Viscerocranium

The viscerocranium, which consists of the bones of the face, is formed mainly from the first two pharyngeal arches (see Chapter 15). The first arch gives rise to a dorsal portion, the maxillary process, which extends forward beneath the region of the eye and gives rise to the maxilla, the zygomatic bone, and part of the temporal bone (Fig. 8.6). The ventral portion, the mandibular process,

Mandible Zygomatic Fracture
Figure 8.6 Lateral view of the head and neck region of an older fetus, showing derivatives of the arch cartilages participating in formation of bones of the face.

contains Meckel's cartilage. Mesenchyme around Meckel's cartilage condenses and ossifies by membranous ossification to give rise to the mandible. Meckel's cartilage disappears except in the sphenomandibular ligament. The dorsal tip of the mandibular process, along with that of the second pharyngeal arch, later gives rise to the incus, the malleus, and the stapes (Fig. 8.6). Ossification of the three ossicles begins in the fourth month, making these the first bones to become fully ossified. Mesenchyme for formation of the bones of the face is derived from neural crest cells, including the nasal and lacrimal bones (Fig. 8.3).

At first the face is small in comparison with the neurocranium. This appearance is caused by (a) virtual absence of the paranasal air sinuses and (b) the small size of the bones, particularly the jaws. With the appearance of teeth and development of the air sinuses, the face loses its babyish characteristics.

CLINICAL CORRELATES Craniofacial Defects and Skeletal Dysplasias

Neural Crest Cells

Neural crest cells originating in the neuroectoderm form the facial skeleton and most of the skull. These cells also constitute a vulnerable population as they leave the neuroectoderm; they are often a target for teratogens. Therefore, it is not surprising that craniofacial abnormalities are common birth defects (see Chapter 15).

Cranioschisis

In some cases the cranial vault fails to form (cranioschisis), and brain tissue exposed to amniotic fluid degenerates, resulting in anencephaly. Cranioschisis is due to failure of the cranial neuropore to close. (Fig. 8.7 A). Children with such severe skull and brain defects cannot survive. Children with relatively small defects in the skull through which meninges and/or brain tissue her-niate (cranial meningocele and meningoencephalocele, respectively) (Fig. 8.7B) may be treated successfully. In such cases the extent of neurological deficits depends on the amount of damage to brain tissue.

Craniosynostosis and Dwarfism

Another important category of cranial abnormalities is caused by premature closure of one or more sutures. These abnormalities are collectively known as craniosynostosis, which occurs in 1 in 2500 births and is a feature of over 100 genetic syndromes. The shape of the skull depends on which of the sutures closed prematurely. Early closure of the sagittal suture (57% of cases) results in frontal and occipital expansion, and the skull becomes long and narrow (scaphocephaly) (Fig. 8.8A). Premature closure of the coronal suture results in a short, high skull, known as acrocephaly, or tower skull (Fig. 8.8B). If the coronal and lambdoid sutures close prematurely on one side only, asymmetric

Figure 8.7 A. Child with anencephaly. Cranial neural folds fail to elevate and fuse, leaving the cranial neuropore open. The skull never forms, and brain tissue degenerates. B. Patient with meningocele. This rather common abnormality frequently can be successfully repaired.

craniosynostosis, known as plagiocephaly, results (Fig. 8.8C). Regulation of suture closure involves secretion of various isoforms of transforming growth factor p (TGFP).

One of the exciting breakthroughs in molecular biology and genetics is the discovery of the role of the fibroblast growth factors (FGFs) and fibro-blast growth factor receptors (FGFRs) in skeletal dysplasias. There are nine members of the FGF family and four receptors. Together they regulate cellular events, including proliferation, differentiation, and migration. Signaling is mediated by the receptors, which are transmembrane tyrosine kinase receptors, each of which has three extracellular immunoglobulin domains, a transmembrane segment, and a cytoplasmic tyrosine kinase domain. FGFR-1 and FGFR-2 are coexpressed in prebone and precartilage regions, including craniofacial

Anomaly Skull Acrocephaly

Figure 8.8 A. Child with scaphocephaly caused by early closure of the sagittal suture. Note the frontal and occipital bossing. B. Radiograph of a child with acrocephaly caused by early closure of the coronal suture. C. Child with plagiocephaly resulting from early closure of coronal and lambdoid sutures on one side of the skull (see sutures in Fig. 8.4).

Figure 8.8 A. Child with scaphocephaly caused by early closure of the sagittal suture. Note the frontal and occipital bossing. B. Radiograph of a child with acrocephaly caused by early closure of the coronal suture. C. Child with plagiocephaly resulting from early closure of coronal and lambdoid sutures on one side of the skull (see sutures in Fig. 8.4).

structures; FGFR-3 is expressed in the cartilage growth plates of long bones. In general, FGFR-2 increases proliferation; FGFR-1 promotes osteogenic differentiation; while the role of FGFR-3 is unclear, but expression is increased in the occipital region. Mutations in these receptors, which often involve only a single amino acid substitution, have been linked to specific types of cran-iosynostosis (FGFR-1 and FGFR-2) and several forms of dwarfism (FGFR-3) (Fig. 8.9; and Table 8.1, p. 181). In addition to these genes, mutations in the transcription factor MSX2, a regulator of parietal bone growth, causes Boston type craniosynostosis, which can affect a number of bones and sutures. The TWIST gene codes for a DNA binding protein and plays a role in regulating proliferation. Mutations in this gene result in proliferation and premature differentiation in the coronal suture causing craniosynostosis.

Achondroplasia (ACH), the most common form of dwarfism (1/26,000 live births), primarily affects the long bones. Other skeletal defects include a large skull with a small midface, short fingers, and accentuated spinal curvature (Fig. 8.10). ACH is inherited as an autosomal dominant, and 80% of cases appear sporadically. Thanatophoric dysplasia is the most common neonatal lethal form of dwarfism (1/20,000 live births). There are two types; both are autosomal dominant. Type I is characterized by short, curved femurs with or without cloverleaf skull; type II individuals have straight, relatively long femurs and severe cloverleaf skull caused by craniosynostosis (Fig. 8.11). Hypochon-droplasia, another autosomal dominant form of dwarfism, appears to be a milder type of ACH. In common to all of these forms of skeletal dysplasias are

Figure 8.9 Faces of children with achondroplasia and different types of craniosynosto-sis. A. Achondroplasia. B. Apert syndrome. C. Pfeiffer syndrome. D. Crouzon syndrome. Underdevelopment of the midfacial area (A) is common to all individuals affected with these syndromes.

mutations in FGFR-3 causing abnormal endochondral bone formation so that growth of the long bones and base of the skull is adversely affected.

Acromegaly is caused by congenital hyperpituitarism and excessive production of growth hormone. It is characterized by disproportional enlargement of the face, hands, and feet. Sometimes, it causes more symmetrical excessive growth and gigantism.

Microcephaly

Microcephaly is usually an abnormality in which the brain fails to grow and the skull fails to expand. Many children with microcephaly are severely retarded.

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Responses

  • siiri
    Which of the following gives rise to bone of the face?
    8 years ago
  • nancy
    What is congenital hyperpituitarism?
    8 years ago

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