Chapter 60. Development of The Lower Respiratory System: The Lungs and Terminal Respiratory Tubes - Review of Medical Embryology Book

Introduction: as the lungs develop, they acquire a layer of visceral pleura from the splanchnic mesenchyme, and with expansion the lungs and pleural cavities grow caudally into the mesenchyme of the body wall and come to lie near the heart. The thoracic cage becomes lined by parietal pleura derived from somatic mesoderm

DURING WEEKS 4 AND 5, the respiratory system roughly mimics that of amphibians (lungs have 2 air sacs); during weeks 7 and 8, when segmental bronchi and bronchopulmonary segments are seen, the lungs resemble those of reptiles. From here, 4 stages of development are described

Pseudoglandular period (weeks 5-17): the developing lung resembles a gland, and during this period, the bronchial divisions are differentiated and the air-conducting system is establishe

By week 17, all elements of the lung are formed-except those related to gas exchang Respiration is not feasible because airways are blind-ending tubules

Canalicular period (weeks 13-25): a time overlap exists because cranial lung segments mature earlier than caudal ones

The lumina of the bronchi and bronchioles get larger, and lung tissue becomes more highly vascularized
By week 24, each terminal bronchiole has formed 2 or more respiratory bronchioles, and respiration is possible by the end of this period since some terminal sacs or primitive alveoli are present at the ends of respiratory bronchioles and vascularization is very good

Terminal sac period (week 24 to birth): many terminal sacs develop, lungs have lost their canalicular appearance, and the sac epithelium becomes thin with capillaries bulging into them

The terminal sacs are lined by a continuous flattened epithelium of entodermal origin and are called type I alveolar epithelial cells
Capillary networks proliferate greatly in the surrounding mesenchyme, and there is a concurrent development of lymphatic capillaries
By weeks 25-28 (1000 g fetus), sufficient terminal sacs are present so that survival of a premature birth is possibl The adequate pulmonary vasculature and the alveolar surface area are the critical considerations
In addition to type I cells, type II alveolar cells (cuboidal cells with a brush border of the terminal sacs) or secretory cells differentiate (weeks 23-24). They secrete surfactant (see later discussion, Section 61 next)

Alveolar period (late fetal to 8 years): terminal sac epithelial lining attenuates to a very thin squamous type

Type I cells become so thin that underlying capillaries bulge into space of each terminal sac
By late fetal period (week 28), the lungs are capable of respiration because alveolar-capillary (respiratory) membrane (blood-gas barrier) is thin enough to allow gas exchange and an adequate amount of surfactant is being produce The lungs begin their vital function at birth
At beginning of this period, respiratory bronchioles terminate in a cluster of thin-walled terminal sacs (future alveolar ducts) separated by loose connective tissu Thus, alveolar ducts probably do not exist before birth
Mature alveoli do not form for some time after birth
At birth, air expands the primitive alveoli slightly and the lungs expan Increase in lung size after birth is really due to an increase in the number of primary alveoli rather than an increase in alveolar siz One-eighth to one-sixth of adult number are seen in the newborn.

From years 3 to 8, the number and size of immature alveoli increase as does the potential to form additional primitive alveol As the primitive alveoli increase in size, they also mature