Maturation of Lung - Surfactant - Respiratory Distress Syndrome - Respiratory System Embryology

In this visual lecture Dr Aizaz from MedicoVisual talks about Maturation of Lung from Respiratory System Embryology course:

00:00 Intro and Embryonic phase of lung development
2:36 Maturation of lung and Pseudoglandular Phase
04:48 Canalicular Phase
08:27 Saccular Phase
11:12 Alveolar Phase
14:52 Full Course on Respiratory System Embryology
15:21 Quick Review
17:03 Something that angers me
19:36 Surfactant and Respiratory Distress Syndrome

Link to the Full Course: https://www.medicovisual.com/courses/...

Maturation of lung is further subdivided into four more stages that we will discuss in the forthcoming discussion
1. Pseudoglandular phase
In this stage/phase, the tertiary bronchi subdivide repeatedly till very minute sized tubes called terminal bronchioles are formed. All the bronchi and terminal bronchioles are involved in bringing the air to structures beyond terminal bronchioles (that we will just discuss). Those structures are involved gaseous exchange and are called respiratory zone.
Since these structure up till terminal bronchioles are involved in conducting the air to the respiratory zone, they are termed as conducting airways. So, in other words, we can say, that all the conducting airways are completely formed till the end of this stage.
The epithelium of this endodermal lining is simple cuboidal and this branching pattern is similar to formation of exocrine glands. If you look at any two terminal bronchioles arising from a common precursor, they give an appearance of typical tubuloacinar gland. Because during this stage, the lung development falsely appears as exocrine gland on histological sections, this stage is termed as pseudoglandular (false glands) phase.
During this stage, the blood vasculature also develops, albeit modestly and no physical contact between airways and these blood vessels has been established yet.
Premature neonate born in this stage cannot survive
2. Canalicular phase
The terminal bronchiole formed in the previous stage then again undergo a few generations of subdivisions to form the respiratory bronchioles which then subdivides to form alveolar ducts.
Respiratory bronchioles are considered as transition between conducting and respiratory airways. As they mature, in the later stages, they will develop few personal alveoli. So essentially, they are conducting the air to alveolar ducts and alveoli plus they also take part in gaseous exchange via small number of their own alveoli.
Initially these airways (respiratory bronchioles and alveolar ducts) are very narrow. But later in this stage, they widen and lengthen. And while doing so they compress the surrounding mesenchyme and they become closer to some of the blood vessels. In this stage more and more blood vessels develop in the mesenchyme so vasculature is marked at this stage. At point where the endodermal epithelium of the airways meet with the Endothelium of blood vessels, primordial blood-air barrier forms and these endothelial cells induces the epithelium to flatten and form type 1 alveolar epithelium, while some other epithelial cells in this area that remain cuboidal forms the type 2 epithelium. They are involved in secretion of surfactant [discussed later].
The widening of these airways in this stage is also called canalization by some authors which is the reason of naming this stage as canalicular stage. However, some others argue that this stage is called canalicular stage because of establishment of an intimate relationship between the airways and these bloody canals, I mean blood vessels.
3. Saccular phase:
In this stage, sac-like extensions arise from the respiratory bronchi and alveolar ducts. These sacs (also called terminal sacs) are the primordium of alveoli. Unlike the seculars who hate any association of state with religion, these sacs formed in saccular stage love being religiously associated with the surrounding mesenchymal blood vessels, so much so that these vessels partially penetrate the sacs.
These sacs may divide into two sacs and if that happens the precursor sac forms the duct. So, some authors recommend that these saccules should be termed as transitory structures.
4. Alveolar stage
The terminals sacs are surrounded by connective tissue rich in capillaries. These sacs are separated from each other by this connective tissue, termed as primary septas.
Through these primary septas, ridges arise that subdivide these previously formed primordial alveoli into definite alveoli. These ridges are termed as secondary septas. The alveolar stage start at about 32 weeks of gestation and continues till 8 years postnatally. The number of alveoli at birth is 50-150 million but adult lung has about 500 million alveoli.
Most of the alveoli are thought to develop by this process of splitting of pre-existing alveoli.