Disturbance of growth of the chest and trunk may lead to disproportion of the body in much the same way as disproportion can be produced by disturbance of limb growth. In addition, disturbed growth of the chest may affect the relationship between the sternum and ribs (sternocostal relationship), producing a variety of pectus deformities and possibly compromising respiratory function.
The ribs develop from the mesenchymal costal processes of the thoracic vertebrae. They become cartilaginous during the embryonic period and later ossify. The original union of the costal process with the vertebra is replaced by a synovial joint. The sternum develops from a pair of mesenchymal sternal bands, which at first are widely separated, and develop ventrolaterally in the body wall, independently of the developing ribs. Chondrification occurs in these bands to form two sternal plates, one on each side of the median plane. The cranial six costal cartilages become attached to them. The plates gradually fuse craniocaudally in the median plane to form cartilaginous models of the manubrium, the sternebrae or segments of the sternal body, and the xiphoid process.
Centers of ossification appear craniocaudally before birth, except for the xiphoid process center of ossification, which appears during childhood.
The development of the vertebral column begins with a precartilagi-nous stage during the fourth week of gestation. Cells from the sclerotomes of the somites are found in three main areas. First, surrounding the noto-chord, some of the densely packed cells give rise to the intervertebral disc. The remaining densely packed cells fuse with the loosely arranged cells of the immediately caudal sclerotome to form the mesenchymal centrum of a vertebra. Thus, each centrum develops from two adjacent sclerotomes and becomes an intersegmental structure. The notochord degenerates and disappears where it is surrounded by the developing vertebral body. Between the vertebrae the notochord expands to form the gelatinous center of the intervertebral disc, called the nucleus pulposus. This nucleus is later surrounded by the circularly arranged fibers of the annulus fibrosus.
Second, cells of the sclerotomes of the somites surround the neural tube and later form the vertebral arch. Third, cells from the sclerotomes of the somites are found in the body wall and form the costal processes, which develop into ribs in the thoracic region. During the sixth week, chondrifi-cation centers appear in each mesenchymal vertebra. Ossification begins during the embryonic period and ends at about the 25th year. Malformations of the axial skeleton include spina bifida occulta, rachis-chisis, accessory or fused ribs, hemivertebrae, and cleft sternum.
Abnormalities of the back may result from congenital malformations of the spine, such as hemivertebrae, or from muscular imbalance, abnormalities of the pelvis, leg length discrepancy, or unusual posture, resulting in lordosis, kyphosis, scoliosis, or a combination of these three.
The appendicular skeleton consists of the pectoral and pelvic girdles and the limb bones. Details of limb bone embryology will not be presented here. The clavicle initially develops by intra-membranous ossification, but it later develops growth cartilages at both ends. The clavicles begin to ossify before any other bones in the body. Formation of the pectoral girdle and the pelvic girdle from the upper and lower limb buds respectively is detailed in Chapter 8.
At about 22 days of gestation, folding of the sides of the embryo produces right and left lateral folds. Each lateral body wall or somatopleure folds toward the midline, rolling the edges of the embryonic disk ventrally and forming a roughly cylindrical embryo. As the lateral and ventral body walls form, part of the yolk sac is incorporated into the embryo as the midgut. After folding, the region of attachment of the amnion to the embryo is reduced to the relatively narrow region—the umbilicus—on the ventral surface. Faulty closure of the lateral body folds during the fifth week produces a large defect in the anterior abdominal wall and results in most of the abdominal viscera developing outside the embryo in a transparent sac of amnion (an omphalocoele). Normally, after the intestines return from the umbilical cord, the rectus muscles approach each other and the linea alba, closing the circular defect. An umbilical hernia differs from an omphalocoele in that the protruding mass is covered by subcutaneous tissue and skin rather than a sac of amnion. The hernia usually does not reach its maximum size until the end of the first month after birth. The defect through which the hernia occurs is in the linea alba.
Gastroschisis is another abdominal wall defect; it is usually sporadic and present as an isolated birth defect. Gastroschisis is a congenital fissure of the abdominal wall. It is most likely caused by a vascular disruption, but primary incomplete folding and formation of the anterior abdominal wall or secondary rupture of the wall have also been proposed. It does not involve the site of insertion of the umbilical cord and usually is accompanied by protrusion of the small intestine and part of the large intestine.
Much information can be obtained about prenatal development by examination of the umbilical cord. The attachment of the umbilical cord is usually near the center of the placenta, but it may be found at any point. As the amniotic sac enlarges, the amnion ensheathes the umbilical cord, forming the cord's epithelial coverings. The umbilical cord usually contains two arteries and one vein surrounded by mucoid connective tissue often called Wharton's jelly. Because the umbilical vessels are longer than the cord, twisting and bending of the vessels is common. The vessels frequently form loops, producing so-called false knots that are of no significance. The umbilical cord is usually 1-2 cm in diameter and 30-90 cm in length (average 55 cm). Growth of the umbilical cord slows after the 28th week of gestation but does not stop before term.
Figure 9.1 Trunk landmarks.
Figure 9.1 Trunk landmarks.
Cord length correlates positively with maternal height, pre-gravid weight, pregnancy weight gain, socioeconomic status, and a male fetus. The finding of a short umbilical cord suggests diminished fetal movement and may be associated with subsequent psychomotor abnormalities. A single umbilical artery is present in approximately 0.5 percent of placentas examined. The presence of a single umbilical artery is thought to correlate with an increased incidence of congenital anomalies. Further details of placental pathology are found in Chapter 16.
Breast development, variation, and measurements are discussed in Chapter 10.
Inspection of the chest should answer the following questions:
1. Is it symmetric?
2. Is one side flatter than the other?
3. What is the shape of the rib cage?
4. Is there evidence for pectus excavatum or pectus carinatum?
5. Is the scapula normal in size and position?
6. Is the scapula elevated (Sprengel's deformity)?
7. Is the spine straight?
8. Is there any evidence for kyphosis, scoliosis, or lordosis?
9. Are the nipples normal?
10. Are they inverted?
11. Are accessory nipples present?
12. Is the umbilicus normal in shape and position?
Examination of the breasts is discussed in Chapter 10. Examination of the genitalia is discussed in Chapter 10. Examination of the skin is described in Chapter 11. Examination of the umbilical cord and umbilical cord length in the newborn is described in Chapter 16.
Following inspection, detailed measurements using the charts in the following pages for comparison should be performed. A glossary of terms used to define specific anomalies of the chest and trunk is included at the end of the book.
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