Development and Congenital Anomalies of the Axial Skeleton Flashcards
State the source of vertebral column development.
Sclerotome of somites derived from paraxial mesoderm (ventromedial part of somite) [Diagram: parts of a somite]
Describe the development of the vertebral column. Start from sclerotome resegmentation onwards. Cartilaginous and bony stages appear in subsequent cards.
✓ each sclerotome consists of loose mesenchymal cells cranially and dense mesenchymal cells caudally
✓ mesenchymal cells migrate ventrally, surrounding the notochord, forming the vertebral body and IVD
✓ the notochord degenerates and disappears, but between the vertebrae, the notochord expands to form the nucleus pulposus
✓ there’s dorsal migration of some mesenchymal cells where they surround the neural tube, forming the neural arch, the primordium of the vertebral arch
✓ mesenchymal cells in the body wall form the costal process, which form the ribs in the thoracic region
Notes:
In a frontal section of a 4-week embryo, the sclerotomes appear as paired condensations of mesenchymal cells around the notochord.
Briefly discuss chordoma.
Remnants of the notochord may persist and form a chordoma, a rare neoplasm (tumor). Approximately one third of these slowly growing, malignant tumors occur at the base of the cranium and extend to the nasopharynx (the part of the pharynx that lies above the soft palate).
Chordomas infiltrate bone and are difficult to remove. They may also develop in the lumbosacral region.
Briefly describe the cartilaginous stage of vertebral development.
✓ During the sixth week, chondrification centers appear in each mesenchymal vertebra.
✓ At the end of the embryonic period, the two centers in each centrum fuse to form a cartilaginous centrum.
✓ Concomitantly, the centers in the neural arches fuse with each other and the centrum. The spinous and transverse processes develop from extensions of chondrification centers in the neural arch.
✓ Chondrification spreads until a cartilaginous vertebral column is formed.
Ossification of typical vertebrae begins during the embryonic period and usually ends by the 25th year. Write notes on the ossification from the embryonic period upto puberty.
☑ There are two primary ossification centers in the centrum—ventral and dorsal, which soon fuse to form one center.
☑ Three primary ossification centers are present by the eighth week: one in the centrum and one in each half of the neural arch. Ossification becomes evident in the neural arches during the eighth week.
☑ At birth, each vertebra consists of three bony parts connected by cartilage.
☑ The bony halves of the vertebral arch usually fuse during the first 3 to 5 years.
☑ The arches first unite in the lumbar region and union progresses cranially. The vertebral arch articulates with the centrum at cartilaginous neurocentral joints, which permit the vertebral arches to grow as the spinal cord enlarges.
☑ These joints disappear when the vertebral arch fuses with the centrum during the third to sixth years.
Discuss ossification of the vertebrae after puberty.
Five secondary ossification centers appear in the vertebrae after puberty:
✓ One for the tip of the spinous process
✓ One for the tip of each transverse process
✓ Two anular epiphyses, one on the superior rim and one on the inferior rim of the vertebral body.
All secondary centers unite with the rest of the vertebrae at approximately 25 years of age.
Variations in the ossification of vertebrae occur in C1 (atlas), C2 (axis), and C7 vertebrae, as well as in the lumbar vertebrae, sacrum, and coccyx.
The vertebral body is a composite of the anular epiphyses and the mass of bone between them.
State the adult derivatives of:
a) centrum
b) neural arches
c) costal processes
a) vertebral body
b) pedicles, laminae, spine, articular processes, and transverse processes
c) costal elements of the transverse processes (those in the thoracic region form ribs)
a) State the embryological basis of spina bifida.
b) State the types of spina bifida.
a) Failure of fusion of neural arches
b) Types:
[I] Spina bifida occulta: defect is covered by skin and hair
[II] Spina bifida with meningocele: defect covered by skin but meninges herniate as well
[III] Spina bifida with meningomyelocele: defect covered by skin but both meninges and cord herniate
[IV] Spina bifida aperta/with rachischisis: no skin cover
(a) State the embryological basis of hemivertebrae.
(b) Presentation of this anomaly?
(a) Unilateral ossification of vertebral body.
(b) Scoliosis
State the embryological basis of spondylolisthesis.
pedicles of vertebral arch fail to fuse with the vertebral body
Describe Klippel-Feil Syndrome (Brevicollis).
The main features of this syndrome are shortness of the neck, low hairline, restricted neck movements, fusion of cervical vertebral bodies, and abnormalities of the brainstem and cerebellum. In most cases the number of cervical vertebral bodies is fewer than normal because of fusion of vertebrae before birth. In some cases, there is a lack of segmentation of several elements of the cervical region of the vertebral column. The number of cervical nerve roots may be normal, but they are small, as are the intervertebral foramina. Persons with this syndrome may have other birth defects, including scoliosis (abnormal lateral and rotational curvature of the vertebral column) and urinary tract disorders.
[Picture 1] [Picture 2] [Picture 3] [Picture 4: X-ray of normal cervical spine for comparison] [Picture 5]
The ribs develop from ________________.
the mesenchymal costal processes of the thoracic vertebrae
Describe the sequence of events leading to the formation of the ribs.
➤ Mesenchymal cells of sclerotome differentiate into chondrocytes in thoracic region.
➤ From the thoracic vertebrae, develop costochondral process moving anteriorly.
➤ These processes are cartilaginous and therefore undergo endochondral ossification up to the costochondral junction.
➤ The cartilage then extends to attach to the sternum for the first- seventh ribs [TRUE RIBS].
➤ The 8th, 9th and 10th rib have their cartilage attaching to that of rib seven which extends to the sternum [false ribs].
➤ 11th and 12th ribs are left floating in the body cavity [FLOATING RIBS].
List congenital anomalies associated with rib development.
(1) Accessory ribs/supernumerary ribs - Cervical, lumbar ribs
(2) Rib synostosis and bridging
(3) Bifid/Forked ribs
