Osteology of vertebrae

Question 1

Describe the development of vertebrae.

Answer 1

• The vertebrae develop from the sclerotome parts of the mesodermal somites.
• The sclerotomes surround the notochord and neural tube in a sheath of mesoderm.
• A series of hyaline cartilaginous rings appears in the mesodermal sheath. * Each ring is formed by fusion of adjacent halves (caudal and cranial) of the original somites. Thus the vertebrae lie not in segments of the body wall, but in the intersegmental planes.
• Each ring ossifies in three centres to form the centrum and the two halves of the neural arch of a vertebra.
• The centre for the centrum is initially double, but the two areas rapidly fuse; failure of one half results in a hemi- vertebra; a series of hemivertebrae is one cause of congenital scoliosis (lateral curvature of the spine).
• By the eighth week of fetal life ossification has commenced in the centrum and the two halves of the neural arch.
• At birth a vertebra consists of three ossifying parts, centrum and two half arches, united by cartilage.
• The half arches unite in the first year after birth, first in the lumbar, then thoracic and finally cervical regions.
• The centra unite with the arches first in the cervical region, about the third year, but in the lumbar region this union is not complete until the sixth year.
• Failure of fusion of the vertebral arches in the midline results in the condition known as spina bifida. When the overlying skin is intact this is qualified as being occult.
• A more severe developmental defect results in a cerebrospinal fluid-filled sac (meningocoele) or a sac containing a part of the spinal cord and nerve roots (myelomeningocoele) being exposed on the surface.
• These defects are more common in the lumbar or lumbosacral region.
• The vertical cylindrical surface of the body is covered with compact bone, but the cancellous bone on the flat upper and lower surfaces remains covered with a layer of hyaline cartilage.
• The epiphyses for the body appear as bony rings, upper and lower, soon after puberty. They are ridged and grooved reciprocally with the margins of the surfaces of the body.
• Fusion of the epiphyseal ring and body occurs in the early twenties.
• Soon after puberty secondary centres appear also at the tip of the spinous process (double in the bifid spines of the cervical vertebrae) and at the tips of the transverse processes of all the vertebrae, and in the mamillary processes of T12 and the lumbar vertebrae. These fuse in the early twenties.
• The costal elements of cervical and lumbar vertebrae do not have a separate bony centre, but ossify by direct extension from the neural arch.
• An occasional centre in the costal element of C7 or L1 vertebra may lead to the formation of a cervical or lumbar rib.
• On the other hand the weight-bearing costal elements of the sacrum have primary ossification centres.
• In general the development of the five segments of the sacrum resembles that of typical vertebrae. After puberty these segments coalesce from below upwards.

Question 2

Describe the development of the atlas vertebra.

Answer 2

• The atlas ossifies in the seventh week of fetal life by a centre in each lateral mass.
• These extend around the posterior arch and unite at the fourth year.
• In the meantime a centre in the anterior arch has appeared at the first year.
• Its junction with the bone of the lateral mass cuts across the anterior part of the upper articular surface; these epiphyses fuse at 7 years. This epiphyseal junction may permanently divide the articular surface.

Question 3

Describe the development of the axis vertebra.

Answer 3

• Each half of the neural arch of the axis ossifies from a primary centre in the second month; a pair of centres for the centrum, which soon fuse, ossify in the fourth month.
• The dens is mainly ossified from a pair of centres which appear in the sixth month and join before birth.
• Thus at birth the axis is in four parts and these fuse with each other in the first few years thereafter; but although the cartilage between the dens and the body ossifies circumferentially, the centre may remain cartilaginous till old age.
• A secondary centre for the tip of the dens appears at about 6 years and unites with the rest of the dens at about 12 years.
• As in the case of a typical vertebra, a secondary centre for the lower surface of the body appears at puberty and fuses in the early twenties.