Embryology of Lung & tracheo bronchial tree

Added: 2026-05-28

[00:00:00]So, today we are going to read embryology of lung and tracheobronchial tree.

[00:00:13]Okay?

[00:00:15]So, the development of lung starts starts at fourth week of gestation.

[00:00:32]Embryology was described by Thurlbeck.

[00:00:36]Okay, this was the name of scientist who described the embryology of lung. It was divided into five phases.

[00:00:46]First one is the embryonic period. Okay?

[00:00:53]First one is the embryonic period.

[00:00:56]Second is the pseudoglandular period.

[00:01:02]Pseudoglandular period.

[00:01:06]Third is the canalicular Fourth is saccular.

[00:01:21]And fifth is alveolar.

[00:01:25]Okay? So, these are the five phases. So, the embryonic period in the lung development varies from 0 to 12 weeks.

[00:01:34]First 3 months First 3 months is the embryonic period. Okay? So, what happens in embryonic period? That Suppose this is primitive foregut gut tube. Okay? So, a laryngotracheal diverticulum arises from the primitive foregut tube.

[00:01:58]And from the caudal end of the diverticulum lung buds arise.

[00:02:13]Okay. So, the laryngotracheal This is the laryngotracheal diverticulum which arises from the primitive foregut tube.

[00:02:21]Okay. This is the development which occurs in embryonic period in the early.

[00:02:27]And what happens in the vascular system?

[00:02:32]In the vascular system in the embryonic period double aorta and primordial pulmonary vein exists the heart tube.

[00:02:50]And from the sixth arch pulmonary artery arises.

[00:02:56]The right and the left pulmonary artery arises.

[00:03:01]So, these are the changes which occur in the airway and the vascular system development during the embryonic period in the 0 to 12 weeks. In the airway, as we already have told that there is a formation of laryngotracheal diverticulum along the primitive foregut tube. This laryngotracheal diverticulum towards the 41 that develops a fold from caudal to the cephalic end. This fold is called as tracheoesophageal fold. It forms either gestational age 41 days.

[00:03:42]And defect in this formation of the fold, when there is formation of tracheoesophageal fold, there is formation of tracheoesophageal septum.

[00:03:54]Towards the completion of the tracheoesophageal fold, there is formation of tracheoesophageal septum.

[00:04:01]Okay, and if there is incomplete fusion of this fold, then there occurs tracheoesophageal fistula.

[00:04:10]They are various sizes, congenital TF.

[00:04:14]Its incidence is one in 3,000 or one in 4,500 live births.

[00:04:23]This is the second phase, which is called as pseudoglandular phase. It occurs from 5 to 16 weeks of gestation period. In the airway system, there is formation of secondary and tertiary bronchi.

[00:04:40]And formation of bronchopulmonary segments.

[00:04:46]In parenchyma, there is formation of glandular appearance.

[00:04:55]And differentiation of mesenchymal cells occur in the vascular phase.

[00:05:08]So, in the canalicular phase, it occurs from 17 to 28 weeks of gestational period.

[00:05:16]What happens in this? Lumen of lumen of bronchi and bronchioles become larger.

[00:05:29]There are development of terminal sacs.

[00:05:38]Asinus becomes defined.

[00:05:44]And single tall columnar cells are present.

[00:05:56]And in this phase, type two pneumocytes also produce surfactant. Okay?

[00:06:09]There are migration of capillaries to the air spaces, capillary migration to air spaces at distal bronchi. Okay?

[00:06:21]These things are in the saccular phase.

[00:06:25]It occurs from 28 weeks till birth.

[00:06:29]What happens in saccular phase? More terminal increase number of terminal acinus. Okay?

[00:06:36]Increase number of terminal acinus.

[00:06:39]Saccules become septated.

[00:06:46]And there is formation of alveoli. Okay?

[00:06:50]So, there are 20 to 50 million alveoli present.

[00:06:59]Which is approximately equal to 8% of the adult alveoli.

[00:07:05]Okay?

[00:07:06]So, the last phase we read is the alveolar phase.

[00:07:12]It occurs from birth till eight years of age.

[00:07:19]Formation of squamous epithelium necessary for border of airway lumen.

[00:07:36]There is increased number of alveoli formation.

[00:07:47]>> Okay.

[00:07:58]There are two type of pneumocytes which are present in the canalicular period.

[00:08:09]Okay.

[00:08:10]So, these two pneumocytes are type one pneumocytes and type two pneumocytes.

[00:08:16]The cuboidal cells which are present they dispose most of their material and convert into ultra-thin squamous epithelium.

[00:08:37]Approximate size is 4 micrometer and they are suitable for gas exchange.

[00:08:45]So, the type one pneumocyte participate in the gas exchange and these account for 80 to 90% of alveolar surface area at birth.

[00:08:57]What is type two pneumocyte? Type two pneumocytes, they are in approximate size of 6 micrometer.

[00:09:04]They retain intracellular glycogen and they produce and store produce and store pulmonary surfactant.

[00:09:21]Okay. So, what is surfactant made up of?

[00:09:23]It is made up of phospholipid cholesterol surfactant protein which is A, B, C, D and the surfactant protein which is hydrophobic is B and C.

[00:09:41]B and C are hydrophobic surfactant proteins.

[00:09:45]Okay, so what is the role of surfactant?

[00:09:47]It decreases the surface tension of the alveoli and prevents collapse.