FORMATION OF THE LUNG PARENCHYMA & BLOOD VESSELS The pulmonary parenchyma is gradually formed around the airways, which leads to thedescription of four successive periods which overlap according to the stage of developmentand the histological appearance. 1. The pseudoglandular period The pseudoglandular period corresponds to the formation of the airways to the bronchioles(from the 5th to the 17th week). These are lined by a cubic epithelium with, at the level of thesocalled respiratory bronchioles, rare alveoli. 2. The root canal period The canal period corresponds to the appearance of the alveolar canals (from the 17th to the25th week): the numerous alveoli circumscribe the canal, the lumen of which is lined with aflattened epithelium at the level of the thin inter-alveolar partitions. 3. The “terminal bag” period The "terminal sac" period corresponds to the formation of the first alveolar sacs made up ofjuxtaposed alveoli (the bronchial border is no longer present). This period begins around the24th week during which the secretion of surfactant by the alveolar cells (type IIpneumocytes) appears. The number of alveoli and the maturation of the secreted productallow the survival of a premature baby from the 26th week. 4. The alveolar period The alveolar period corresponds to the end of pregnancy and continues beyond birth. It ischaracterized by the formation of definitive alveolar sacs and the progressive increase inlung volume. The very small spaces remaining between the alveoli are occupied by theremains of the mesenchyme which will give fine connective tissue,the interstitium, where theblood vessels run. 5. The ships before the second month Before the second month: At the time of individualization of the respiratory diverticulum, itsvascularization depends on that of the anterior primitive intestine from which it derives. Itconsists of an afferent plexus coming from the ventral branches of the dorsal aortas and anefferent network drained by the branches of the anterior cardinal veins. These networksbranch into the mesenchyme trabeculae during airway branching and parenchymaorganization. 6. During the second month During the second month: the afferent and efferent vessels will change:
The segmentation of the conotruncus leads to the constitution of the trunk of the pulmonaryartery which enters into communication with the proximal part of the sixth left aortic arch.This new afferent path ends at the initial plexus of the respiratory diverticulum of which itbecomes the dominant source constituting the path of the pulmonary arteries. The afferentbranches coming from the dorsal aortas regress, except the most cranial of them whichbecome the bronchial arteries. The dorsal wall of the auricle gives rise to four evaginations (two on the right and two on theleft) which connect with the efferent network of the pulmonary anlage. These evaginationscorrespond to the pulmonary veins which preferentially drain the efferent circulation towardsthe left atrium. Some primitive veins of the anlage persist and become the bronchial veinswhich open into the superior vena cava. From then on, the outline of the respiratory systemis the seat of a double circulation, one passing through the pulmonary vessels, the otherthrough the bronchial vessels. During the rest of the pregnancy this circulation remains onlynourishing, gas exchanges take place at the level of the placenta. 7. At birth At birth: The first cries and respiratory movements of the newborn cause the swelling and unfoldingof the alveoli, which brings the pneumocytes which line their lumen into contact with thewalls of the capillaries located in the interstitium and allows respiratory exchanges. Thepulmonary circulation becomes functional and drains oxygenated blood towards the heart,the bronchial circulation will only have a nourishing role.