Anatomy of the Low Back

The anatomy of the low back begins in embryonic development, when the cells within the notochord, mesoderm, and entoderm develop into the vertebral bodies and intervertebral discs. The main structure of the spine and its ancillary structures that support it, involve the vertebral bones, and sacrum and coccyx which are present as individual bones and birth, and become fused by the time we reach physical maturity. We may experience back pain when these bony elements become injured or weakened, or when their ancillary structures that are responsible from moving and protecting the spine become injured. Before we take a closer look at back physiology, let's go back and time and observe some of these structures as they evolve in early fetal development.

Development of the Low Back: Development of the Fetus, Nervous System, and Vertebral Bodies - When the fetus is first fertilized, it exists as one cell, whose DNA is comprised of 50% genetic material from the male, and 50% from the female. The genetic information in this single cell guides the cells growth as it continued to divide and differentiate into specialized organ systems and structures.

A week after fertilization the embryo comes into direct contact with the uterine mucosa and adheres to it. The embryo attaches the uterine wall and begins digesting materials within the endometrium that lines this wall, in order to grow. The embryo burrows further and further into the endometrium as it digests materials from it, and by the second week the embryo becomes completely enclosed inside the uterine epithelium.

The first signs of the embryo existing as separate specialized sections occur with the formation of the flat oval embryonic disc formed from the thickened layers of the germ disc.

The primitive groove occupies the midline in the caudal portion of the embryonic disc. The primitive node, or Hensen's node, is a clump of cells located at the cephalic end of the primitive groove. This node forms a specialized column of cells in the midline, which becomes the notochord. The notochord will later develop into the vertebral column.

Parallel to the long axis of the embryo, the thickened lateral ectoderm rises into rounded edges on each side of the notochord forming the neural groove. The neural groove will develop into the central nervous system, which includes the brain and spinal cord.

By the fourth week of embryonic development the column of mesoderm, which lies on both sides of the notochord, becomes organized into somites or primitive segments which are precursors of body segmentation for the skeletal, muscular and nervous systems.

The notochord continues to divide, and begins to differentiate into individual sections that will later become the vertebrae (spinal bones) and intervertebral discs. The individual sections become known as a caudal mass once upon differentiation. The primitive segments are separated by intersegmental arteries - direct arterial branches of the aorta. As the distinct segments of the vertebral masses develop, each one consists of a dark caudal half and a lighter cephalic half.

The cells closer to the intersegmental arteries receive more nutrition than those located further away from these sources of blood, oxygen, and nutrition. Because of this factor, the cells closer to the intersegmental arteries experience more rapid growth and differentiation to fuse and become the precursor to the vertebral bodies. The dark caudal masses, being further from this blood and nutrient source, remain undifferentiated as the precursor to the intervertebral disc. The largely avascular development of the intervertebral disc continues throughout childhood and adult life, which is the main reason why our discs begin to degenerate by our late 20s and early 30s, and have such a limited ability to heal and regenerate themselves. The intervertebral discs, throughout embryonic development and adult life, have limited nutritional demands and receive maintain its processes as a result of the diffusion of lymph.

Click here to learn more about the development of the spine and vertebral column.