Anatomy of the Vertebrae
The anatomy of the vertebrae in the human spine includes the development of the notochord into the intervertebral discs and the vertebral masses of the provertebra into the vertebral bodies of the spine.
Before getting further into the anatomy and physiology of the developed spine, let us review the early development of the fetus and nervous system.
During early development, the fetus begins to specialize into sections that include:
The two masses of the provertebra grow, develop, and quickly fuse due to their rich supply of blood and cytoplasm. During this development, the notochord, which innervated all the vertebral bodies in one long cord, begins to become fractured and obliterated. The notochord cells within the vertebral bodies become obliterated and migrate to the intervertebral region. The accumulation of notochord cells develops into the nucleus pulposus, the inner layer (or nucleus) of the intervertebral discs. In the tenth week of fetal development, the cells around the periphery of the intervertebral region begin to differentiate into an elongated fibroblastic type and are arranged along a vertical axis, attaching above and below the vertebral bodies. These cells are formed from the cranial portion of the dark masses and develop into the annulus fibrosus. Thus, the two main sections if the discs develop from two distinct regions. The nucleus pulposus is derived from the notochord, and the annulus from the fibroblastic extensions of the vertebral bodies.
- The notochord, which becomes the central nervous system (the brain and spinal cord)
- The aorta, with intersegmental arteries that branch off of it to nourish the developing nervous system and precursors of the spine and intervertebral discs
- The provertebra, which consist of a dark cephalic half and a lighter caudal halves. Both of these halves will later develop into individual vertebral bodies. The intersegmental arteries supply blood into each of these individual provertebra. The sections of these provertebra closest to the intersegmental arteries will fully develop into the vertebral bodies, as they are richly supplied with oxygen and nutrient carrying blood. The sections of these cell masses furthest from these blood sources will undergo mostly avascular development - into the intervertebral discs. This avascular development of the discs continues throughout fetal and childhood development and into adulthood, accounting for the slow healing of discs that have become injured due to conditions such as herniated discs and degenerative disc disease.
Development and Aging of the Intervertebral Discs: The intervertebral disc undergoes progressive changes throughout fetal development, and at every stage of life. The physical health and cellular composition discs are specifically age related. At birth, the notocordal cells are present within the nucleus pulposus with fine strands of fibrocartilage interlacing the mucoid center. By the age of 4 years the notocordal cells may be present but are increasingly difficult to find. The fibrous element and cartilage cells are increasingly prominent. By the age of 12 years, the notocordal cells have been completely replaced by loose fibrocartilage with abundant gelatinous matrix. This gelatinous matrix gives the discs much of its volume, ability to allow from some movement between the vertebral bones, and ability to cushion the spine against the effects of gravity. As we get older, the fibrocartilage begins to replace the gelatinous mucous material of the nucleus pulposus. This replacement of the gelatinous material with fibrocartilage continues progressively as we get older. Also as we get older, the volume of water the nucleus is able to absorb and retain decreases as well. As the discs age, back pain may develop at levels of the spine have changed their composition to the point that they can no longer separate the vertebral bones they lie between.
The Vertebral Bones and The Vertebral Column: After the development of the individual vertebral masses, and their continued fusion and development, the later fetal spine is formed from 33 total, distinct vertebrae - 7 cervical, 12 thoracic, 5 lumbar, and 4 coccygeal. The sacrum and coccyx each become fused into one combined vertebral mass after birth. An individual vertebral bone (spinal bone) is known as a vertebra, and two or more of these bones are known as vertebrae. The vertebrae of the 7 cervical, thoracic, and lumbar spine are known as "true vertebrae" because they remain distinct elements throughout life, and the sacral and coccygeal elements are known as "false vertebrae", or pseudo vertebrae, because of their fusion after birth. The bodies of the true vertebrae are separated by each other by the discs, with the exception of the first and second vertebrae. The first cervical vertebra articulates (meets up with) the occipital bone of the skull. The first two cervical vertebrae, the Atlas and Axis, connect directly with one another, as the Dens (upward projection of the Axis) is received by the Atlas in its posterior arch.