Laryngeal Orifice

Figure 12.3 Various types of esophageal atresia and/or tracheoesophageal fistulae.

A. The most frequent abnormality (90% of cases) occurs with the upper esophagus ending in a blind pouch and the lower segment forming a fistula with the trachea.

B. Isolated esophageal atresia (4% of cases). C. H-type tracheoesophageal fistula (4% of cases). D and E. Other variations (each 1% of cases).

pouch and the lower segment forming a fistula with the trachea (Fig. 12.3 A). Isolated esophageal atresia (Fig. 12.3B) and H-type TEF without esophageal atresia (Fig. 12.3C) each account for 4% of these defects. Other variations (Fig. 12.3, D and E) each account for approximately 1 % of these defects. These abnormalities are associated with other birth defects, including cardiac abnormalities, which occur in 33% of these cases. In this regard TEFs are a component of the VACTERL association (Vertebral anomalies, Anal atresia, Cardiac defects, Tracheoesophageal fistula, Esophageal atresia, Renal anomalies, and Limb defects), a collection of defects of unknown causation, but occurring more frequently than predicted by chance alone.

A complication of some TEFs is polyhydramnios, since in some types of TEF amniotic fluid does not pass to the stomach and intestines. Also, gastric contents and/or amniotic fluid may enter the trachea through a fistula, causing pneumonitis and pneumonia.

Larynx

The internal lining of the larynx originates from endoderm, but the cartilages and muscles originate from mesenchyme of the fourth and sixth pharyngeal

Laryngeal Orifice
Figure 12.4 Laryngeal orifice and surrounding swellings at successive stages of development. A. 6 weeks. B. 12 weeks.

arches. As a result of rapid proliferation of this mesenchyme, the laryngeal orifice changes in appearance from a sagittal slit to a T-shaped opening (Fig. 12.4A). Subsequently, when mesenchyme of the two arches transforms into the thyroid, cricoid, and arytenoid cartilages, the characteristic adult shape of the laryngeal orifice can be recognized (Fig. 12.4B).

At about the time that the cartilages are formed, the laryngeal epithelium also proliferates rapidly, resulting in a temporary occlusion of the lumen. Subsequently, vacuolization and recanalization produce a pair of lateral recesses, the laryngeal ventricles. These recesses are bounded by folds of tissue that differentiate into the false and true vocal cords.

Since musculature of the larynx is derived from mesenchyme of the fourth and sixth pharyngeal arches, all laryngeal muscles are innervated by branches of the tenth cranial nerve, the vagus nerve. The superior laryngeal nerve innervates derivatives of the fourth pharyngeal arch, and the recurrent laryngeal nerve innervates derivatives of the sixth pharyngeal arch. (For further details on the laryngeal cartilages, see Chapter 15.)

Trachea, Bronchi, and Lungs

During its separation from the foregut, the lung bud forms the trachea and two lateral outpocketings, the bronchial buds (Fig. 12.2, B and C). At the beginning of the fifth week, each of these buds enlarges to form right and left main bronchi. The right then forms three secondary bronchi, and the left, two (Fig. 12.5A), thus foreshadowing the three lobes on the right side and two on the left (Fig. 12.5, B and C).

With subsequent growth in caudal and lateral directions, the lung buds expand into the body cavity (Fig. 12.6). The spaces for the lungs, the pericardioperitoneal canals, are narrow. They lie on each side of the foregut (Fig. 10.4)

Laryngeal Orifice
Figure 12.5 Stages in development of the trachea and lungs. A. 5 weeks. B. 6 weeks. C. 8 weeks.

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Pleuro Peritoneal Folds

Lung bud

Pleuro-

pericardial fold

Phrenic nerve

Common cardinal vein

Heart

Figure 12.6 Expansion of the lung buds into the pericardioperitoneal canals. At this stage the canals are in communication with the peritoneal and pericardial cavities. A. Ventral view of lung buds. B. Transverse section through the lung buds showing the pleuropericardial folds that will divide the thoracic portion of the body cavity into the pleural and pericardial cavities.

Lung bud

Pleuro-

pericardial fold

Phrenic nerve

Common cardinal vein

Heart

Figure 12.6 Expansion of the lung buds into the pericardioperitoneal canals. At this stage the canals are in communication with the peritoneal and pericardial cavities. A. Ventral view of lung buds. B. Transverse section through the lung buds showing the pleuropericardial folds that will divide the thoracic portion of the body cavity into the pleural and pericardial cavities.

and are gradually filled by the expanding lung buds. Ultimately the pleuroperi-toneal and pleuropericardial folds separate the pericardioperitoneal canals from the peritoneal and pericardial cavities, respectively, and the remaining spaces form the primitive pleural cavities (see Chapter 10). The mesoderm, which covers the outside of the lung, develops into the visceral pleura. The somatic mesoderm layer, covering the body wall from the inside, becomes the parietal pleura (Fig. 12.6A). The space between the parietal and visceral pleura is the pleural cavity (Fig. 12.7).

Trachea

Pseudoglandular Period
Figure 12.7 Once the pericardioperitoneal canals separate from the pericardial and peritoneal cavities, respectively, the lungs expand in the pleural cavities. Note the visceral and parietal pleura and definitive pleural cavity. The visceral pleura extends between the lobes of the lungs.

During further development, secondary bronchi divide repeatedly in a di-chotomous fashion, forming 10 tertiary (segmental) bronchi in the right lung and 8 in the left, creating the bronchopulmonary segments of the adult lung. By the end of the sixth month, approximately 17 generations of subdivisions have formed. Before the bronchial tree reaches its final shape, however, an additional 6 divisions form during postnatal life. Branching is regulated by epithelial-mesenchymal interactions between the endoderm of the lung buds and splanchnic mesoderm that surrounds them. Signals for branching, which emit from the mesoderm, involve members of the fibroblast growth factor (FGF) family While all of these new subdivisions are occurring and the bronchial tree is developing, the lungs assume a more caudal position, so that by the time of birth the bifurcation of the trachea is opposite the fourth thoracic vertebra.

Maturation of the Lungs (Table 12.1)

Up to the seventh prenatal month, the bronchioles divide continuously into more and smaller canals (canalicular phase) (Fig. 12.8A), and the vascular table 12.1 Maturation of the Lungs

Pseudoglandular period

5-16 weeks

Canalicular period 16-26 weeks

Terminal sac period

Alveolar period

26 weeks to birth

8 months to childhood

Branching has continued to form terminal bronchioles. No respiratory bronchioles or alveoli are present.

Each terminal bronchiole divides into 2 or more respiratory bronchioles, which in turn divide into 3-6 alveolar ducts.

Terminal sacs (primitive alveoli) form, and capillaries establish close contact.

Mature alveoli have well-developed epithelial endothelial (capillary) contacts.

Respiratory bronchiolus

Blood capillaries

Cuboidal epithelium

Respiratory bronchiolus

Blood capillaries

Cuboidal epithelium

Laryngeal Orifice
Terminal bronchiolus

Thin squamous epithelium

Thin squamous epithelium

Laryngeal Oriface

Cuboidal epithelium

Flat endothelium cell of blood capillary

Cuboidal epithelium

Figure 12.8 Histological and functional development of the lung. A. The canalicular period lasts from the 16th to the 26th week. Note the cuboidal cells lining the respiratory bronchioli. B. The terminal sac period begins at the end of the sixth and beginning of the seventh prenatal month. Cuboidal cells become very thin and intimately associated with the endothelium of blood and lymph capillaries or form terminal sacs (primitive alveoli).

supply increases steadily Respiration becomes possible when some of the cells of the cuboidal respiratory bronchioles change into thin, flat cells (Fig. 12.8B). These cells are intimately associated with numerous blood and lymph capillaries, and the surrounding spaces are now known as terminal sacs or primitive alveoli. During the seventh month, sufficient numbers of capillaries are present to guarantee adequate gas exchange, and the premature infant is able to survive.

During the last 2 months of prenatal life and for several years thereafter, the number of terminal sacs increases steadily. In addition, cells lining the sacs,

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Laryngeal Oriface

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Figure 12.9 Lung tissue in a newborn. Note the thin squamous epithelial cells (also known as alveolar epithelial cells, type I) and surrounding capillaries protruding into mature alveoli.

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Figure 12.9 Lung tissue in a newborn. Note the thin squamous epithelial cells (also known as alveolar epithelial cells, type I) and surrounding capillaries protruding into mature alveoli.

known as type I alveolar epithelial cells, become thinner, so that surrounding capillaries protrude into the alveolar sacs (Fig. 12.9). This intimate contact between epithelial and endothelial cells makes up the blood-air barrier. Mature alveoli are not present before birth. In addition to endothelial cells and flat alveolar epithelial cells, another cell type develops at the end of the sixth month. These cells, type II alveolar epithelial cells, produce surfactant, a phospholipid-rich fluid capable of lowering surface tension at the air-alveolar interface.

Before birth the lungs are full of fluid that contains a high chloride concentration, little protein, some mucus from the bronchial glands, and surfactant from the alveolar epithelial cells (type II). The amount of surfactant in the fluid increases, particularly during the last 2 weeks before birth.

Fetal breathing movements begin before birth and cause aspiration of amniotic fluid. These movements are important for stimulating lung development and conditioning respiratory muscles. When respiration begins at birth, most of the lung fluid is rapidly resorbed by the blood and lymph capillaries, and a small amount is probably expelled via the trachea and bronchi during delivery. When the fluid is resorbed from alveolar sacs, surfactant remains deposited as a thin phospholipid coat on alveolar cell membranes. With air entering alveoli during the first breath, the surfactant coat prevents development of an air-water (blood) interface with high surface tension. Without the fatty surfactant layer, the alveoli would collapse during expiration (atelectasis).

Respiratory movements after birth bring air into the lungs, which expand and fill the pleural cavity. Although the alveoli increase somewhat in size, growth of the lungs after birth is due primarily to an increase in the number of respiratory bronchioles and alveoli. It is estimated that only one-sixth of the adult number of alveoli are present at birth. The remaining alveoli are formed during the first 10 years of postnatal life through the continuous formation of new primitive alveoli.

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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