Chapter 1 fetal lung development

In the presentation we discuss about the stages of prenatal and postnatal lung development

1. FETAL LUNG DEVELOPMENT Reference: PERINATAL and PEDIATRIC RESPIRATORY CARE 3 rd Edition Brian K. Walsh

2. LEARNING OBJECTIVES PHASES OF LUNG DEVELOPMENT Embryonal Phase Pseudoglandular Phase Canalicular Phase Saccular Phase Alveolar Phase POSTNATAL LUNG GROWTH Factors Affecting Prenatal and Postnatal Lung Growth Abnormal Lung Development Pulmonary Hypoplasia Alveolar Cell Development and Surfactant Production Fetal Lung Liquid

3. Fetal lung development is a gradual and progressive process and at birth the lungs have reached only that degree of morphological, physiological, and biochemical maturity required for basic functioning to support extrauterine life. Lung development continuous even after birth. The alveoli develop in increasing numbers until the age of 8 years and increase in size until growth of the chest wall is finished.

4. It’s a complex process of growth occurs after birth, accommodating differing proportions of airway size, alveolar size, and surface area. The full-term infant , with an estimated 50 million alveoli, has the potential to add another 250 million alveoli and increase its total alveolar surface area from approximately 3 to 70 m2 at maturity. More than 40 different cell types, with many different functions, are found in the lungs.

5. Embryonal s tage It includes a primitive lung development , encompass to the first 2 month of gestation. The lung begins to emerge as a bud from the pharynx 26 days after conception, this lung bud elongates and forms trachea and two bronchial buds , which then separated from the esophagus forming a tracheoesophageal septum . Further subdivision occurs in irregular dichotomous ways. The left and right pulmonary artery and vein develops at about 5 weeks of gestation The respiratory epithelium is developed from endoderm referred as foregut bud . The diaphragm also develop during the embryonal stage of lung development. Complete development of diaphragm occurs by approx. week 7 of gestation.

6. Pseudoglandular stage Named after the distinct glandular appearance of the developing lung, extends to week 16 of gestation. Conducting airways continue to develop, extensive subdivision of the conducting airway system. Distal strutures are the terminal bronchioles, which likely to differentiate into respiratory bronchioles and alveolar ducts. Once pattern is laid, the subsequent growth of airways is in size only.

7. STRUTURE DEVELOPMENT GESTATIONAL AGE Cilia appears on surface of epithelium of the trachea and the mainstem bronchi 10 week of gestation Globlet cells appears 13-14 week gestation Submucosal glands arise as solid buds from basal layer of epithelium 15-16 week gestation The development of cartilage 10-24 week gestation Lymphatics appear 8-10 week gestation

8. Canalicular stage Approximately 17 weeks to 26 weeks of gestation This stage is named so because of presence of vascular channels or capillary network around the passages. This capillary development with sufficient surface area adjacent to the airspaces for efficient gas exchange and presence of surfactant is critical for the extra uterine survival of the immature fetus. Pulmonary acinar units are also formed, each acinus consist of respiratory bronchioles, alveolar ducts and alveolar sacs. Differentiation of type I and type II cells . By the end of canalicular stage , the development of air-blood barrier is thin enough to support gas exchange. The survival of the fetus become possible in canalicular stage at 22-24 weeks gestation .

9. Saccular phase Approximately 26 weeks to 36 weeks of gestation. At the beginning of this phase, about 26 weeks of gestation, the terminal structures are referred to as saccules and are relatively smooth-walled, cylindrical structures. They then become subdivided by ridges known as secondary crests. As the crests protrude into the saccules, part of the capillary net is drawn in with them , forming a double capillary layer. During the saccular phase, there is a marked increase in the potential gas-exchanging surface area.

10. Alveolar stage Distinction between the saccular and alveolar phases is somewhat arbitrary. Alveolar maturation and proliferation are primarily a postnatal event, extending beyond birth with rapid growth up to 18 months post gestation. Alveologenesis is characterized by a complex interaction of epithelial, fibroblast, and vascular growth factors with extracellular matrix components. It has been estimated that only 15% to 20% of the adult number of alveoli are present at birth, and thus alveologenesis is largely a postnatal event.

11. Post natal lung development More than 80% of the eventual total number of alveoli—about 300 million—will form after birth. Lung volume will increase 23-fold, alveolar number will increase 6-fold, alveolar surface area will increase 21-fold , and lung weight will increase 20-fold . Lung volume increases disproportionally to alveolar number . At 2 years of age, the number of alveoli varies substantially among individuals. After 2 years of age, males have more alveoli than do females. After the end of alveolar multiplication, the alveoli continue to increase in size until thoracic growth is completed.

12. Alveolar cell development and Surfactant production

13. The primitive alveolar epithelium evolves, into the highly specified type I and type II pneumocytes . Type I cells constitutes 97% of the alveolar surface area, helps in gas diffusion and prevent water and solute leakage. Type II cells produces and stores surfactant. Surfactant reduces surface tension within alveolus.

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