Lung Development

Week 6-Highlights: Lungs and digestive system are beginning to take shape, the heart begins to beat, facial components-eyes, nose, mouth and ears begin to form. The whole body is about 1/2 an inch long.

Around week 6 lung development beings to pick up, with the necessary background work done in weeks 4 &5. A unique property of this organ is it doesn’t really have to hurry to become functional, it can take the entire gestational period and beyond (to after birth) to fully develop. This is because the lungs take care of respiration, which of course involves breathing air in and out-something our little developing organism won’t have the chance to do until it is done incubating in the womb. The other major function, but certainly tied to respiration, is gas exchange, which we learned is being covered by the placenta until further notice. Nonetheless the structure of the lungs starts to develop around the 5-6 week mark when an off-growth, the true lung primordium, of the foregut develops. And yes it starts as a tube as most other organs but the lung actually retains its tube-like structure in adult form. The lungs are series of branched tubes starting with the trachea, that branches to the two lungs- each lung is further subdivided into bronchi and these bronchi lead to bronchioles that leave to alveolar sacs and then the individual unit of gas exchange, alveoli. Alveoli complex with the pulmonary veins and arteries that deliver the blood in its two forms (deoxygenated and oxygenated- depending on which vessel it is in and thus which direction it is going) Thus the alveoli complete the pulmonary circuit we discussed in the heart development section. Because the lung has many multi-functional areas it should come as no surprise that all three of the primitive embryonic cell layers contribute different components to the lung- the endoderm gives the respiratory epithelium that lines all the divisions of the lung; the mesoderm gives the blood vessels, smooth muscle, cartilage, and other connective tissue; while the ectoderm gives the nervous system component.

 Around week 6 lung development beings to pick up, with the necessary background work done in weeks 4 &5. A unique property of this organ is it doesn’t really have to hurry to become functional, it can take the entire gestational period and beyond (to after birth) to fully develop. This is because the lungs take care of respiration, which of course involves breathing air in and out-something our little developing organism won’t have the chance to do until it is done incubating in the womb. The other major function, but certainly tied to respiration, is gas exchange, which we learned is being covered by the placenta until further notice. Nonetheless the structure of the lungs starts to develop around the 5-6 week mark when an off-growth, the true lung primordium, of the foregut develops. And yes it starts as a tube as most other organs but the lung actually retains its tube-like structure in adult form. The lungs are series of branched tubes starting with the trachea, that branches to the two lungs- each lung is further subdivided into bronchi and these bronchi lead to bronchioles that leave to alveolar sacs and then the individual unit of gas exchange, alveoli. Alveoli complex with the pulmonary veins and arteries that deliver the blood in its two forms (deoxygenated and oxygenated- depending on which vessel it is in and thus which direction it is going) Thus the alveoli complete the pulmonary circuit we discussed in the heart development section. Because the lung has many multi-functional areas it should come as no surprise that all three of the primitive embryonic cell layers contribute different components to the lung- the endoderm gives the respiratory epithelium that lines all the divisions of the lung; the mesoderm gives the blood vessels, smooth muscle, cartilage, and other connective tissue; while the ectoderm gives the nervous system component.

Source: http://discovery.lifemapsc.com/library/images/lung-anatomy-and-structure

 

Lung development is broken up into several stages-the Embryonic Stage which last up until about the 7th week, Pseudoglandular Stage (weeks 7-16), Canalicular Stage (weeks 16-25), Terminal Sac/Saccular Stage (week 26 until birth) and the Alveolar Stage (eight months until childhood). 

In the Pseudoglandular stage, the functional development of the trachea and lungs are started, with the branching of the bronchial trees and respiratory parenchyma, it also marks the development of acinus (this is another term of classification, the structure that houses the bronchi, alveolar ducts and alveoli). The differentiation of the lung cells will also begin in this stage as the cells are more specialized as we move down to the alveoli. The next stage, the Canalicular stage marks the end of the structure formation, epithelial differentiation takes place now and the air-blood barrier is formed.  The capillary network of the alveoli is set up and the lamellar bodies which will store surfactant are formed. Surfactant is a macromolecular complex of phospholipids and hydrophobic proteins present in the alveoli, the function of this complex is to decrease surface tension and prevent alveolar collapse during exhalation. Surfactant is produced by Type II cells/pneumocytes (these cells are found in the alveoli), while the type I pneumocyte is found in the airways and alveoli as well. Surfactant production begins, in modest amounts, once the type II cells are differentiated, during the Canalicular stage (16 to 25 weeks) but the real quantities are not made until week 35. After the Canalicular stage is the Terminal Sac/Saccular stage which goes from 26 weeks to birth- there is continued development of the alveoli area but even at birth this area is not fully developed, it continues to maturity after birth. The alveolar stage after birth finishes up the task of completing the alveoli for gas exchange.

Source: http://www.ehow.com/how-does_5220795_lung-development-fetus.html