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Studying the developmental history of our intervertebral discs can help us to understand segmentation anomalies (Klippel-Feil, bar formation, etc.) as well as simply the biological construction principle of our spine.
Embryology of the movement segment
The spinal motion segment consists of the intervertebral disc and vertebral joints in the center and vertebral bodies at the ends. The nucleus pulposus, the core of the intervertebral disc tissue, produces proteoglycans to build up hydrostatic pressure. These cells arise from the primitive axial organ, the chorda dorsalis. Paired segmented cell clusters – somites – contribute the cartilage-producing inner layer of the annulus fibrosus from their caudal center (sclerotome). The outer fibrous layer of the annulus fibrosus is formed around this cartilage layer. 1Alkhatib B, Ban GI, Williams S, Serra R. IVD development: nucleus pulposus development and sclerotome specification. Curr Mol Biol Rep. 2018 Sep;4(3):132-141. doi: 10.1007/s40610-018-0100-3. Epub 2018 Jul 13. PMID: 30505649; PMCID: PMC6261384.
More dorsally located cells in the somites (arthrotome) form the vertebral joint at the same level. 2Mittapalli VR, Huang R, Patel K, Christ B, Scaal M. Arthrotome: a specific joint forming compartment in the avian somite. Dev Dyn. 2005 Sep;234(1):48-53. doi: 10.1002/dvdy.20502. PMID: 16028274. Only the environment of these somite cells allows the spinal nerve to grow at the same level3.Agnieszka M. Piatkowska, Susan E. Evans, Claudio D. Stern, Cellular aspects of somite formation in vertebrates, Cells & Development, Volume 168, 2021, 203732, ISSN 2667-2901, https://doi.org/10.1016/j.cdev.2021.203732.
In summary, the central motor organs of our spine are formed from the caudal somite sections.
Embryology of the spinal sheaths
Embryological organs such as the neural tube control the formation of their containers – i.e. meninges and skeletal elements such as calvaria (for the brain) and vertebral arches (spinal cord). Embryologically, these organs develop from the rostral (cf. cranial) somite sections through the formation of pedicels and vertebral arches. The formation of meninges and the components of the peripheral nervous system (spinal ganglion) takes place through the neural crest, which only occurs in vertebrates.4Batarfi, M., Valasek, P., Krejci, E., Huang, R. and Patel, K.(2017) The development and origins of vertebrate meninges.
Biological Communications, 62 (2). pp. 73-81. ISSN 2542-2154 doi:https://doi.org/10.21638/11701/spbu03.2017.203, Available at https://centaur.reading.ac.uk/70692/
The pial (inner) meninges in turn have a significant influence on the positioning of the neurons. 5Dasgupta K, Jeong J. Developmental biology of the meninges. Genesis. 2019 May;57(5):e23288. doi: 10.1002/dvg.23288. Epub 2019 Mar 13. PMID: 30801905; PMCID: PMC6520190.
The integration of all 3 systems, passive movement segment (intervertebral discs, vertebral joints), protective structures (vertebral body, vertebral arch) and control (spinal cord, peripheral nerves) then takes place via the outer section of the somites (myotome), from which the tendinous attached musculature emerges.
Ossification
Cartilaginous precursors of the vertebral bodies can be detected in the human embryo from the 6th week, ossification probably begins from the 8th week in the primary ossification centers in the vertebral body and in the vertebral arches. 6
Ossification of the vertebral column in human foetuses: histological and computed tomography studies
A. Skórzewska, M. Grzymisławska, M. Bruska, J. Łupicka, W. Woźniak, Folia Morphol. Vol. 72, No. 3, pp. 230-238 DOI: 10.5603/FM.2013.0038 Secondary ossification centers are then detectable at the annular apophyses, transverse process and spinous process apophyses into adulthood (25-30Y) 7Knipe H, Chmiel-Nowak M, Hacking C, et al. Ossification centers of the vertebral column. Reference article, Radiopaedia.org (Accessed on 23 Feb 2024) https://doi.org/10.53347/rID-24695
Vascular supply
The nucleus pulposus is the largest completely avascular structure in the human body during the entire lifespan. While good vascularization of the inner annulus fibrosus and the cartilaginous end plates is detectable in the embryo until the first two years of life, these areas retract into the outer annulus in adults. Only in the context of degeneration or focal damage, e.g. in the case of a herniated disc, does a new ingrowth of vessels and nerves occur – possibly through VEGF-mediated signals (→ discogenic pain). 8Fournier DE, Kiser PK, Shoemaker JK, Battié MC, Séguin CA. Vascularization of the human intervertebral disc: A scoping review. JOR Spine. 2020 Sep 15;3(4):e1123. doi: 10.1002/jsp2.1123. PMID: 33392458; PMCID: PMC7770199
