formed at the midline of the mesoderm—the anterior notochordal disc and notochordal process. The notochordal process is originally a hollow mesodermal tube, and it continues to develop into a solid structure called notochord. Notochord induces vertebral body formation, and when the vertebral body is formed around the notochord, nucleus pulposus is formed. After the development of notochord, three structures have been formed in the mesoderm, that is, the paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm. Paraxial mesoderm is located adjacent to the notochord, with its cells able to form somites, which will be differentiate into bone, voluntary muscle, and skin. The intermediate mesoderm differentiates into the urinary and reproductive systems. The lateral plate mesoderm can be divided into ventral and dorsal layers. The ventral cells will differentiate into mesothelial tissues of internal organs and the dorsal layer cells will differentiate into skin and body wall.