Figure 8.21 Formation of the vertebral column at various stages of development. A. At the fourth week of development, sclerotome segments are separated by less dense intersegmental tissue. Note the position of the myotomes, intersegmental arteries, and segmental nerves. B. Condensation and proliferation of the caudal half of one sclero-tome proceed into the intersegmental mesenchyme and cranial half of the subjacent sclerotome (arrows in A and B). Note the appearance of the intervertebral discs. C. Precartilaginous vertebral bodies are formed by the upper and lower halves of two successive sclerotomes and the intersegmental tissue. Myotomes bridge the intervertebral discs and, therefore, can move the vertebral column.
vertebra becomes intersegmental. Patterning of the shapes of the different vertebra is regulated by HOX genes.
Mesenchymal cells between cephalic and caudal parts of the original sclerotome segment do not proliferate but fill the space between two precartilaginous vertebral bodies. In this way they contribute to formation of the intervertebral disc (Fig. 8.21 B). Although the notochord regresses entirely in the region of the vertebral bodies, it persists and enlarges in the region of the intervertebral disc. Here it contributes to the nucleus pulposus, which is later surrounded by circular fibers of the annulus fibrosus. Combined, these two structures form the intervertebral disc (Fig. 8.21 C).
Rearrangement of sclerotomes into definitive vertebrae causes the my-otomes to bridge the intervertebral discs, and this alteration gives them the capacity to move the spine (Fig. 8.21 C). For the same reason, interseg-mental arteries, at first lying between the sclerotomes, now pass midway over the vertebral bodies. Spinal nerves, however, come to lie near the intervertebral discs and leave the vertebral column through the intervertebral foramina.
The process of formation and rearrangement of segmental sclerotomes into definitive vertebrae is complicated, and it is fairly common to have two successive vertebrae fuse asymmetrically or have half a vertebra missing, a cause of scoliosis (lateral curving of the spine). Also, the number of vertebrae is frequently more or less than the norm. A typical example of these abnormalities is found in patients with Klippel-Feil anomaly. These patients have fewer than normal cervical vertebrae, and often other vertebrae are fused or abnormal in shape. This anomaly is usually associated with other abnormalities.
One of the most serious vertebral defects is the result of imperfect fusion or nonunion of the vertebral arches. Such an abnormality, known as cleft vertebra (spina bifida), may involve only the bony vertebral arches, leaving the spinal cord intact. In these cases the bony defect is covered by skin, and no neurological deficits occur (spina bifida occulta). A more severe abnormality is spina bifida cystica, in which the neural tube fails to close, vertebral arches fail to form, and neural tissue is exposed. Any neurological deficits depend on the level and extent of the lesion. This defect, which occurs in 1/1000 births, may be prevented, in many cases, by providing mothers with folic acid prior to conception. Spina bifida can be detected prenatally by ultrasound (Fig. 8.22), and if neural tissue is exposed, amniocentesis can detect elevated levels of «-fetoprotein in the amniotic fluid. (For the various types of spina bifida, see Figs. 19.15 and 19.16.)
Ribs and Sternum
Ribs form from costal processes of thoracic vertebrae and thus are derived from the sclerotome portion of paraxial mesoderm. The sternum develops independently in somatic mesoderm in the ventral body wall. Two sternal bands are formed on either side of the midline, and these later fuse to form cartilaginous models of the manubrium, sternebrae, and xiphoid process.
Summary rThe skeletal system develops from mesenchyme, which is derived from the mesodermal germ layer and from neural crest. Some bones, such as the flat bones of the skull, undergo membranous ossification; that is, mesenchyme cells are directly transformed into osteoblasts (Fig. 8.2). In most bones, such as the long bones of the limbs, mesenchyme condenses and forms hyaline cartilage models of bones (Fig. 8.15). Ossification centers appear in these cartilage models, and the bone gradually ossifies by endochondral ossification.
The skull consists of the neurocranium and viscerocranium (face). The neurocranium includes a membranous portion, which forms the cranial vault, and a cartilaginous portion (chondrocranium), which forms the base of the skull. Neural crest cells form the face, most of the cranial vault, and the prechordal part of the chondrocranium (the part that lies rostral to the notochord). Paraxial mesoderm forms the remainder of the skull.
Limbs form as buds along the body wall that appear in the fourth week. Lateral plate mesoderm forms the bones and connective tissue, while muscle cells migrate to the limbs from the somites. The AER regulates limb outgrowth, and the ZPA controls anteroposterior patterning. Many of the genes that regulate limb growth and patterning have been defined (see Fig. 8.16).
The vertebral column and ribs develop from the sclerotome compartments of the somites, and the sternum is derived from mesoderm in the ventral body wall. A definitive vertebra is formed by condensation of the caudal half of one sclerotome and fusion with the cranial half of the subjacent sclerotome (Fig. 8.21).
The many abnormalities of the skeletal system include vertebral (spina bifida), cranial (cranioschisis and craniosynostosis), and facial (cleft palate) defects. Major malformations of the limbs are rare, but defects of the radius and digits are often associated with other abnormalities (syndromes).
Problems to Solve
1. Why are cranial sutures important? Are they involved in any abnormalities?
2. If you observe congenital absence of the radius or digital defects, such as absent thumb or polydactyly, would you consider examining the infant for other malformations? Why?
3. Explain the origin of scoliosis as a vertebral anomaly. What genes might be involved in this abnormality?
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