I. SKULL (Figure 17-1). The skull can be divided into two parts: the neurocianium and the viscerocranium.
A- Neurocranium. The neurocranium consists of the flat bones of the skull (cranial vault) and the base of the skull. The neurocranium develops from neural crest cells, except for the basilar part of the occipital bone, which forms from mesoderm of the occipital sclerotomes.
B. Viscerocranium. The viscerocranium consists of the bones of the face involving the pharyngeal arches, which have been discussed in Chapter 11. This part develops from neural crest cells, except for the laryngeal cartilage, which forms from mesoderm within pharyngeal arches 4 and 6.
1. During fetal life and infancy, the flat bones of the skull are separated by dense connective tissue (fibrous joints) called sutures. There are five sutures: the frontal suture, sagittal suture, lambdoid suture, coronal suture, and squamosal suture.
2. Sutures allow the flat bones of the skull to deform during childbirth (called molding) and to expand during childhood as the brain grows. Molding may exert considerable tension at the 'obstetric hinge" (junction of the squamous and lateral parts ot the occipital bone) such that the great cerebral vein (of Galen) is ruptured during childbirth.
1. The anterior fontanelle is the largest fontanelle and is readily palpable in the infant. It pulsates because of the underlying cerebral arteries and can be used to obtain a blood sample from the underlying superior sagittal sinus.
2. The anterior fontanelle and the mastoid fontanelles close at about 2 years of age when the main growth of the brain ceases.
3. The posterior fontanelle and the sphenoid fontanelles close at approximately 6 months of age.
1. Abnormalities in the shape of the skull may result from failure of cranial sutures to form or from premature closure of sutures (craniosynostoses). a. Microcephaly results from failure of the brain to grow and is usually associated with mental retardation.
Figure 17-1. Schematic diagram of the newborn skull indicating the neurocranium (lighter shaded area) and the visce-rocranium (darker shaded area). The bones of the neurocranium and viscerocranium are derived almost entirely from neural crest cells, except for the basilar part of the occipital bone (*), which forms from mesoderm of the occipital sclerotomes, and the laryngeal cartilages (A), which form from mesoderm within pharyngeal arches 4 and 6. (Modified from Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 201.)
b. Oxycephaly (turricephaly or acrocephaly) is a tower-like skull caused by premature closure of the lambdoid and coronal sutures. It should be differentiated from Crouzon's syndrome, which is a dominant genetic condition with a presentation quite similar to that of oxycephaly but which is accompanied by malformations of the face, teeth, and ears. C. Plagiocephaly involves an asymmetric skull that is caused by premature closure of the lambdoid and coronal sutures on one side of the skull, d. Scaphocephaly is a long skull (in the anterior/posterior plane) and is caused by premature closure of the sagittal suture.
2. Temporal bone formation a. Mastoid process. This portion of the temporal bone is absent at birth, which leaves the facial nerve (CN VII) relatively unprotected as it emerges from the stylomastoid foramen. In a difficult delivery, forceps may damage CN VII. The mastoid process forms by 2 years of age.
b. Petrosquamous fissure. The petrous and squamous portions of the temporal bone are separated by the petrosquamous fissure, which opens directly into the mastoid antrum of the middle ear. This fissure, which may remain open until 20 years of age, provides a route for the spread of infection from the middle ear to the meninges.
3. The spheno-occipital joint is a site of growth up to approximately 2C years of age. II. VERTEBRAL COLUMN
A. Vertebrae in general (Figure 17-2). Mesodermal cells from the sclerotome migrate and condense around the notochord to form the centrum, around the neural tube to form the vertebral arches, and in the body wall to form the costal processes.
1. The centrum forms the vertebral body.
2. The vertebral arch forms the pedicles, laminae, spinous process, articular processes, and transverse processes.
3. The costal processes form die ribs.
Figure 17-2. Schematic diagram depicting the development of a typical thoracic vertebra. (A) At approximately weeks 5—7, mesodermal cells from the sclerotome demonstrate three distinct condensations: a centrum, a vertebral arch, and a costal process. From 3-5 years of age, the vertebral arches fuse with each other and also fuse with the centrum. Ossification ends when the person is about 25 years of age. (B) In an adult, each condensation develops into distinct components of the adult vertebrae as indicated by the shading. (Modified from Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams Wilkins, 1998, p 204.)
B. The axis (CI) and the atlas (C2) are highly modified vertebrae.
1. The atlas has no vertebral body.
2. The axis has an odontoid process (dens) that represents the vertebral body of the atlas.
C. The sacrum is a large triangular fusion of five sacral vertebrae that forms the posterior/superior wall of the pelvic cavity.
D. The coccyx is a small triangular fusion of four rudimentary vertebrae.
E. Intersegmental position of the vertebrae (Figure 17-3)
1. As mesodermal cells from the sclerotome migrate toward the notochord and neural tube, they split into a cranial portion and a caudal portion. The caudal portion of each sclerotome fuses with the cranial portion of the succeeding sclerotome, which results in the intersegmental position of the vertebrae. The splitting of the sclerotome is important because it allows the developing spinal nerve to have a route of access to the myotome, which it must innervate.
2. In the cervical region, the caudal portion of the fourth occipital sclerotome (04) fuses with the cranial portion of the first cervical (CI) sclerotome to form the base of the occipital bone, which allows the CI spinal nerve to exit between the base of the occipital bone and the CI vertebrae.
1. The primary curves of the spine are the thoracic and sacral curvatures that form during the fetal period.
Future-fl spinal W Figure 17-3. Schematic diagrams nerve depicting the splitting of the sclero- JpHj tome into caudal and cranial portions fl^H as the spinal nerves grow out to inner' iBm vate the myotome. The dotted lines in A indicate where the sclerotome splits, thus allowing the growing spinal ^HB nerve to reach the myotome. (Modified from Dudek RW, Fix J D: DRS Em- A ^P bryology, 2nd ed. Baltimore, Williams
& Wilkins, 1998, p 205.) Sclerotome/ Myotome
2. The secondary curves of the spine are the cervical and lumbar curvatures that form after birth as a result of lifting the head and walking, respectively.
G. Joints of the vertebral column
1. Synovial joints a. The atlanto-occipital joint lies between CI (atlas) and the occipital condyles.
b. The atlanto-axial joint occurs between CI (atlas) and C2 (axis).
C. Facets (zygapophyseal) are joints between the inferior and superior articular facets.
2. Secondary cartilaginous joints (symphyses) are the joints between the vertebral bodies in which the intervertebral disks play a role. An intervertebral disk consists of the nucleus pulposus and the annulus fibrosus.
a. Nucleus pulposus. This is a remnant of the embryonic notochord. By 20 years of age, all notochordal cells have degenerated such that all notochordal vestiges in the adult are limited to just a noncellular matrix.
b. Annulus fibrosus. This is an outer rim of fibrocartilage derived from mesoderm found between the vertebral bodies.
H. Clinical correlations (Figure 17-4)
1. Congenital brevicollis (Klippel-Feil syndrome) results from fusion and shortening of the cervical vertebrae. It is associated with shortness of the neck, a low hairline, and limited motion of the head and neck.
2. Intervertebral disk herniation involves the prolapse of the nucleus pulposus through the defective annulus fibrosus into the vertebral canal. The nucleus pulposus impinges on the spinal roots and results in root pain or radiculopathy.
3- Spina bifida occulta results from failure of the vertebral arches to form or fuse (see Chapter 12).
4. Spondylolisthesis occurs when the pedicles of the vertebral arches fail to fuse with the vertebral body. This allows the vertebral body to move anteriorly with respect to the vertebrae below it, causing loidosis. Congenital spondylolisthesis usually occurs at the L5-S1 vertebral level.
5- Hemivertebrae occur when wedges of vertebrae appear that are usually situated laterally between two other vertebrae.
Coronal and sagittal cleft vertebrae
Coronal and sagittal cleft vertebrae
Figure 17-4, Malformations of the vertebral column. (Modified from Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 207.)
6. Vertebral bar occurs when there is a localized failure of segmentation on one side of the column, usually on the posterolateral site.
7. Block vertebra occurs when there is a lack of separation between two or more vertebrae, usually in the lumbar region.
8. Cleft vertebra occurs when a fissure develops in the vertebra, usually in a coronal or sagittal plane in the lumbar region.
9. Idiopathic scoliosis is a lateral deviation of the vertebral column that involves both deviation and rotation of vertebral bodies.
Was this article helpful?
If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?