Musculoskeletal System - Axial Skeleton Development
Introduction
During the 3rd week the paraxial mesoderm forms into "balls" of mesoderm paired either side of the neural groove, called somites. Different regions of the somite differentiate into dermomyotome (dermal and muscle component) and sclerotome (forms vertebral column). Vertebral bone is formed through a lengthy process involving endochondrial ossification of a cartilage formed from mesenchyme.
The vertebral body begins as a bony collar that expands into regions of dying cartilage. The bony vertebral arch, enclosing the spinal cord, forms later and the arch remains open dorsally (linked by a ligament) to allow growth of the spinal cord.
The axial skeleton consists of: Skull, Auditory Ossicles, Hyoid bone, Vertebral column, Chest (sternum, ribs)
The vertebral column is a series of bone segments (vertebra) separated by specialized joints (intervertebral disc).
In the adult, the vertebra form rostro-caudally: 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum, coccyx. There has been identified a population variability in the final number of vertebra.
Skeletal ossification continues postnatally, through puberty until mid 20s. Abnormalities of vertebral column development can lead to defects including scoliosis.
Some Recent Findings
|
Textbooks
- The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Moore & Persaud Chapter 15 the skeletal system
- Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Chapter 11 Limb Dev (bone not well covered in this textbook)
- Before we Are Born (5th ed.) Moore and Persaud Chapter 16,17: p379-397, 399-405
- Essentials of Human Embryology Larson Chapter 11 p207-228
Objectives
- Identify the components of a somite and the adult derivatives of each component.
- Give examples of sites of (a) endochondral and (b) intramembranous ossification and to compare these two processes.
- Identify the general times (a) of formation of primary and (b) of formation of secondary ossification centres, and (c) of fusion of such centres with each other.
- Briefly summarise the development of the limbs.
- Describe the developmental abnormalities responsible for the following malformations: selected growth plate disorders; congenital dislocation of the hip; scoliosis; arthrogryposis; and limb reduction deformities.
Development Overview
Below is a very brief overview using simple figures of 3 aspects of early musculoskeletal development. More detailed overviews are shown on other notes pages Mesoderm and Somite, Vertebral Column, Limb in combination with serial sections and Carnegie images.
Mesoderm Development
Cells migrate through the primitive streak to form mesodermal layer. Extraembryonic mesoderm lies adjacent to the trilaminar embryo totally enclosing the amnion, yolk sac and forming the connecting stalk. | |
Paraxial mesoderm accumulates under the neural plate with thinner mesoderm laterally. This forms 2 thickened streaks running the length of the embryonic disc along the rostrocaudal axis. In humans, during the 3rd week, this mesoderm begins to segment. The neural plate folds to form a neural groove and folds. | |
Segmentation of the paraxial mesoderm into somites continues caudally at 1 somite/90minutes and a cavity (intraembryonic coelom) forms in the lateral plate mesoderm separating somatic and splanchnic mesoderm.
Note intraembryonic coelomic cavity communicates with extraembryonic coelom through portals (holes) initially on lateral margin of embryonic disc. | |
Somites continue to form. The neural groove fuses dorsally to form a tube at the level of the 4th somite and "zips up cranially and caudally and the neural crest migrates into the mesoderm. |
Somite Development
Mesoderm beside the notochord (axial mesoderm, blue) thickens, forming the paraxial mesoderm as a pair of strips along the rostro-caudal axis. | |
Paraxial mesoderm towards the rostral end, begins to segment forming the first somite. Somites are then sequentially added caudally. The somitocoel, is a cavity forming in early somites, which is lost as the somite matures. | |
Cells in the somite differentiate medially to form the sclerotome (forms vertebral column) and dorsolaterally to form the dermomyotome. | |
The dermomyotome then forms the dermotome (forms dermis) and myotome (forms muscle).
Neural crest cells migrate beside and through somite. | |
The myotome differentiates to form 2 components dorsally the epimere and ventrally the hypomere, which in turn form epaxial and hypaxial muscles respectively. The bulk of the trunk and limb muscle coming from the Hypaxial mesoderm. Different structures will be contributed depending upon the somite level. |
Embryonic Development
Week 8
Human embryo (Stage 22) vertebra and spinal cord development. Note the structure of the vertebral arch, the dorsal ligament allowing expansion of the arch to accommodate spinal cord growth.
Vertebral Column Changes
Cervical Vertebra
A comparison of vertebral element ossification between different species also in relation to Hox gene expression.[3]
- For each taxon, circles indicate centra, and squares indicate left and right neural arches.
- Colours represent the order of ossification.
- Hox expression boundaries in the mouse (placentals) and vertebral segment identity are shown at right.
- Conserved timing of V7 centrum ossification across mammals, including sloths (bradypus)
- Overlap in Hox5-6 expression in the V6–V9 region of the sloth neck.
bradypus - three-toed sloths are the only members of this genus
dasypus - armadillo genus in the Dasypodidae family
Sacrum
Evolution of the sacrum[4] "In order to study the formation of the sacrum during the primate evolution, a new way of numbering mammalian vertebrae is presented; this demonstrates that the thoracolumbosacral complex is fixed at 22 vertebrae in 80% and at 22 +/- 1 in 100% of the cases. The shift of a vertebra from one type to another occurs either at the thoracolumbar or at the lumbosacral junction and not at the cervicothoracic junction. Rarely does the shift take place at the sacrococcygeal junction. Data from 318 primates reveal that the seven original lumbar vertebrae of the Old World monkeys are reduced in the great apes by a caudad "thoracization" of one to two lumbar vertebrae and a cephalad sacralization of one to four lumbar vertebrae. In the apes, sacralization is not total and different stages that are intermediate between lumbar and sacral are described. In Homo sapiens there is a total sacralization of the last two original lumbar vertebrae. In addition, development of the sacral wings (alae) is minimal in apes and reaches its maximum in hominids. The tendency of the hominoid sacrum to incorporate the last lumbar vertebrae and to widen markedly provides for an enhanced articulation of the sacrum with the ilium and offers a firm base of support for the trunk during erect posture. This is necessary for the support of the weight of the trunk above the sacrum and for the stabilization of the body during bipedal posture and locomotion. Encephalization did not play any major role in the widening of the sacrum since the former by far preceded the latter."
Intervertebral Disc
The adult intervertebral disc (IVD) has to bear the same loads as the vertebra and also have flexibility to allow axial column movement. This is achieved by a complex structure (cartilaginous end-plate) that links the vertebra above and below the disc to a outer dense fibrous structure (annulus fibrosus) containing a gel-like core region (nucleus pulposus). Some research in this area focusses on the degeneration of the IVD with ageing.
- cartilaginous end-plate - that anchor the discs to the adjacent vertebral body bones
- annulus fibrosus - cells are derived from the sclerotome
- nucleus pulposus - cells are derived from the notochord[6]
Annulus Fibrosus
- cells are derived from the sclerotome
Nucleus Pulposus
- cells are derived from the notochord[6]
- notochordal marker brachyury
- proteoglycan rich extracellular matrix
Molecular
Like many other embryonic structures there are two separate considerations:
- Pattern Formation - sclerotome differentiation and segmentation
- Overt Differentiation - mesoderm differentiation to cartilage differentiation and ossification to bone.
Notch
- Links: Notch
Nuclear factor of activated T-cells, cytoplasmic 1
- Transcription factor (Nfatc1) has been identified in developing mouse intervertebral disc (IVD).
- The NFAT family of transcription factors regulates cytokine gene expression by binding to the promoter/enhancer regions of antigen-responsive genes, usually in cooperation with heterologous DNA-binding partners.
- The activation of NFAT proteins is controlled by calcineurin, the calmodulin-dependent phosphatase.
- Links: OMIM - NFATC1
Transforming growth factor beta
Two functions in IVD development:[5]
- prevent chondrocyte differentiation in the presumptive IVD
- promote differentiation of annulus fibrosus from sclerotome
Abnormalities
Spina Bifida
Folic Acid and Neural Tube Defects
Absent Cervical Spine Pedicle
Absent cervical spine pedicle[7]
The vertebral pedicles (Latin, pediculus ="small foot") are paired processes that project dorsally and connect the body of the spinal vertebra to the arch. Absence is a rare abnormality characterized by the absence of a pedicle of the affected vertebral body, seen most frequently at the level C6 followed by the level C5 and C7.
References
- ↑ <pubmed>20544699</pubmed>
- ↑ <pubmed>21193993</pubmed>| PMC3055990 | {http://www.springerlink.com/content/yjn47x15520jn728/fulltext.html Childs Nerv Syst.]
- ↑ 3.0 3.1 <pubmed>20956304</pubmed>| Proc Natl Acad Sci U S A.
- ↑ <pubmed>3688211</pubmed>
- ↑ 5.0 5.1 5.2 <pubmed>20214815</pubmed>| PMC2848151 | BMC Dev Biol.
- ↑ 6.0 6.1 <pubmed>20568241</pubmed>
- ↑ <pubmed>21062465</pubmed>| BMC Med Imaging.
Reviews
<pubmed>21193993</pubmed> <pubmed>19651306</pubmed> <pubmed>19575673</pubmed> <pubmed>19247958</pubmed> <pubmed>18157900</pubmed>
Articles
<pubmed>19590010</pubmed> <pubmed>19520072</pubmed> <pubmed>19395637</pubmed>
Search PubMed
Search Pubmed: Axial Skeleton Development | Vertebra Development | Intervertebral Disc Development | Axial Skeleton Abnormalities
Additional Images
Terms
Glossary Links
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2024, June 14) Embryology Musculoskeletal System - Axial Skeleton Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Musculoskeletal_System_-_Axial_Skeleton_Development
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G