Luisa Vera MD
Médico Radiólogo
Radiologist
Caracas, Venezuela February 17 -2022
DEVELOPMENT OF THE LOWER RESPIRATORY SYSTEM
Development of the lower respiratory tract begins on day 22 and continues to form the trachea, lungs, bronchi, and alveoli. The process divides into five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar stage. Although the process begins early on in fetal development, complete maturation does not take place until the child is approximately 8 years of age. This developmental delay is vital in premature babies where their survival is intricately linked to which developmental stage their respiratory tract has reached at the time of birth.
Embryonic Stage – 3-6 weeks
The respiratory diverticulum appears on the ventral wall of the primitive foregut endoderm, posterior to the pharynx.
Following the fourth week of development, the caudal end of the trachea bifurcates to form the left and right primary bronchial buds which continue to grow into the adjacent layer of splanchnopleuric mesoderm-derived pleural mesenchyme.
At the end of the fifth week, the primary bronchial buds divide asymmetrically to form the secondary bronchial buds, two on the left and three on the right, which will give rise to future lobes of each lung.
At the end of the sixth week of development with the division of the secondary bronchial buds into tertiary bronchial buds on each side which will eventually give rise to the bronchopulmonary segments of the mature lung.
Parietal and visceral pleura of the lungs form from somatopleuric and splanchnopleuric layers of mesoderm respectively during weeks 5 to 7.
Canalicular Stage – 16-25 weeks
This stage marks the division between the conducting and respiratory units in the respiratory tree. Elongation and growth of the existing terminal bronchioles form an acinus composed of respiratory bronchioles, each of which gives rise to 3 to 6 alveolar ducts.
During week 20, lamellar bodies begin to appear in the cytoplasm of the cuboidal type II pneumocytes lining the distal epithelium. Lamellar bodies store pulmonary surfactant, composed of lipids and Surfactant Proteins A-C, before exocytotic release into the alveoli.
Saccular Stage – 24 weeks-birth
During this stage, the gas-exchange surface area of the lungs significantly expands.
Maturation and differentiation of type II pneumocytes into type I pneumocytes results in thin-walled terminal sacs. Capillaries invade the thin walls of the sacculi to form the blood-air barrier composed of type I pneumocytes, thin basement membrane of the capillaries, and endothelium, producing a functional surface for efficient gas exchange.
Pulmonary surfactant production begins at 24 weeks; however, the production of adequate amounts to prevent atelectasis is not until 32 weeks. Therefore infants born after 32 weeks have a much higher chance of survival than those born at 24 weeks.
Alveolar Stage – 36 weeks – 8 years
Prior to birth, immature alveoli appear as bulges from the sacculi which invade the primary septa. As the sacculi continue to increase in size, the protrusions in the primary septa become larger; these new longer and thinner septations are known as secondary septa and are responsible for the final division of the respiratory tree of sacculi into alveoli. Septation occurs at sites where there is increased fibroblast activity and secretion of collagen and elastin fibers into the interstitium.
The process of alveolar division continues until 3 years of age, with the majority of divisions occurring within the first 6 months.
Until the third year of life, enlargement of lungs is a consequence of the increasing number of alveoli; after this point, both the number and size of alveoli increases until the mature lungs form at around 8 years of age.
REFERENCES
1.- Rehman S, Bacha D. Embriology Pulmonary: Treasure Island (FL): StatPearls Publishing; 2022 Jan-.
2.- Kaminsky D. Respiratory system. The Netter Collection of Medical Illustrations. Vol.3 2nd edition
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