TRACHEOESOPHAGEAL FISTULA - Before We Are Born

C H A P T E R 11 RESPIRATORY SYSTEM 131

Figure 11–5 The four main varieties of tracheoesophageal fistula (TEF) are shown in order of frequency. Possible directions of the flow of the contents are indicated by arrows. A, Esopha- geal atresia is associated with TEF in more than 85% of cases.

B, Fistula between the trachea and esophagus; this type of birth defect accounts for approximately 4% of cases. C, Atresia of the proximal esophagus ending in a tracheoesophageal fistula with the distal esophagus having a blind pouch. Air cannot enter the distal esophagus and stomach. D, Atresia of the proximal segment of the esophagus with fistulas between the trachea and both the proximal and distal segments of the esophagus. Air can enter the distal esophagus and stomach. All neonates born with TEF have esophageal dysmotility disorders, and most have reflux (regurgitation of contents of the stomach).

Esophageal

atresia Trachea

Fistula

Fistula Esophagus

Trachea

Esophagus Fistula

Figure 11–6 A, Tracheoesophageal fistula in a 17-week fetus.

The upper esophageal segment ends blindly (arrow). (A, From Kalousek DK, Fitch N, Paradice BA: Pathology of the Human Embryo and Previable Fetus. New York, Springer Verlag, 1990.) B, Radiograph of an infant with esophageal atresia. Air in the distal gastrointestinal tract indicates the presence of a tracheoesophageal fistula (arrow, blind proximal esophageal sac).

C H A P T E R 11 RESPIRATORY SYSTEM 131.e1 (B, Courtesy Dr. J. Been, Dr. M. Shuurman, and Dr. S. Robben,

Maastricht University Medical Centre, Maastricht, Netherlands.)

132 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS

right lung and the other connecting to the lower (inferior) lobe. On the left, the two secondary bronchi supply the upper and lower lobes of the lung. Each secondary bronchus undergoes progressive branching.

The segmental bronchi, 10 in the right lung and 8 or 9 in the left lung, begin to form by the seventh week. As this occurs, the surrounding mesenchyme also divides.

Each segmental bronchus, with its surrounding mass of mesenchyme, is the primordium of a bronchopulmonary segment. By 24 weeks, approximately 17 orders of branching have occurred and respiratory bronchioles have developed (Fig. 11-9B). An additional seven orders of airways develop after birth.

As the bronchi develop, cartilaginous plates are formed from the surrounding splanchnic mesenchyme. The bronchial smooth muscle and connective tissue and the pul- monary connective tissue and capillaries are also derived Together with the surrounding splanchnic mesoderm,

the bronchial buds differentiate into the bronchi and their ramifications in the lungs (see Fig. 11-7B). Early in the fifth week, the connection of each bronchial bud with the trachea enlarges to form the primordia of main bronchi (Fig. 11-8).

The embryonic right main bronchus is slightly larger than the left one and is oriented more vertically. This embryonic relationship persists in adults; consequently, a foreign body is more liable to enter the right main bronchus than the left one.

The main bronchi subdivide into secondary bronchi that form lobar, segmental, and intrasegmental branches (see Fig. 11-8). On the right, the superior secondary bronchus supplies the upper (superior) lobe of the lung, whereas the inferior secondary bronchus subdivides into two bronchi, one connecting to the middle lobe of the

Figure 11–7 Illustrations of the growth of the developing lungs into the splanchnic mesoderm adjacent to the medial walls of the pericardioperitoneal canals (primordial pleural cavities). Development of the layers of the pleura is also shown. A, 5 weeks. B, 6 weeks.

Pericardioperitoneal canal Trachea

Visceral pleura Parietal pleura

Splanchnic mesoderm Somatic mesoderm

Pleural cavity Secondary bronchial buds

A B

Figure 11–8 Successive stages in the development of the bronchial buds, the bronchi, and the lungs.

28 days Trachea

B B

E E

C 35 days

42 days

56 days

Right main bronchus Left main bronchus

Mesenchyme

Left secondary bronchus Right secondary bronchus

A. Right upper (superior) lobe B. Right middle lobe C. Right lower (inferior) lobe

D. Left upper (superior) lobe E. Left lower (inferior) lobe Secondary

bronchial buds

C H A P T E R 11 RESPIRATORY SYSTEM 133

exchange. Respiration is not possible; hence, fetuses born during this period are unable to survive.

Canalicular Period (16–25 Weeks)

This period overlaps the pseudoglandular period because cranial segments of the lungs mature faster than caudal segments. During the canalicular period, the lumina of the bronchi and the terminal bronchioles become larger and the lung tissue becomes highly vascular (see Fig.

11-9B). By 24 weeks, each terminal bronchiole has given rise to two or more respiratory bronchioles, each of which then divides into three to six tubular passages—

primordial alveolar ducts.

Respiration is possible toward the end of the canalicular stage because some thin-walled terminal sacs (primordial alveoli) have developed at the ends of the respiratory bronchioles and the lung tissue is well vascularized (ren- dered vascular by formation of new vessels). Although a fetus born at 24 to 26 weeks may survive if given inten- sive care, it often dies because its respiratory and other systems are relatively immature.

from this mesenchyme. As the lungs develop, they acquire a layer of visceral pleura from the splanchnic mesoderm (see Fig. 11-7). With expansion, the lungs and pleural cavities grow caudally into the mesenchyme of the body wall and soon lie close to the heart. The thoracic body wall becomes lined by a layer of parietal pleura derived from the somatic mesoderm (see Fig. 11-7B). The space between the visceral and parietal pleura is the pleural cavity.

Maturation of Lungs

Maturation of the lungs is divided into four histologic stages: pseudoglandular, canalicular, terminal saccular, and alveolar.

Pseudoglandular Period (5–17 Weeks)

The developing lungs resemble, on a histologic basis, an exocrine gland during the early part of this period (see Fig. 11-9A). By 16 weeks, all of the major elements of the lung have formed, except those involved with gas

Figure 11–9 Diagram of histologic sections showing progressive stages of lung development. A and B, Early stages of lung development. In C and D, note that the alveolocapillary membrane is thin and that some capillaries bulge into the terminal saccules.

Connective tissue cells

Capillaries

Connective tissue Terminal bronchiole

Terminal bronchiole Alveolocapillary

membrane

Terminal bronchiole

Terminal sac

Terminal sacs

Respiratory bronchioles

Respiratory

bronchiole Respiratory

bronchiole Squamous epithelium Fibroblasts

Alveolus

Capillary Primordial

alveoli Respiratory

bronchiole Smooth muscle cell Elastin

fiber

A Pseudoglandular period (5–17 weeks) B Canalicular period (16–25 weeks)

C Terminal saccular period (24 weeks–late fetal period) D Alveolar period (late fetal period–8 years) Terminal bronchiole

134 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS

results from a continued increase in the number of respiratory bronchioles and primordial alveoli rather than from an increase in the size of the alveoli. Alveolar development is largely complete by 3 years of age, but new alveoli may be added until approximately 8 years of age.

Unlike mature alveoli, immature alveoli have the poten- tial for forming additional primordial alveoli.

Approximately 150 million primordial alveoli, one half the number in adults, are present in the lungs of full- term neonates. On chest radiographs, therefore, the lungs of neonates appear denser than adult lungs. Between the third and eighth year, the adult complement of 300 million alveoli is achieved.

Three factors that are essential for normal lung development are:

•

Adequate thoracic space for lung growth

•

Adequate amniotic fluid volume

•

Fetal breathing movements

The mechanism modulating lung morphogenesis and formation of blood vessels in the lungs involves the tran- scription factors Sox17 and Wnt signaling.

Fetal breathing movements occur before birth, exert- ing sufficient force to cause aspiration of some amniotic fluid into the lungs. These fetal breathing movements occur approximately 50% of the time and only during rapid eye movement sleep. These movements stimulate

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