The clustering of sclerotomal cells to form the bodies of the vertebrae establishes intervertebral fissures that fill with mesenchymal cells to become the intervertebral discs (see Plate 1-3). The notochord in the center of the developing intervertebral disc expands as its cells produce a large amount of mucoid semifluid matrix to form the nucleus pulposus. The mesenchymal cells surrounding the nucleus pulposus produce proteoglycans and collagen fibers to become the fibrocartilage anulus fibrosus of the intervertebral disc. At birth, the nucleus pulposus makes up the bulk of an intervertebral disc. From birth to adulthood, it serves as a shock-absorbing mechanism, but by 10 years of age the notochordal cells have disappeared and the surrounding fibrocartilage begins to gradually replace the mucoid matrix. The water-binding capacity and elasticity of the matrix are also gradually reduced.
The portion of the notochord surrounded by the developing body of a vertebra usually disappears completely before maturity. This is also true of the portions that become incorporated into the body of the sphenoid and the basilar part of the occipital bone. However, the portion of the notochord that normally becomes the nucleus pulposus in the intervertebral discs becomes the apical dental ligament, connecting the dens of the axis with the occipital bone. The dens evolved as an addition to the body of the first cervical vertebra, the atlas, in those reptiles that gave rise to mammals. The most primitive of mammals, the duck-billed platypus and the spiny anteater, have a large atlas body and a dens. In the human embryo, the atlas body and dens become dissociated as a unit from the rest of the atlas and fuse with the body of the second cervical vertebra, the axis (see Plate 1-5). This fusion results in a mature ring-shaped atlas with an anterior arch lacking a body.
At 5 weeks, a prominent tail containing coccygeal vertebrae is present in the human embryo (see Plate 1-3). A free-moving tail is characteristic of most adult vertebrates. However, the human tail is concealed by the growing buttocks and actually regresses to become the coccyx, which consists of four or five rudimentary vertebrae fused together.
At 6 weeks, a pair of bands of mesenchymal cells, the sternal bars, appear ventrolaterally in the body wall (see Plate 1-5). They have no connection with the ribs or with each other, and their formation is independent of any sclerotomal derivatives. After the attachment of the upper ribs to the sternal bars, they fuse together progressively in a craniocaudal direction. At 9 weeks, the union of the bars, which have become cartilaginous, is complete. At the cranial end of the sternal bars, two suprasternal masses form and fuse with the future manubrium to serve as sites where the clavicles articulate. Influenced by the ribs, the cartilaginous body of the sternum becomes secondarily segmented into six sternebrae. Faulty fusion of the sternal bars in the midline results either in a cleft or perforated sternum or in a bifid xiphoid process.
The skeleton of the head consists of three primary components: (1) the capsular investments of the sense organs, (2) the brain case, and (3) the branchial arch skeleton (see Plate 1-6). Other than some exceptions of the branchial arch skeleton, these three primary components unite into a composite mammalian skull.
The notochord originally extends into the head of the embryo as far as the oropharyngeal membrane. Its termination later shifts to the caudal border of the hypophyseal fossa of the sphenoid bone. (The replacement of the notochord in the head region during evolution involved the formation of a cartilaginous cranium similar to that in the primitive fish of the shark type, which had a skeleton composed of only cartilage.) The earliest indication of skull formation in the human embryo is the concentration of mesenchyme about the notochord at the level of the hindbrain during the fifth and sixth weeks (see Plate 1-3). This mesenchymal skull formation extends forward to form a floor for the developing brain. By the seventh week, the skull begins to become cartilaginous as it completely or incompletely encapsulates the organs of olfaction (nasal capsule), vision (orbitosphenoid), and audition and equilibrium (otic capsule). This chondrocranium is essentially roofless.
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