Development of the Respiratory System | Stages of Lung Development | Embryology

Summary

This educational video script delves into the intricate development of the respiratory system, starting from the final structures and tracing back to the embryonic stage. It outlines the formation of the respiratory tree, lungs, and pleural cavities, highlighting the roles of the three germ layers. The script explains the stages of lung maturation, from the pseudoglandular to the alveolar stage, and the crucial transition at birth when the lungs shift from fluid-filled to air-filled, enabling gas exchange. A must-watch for those curious about the complex journey of respiratory system development.

Takeaways

• 🌟 The respiratory system develops from the lower airways, starting from the trachea, which divides into bronchi, bronchioles, and finally into alveoli where gas exchange occurs.
• 🔍 The respiratory tree includes the trachea, bronchi, bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveolar sacs, with the latter two being part of the respiratory zone.
• 🏠 The lungs are situated within the pleural cavities, which are lined by two layers of pleura: the visceral pleura attached to the lung and the parietal pleura attached to the chest wall.
• 🧬 The development of the respiratory system involves the interaction of the three germ layers: ectoderm, mesoderm, and endoderm, with different structures of the airways deriving from these layers.
• 🌱 The respiratory system begins to form at week four of development with the outgrowth of the respiratory diverticulum from the primitive gut tube, leading to the formation of the lung bud.
• 🔄 The trachea and esophagus initially share a connection, which is later separated by the formation of the tracheoesophageal septum, preventing a potential tracheal-esophageal fistula.
• 🌀 The bronchial buds from the lung bud grow into the pericardial peritoneal canals, which eventually form the pleural cavities, with the communication between these cavities closing off through the formation of membranes.
• 🌿 The bronchi further branch into secondary and tertiary bronchi, which supply individual lung lobes and bronchopulmonary segments, respectively.
• 📈 Lung development is divided into four histological stages: the pseudoglandular stage, the canalicular stage, the saccular stage, and the alveolar stage, each characterized by specific developmental milestones.
• 👶 At birth, the transition from fetal life to independent breathing involves the lungs filling with air, a decrease in pulmonary vascular resistance, and the closure of cardiac shunts, enabling gas exchange.
• 📚 Understanding the stages of lung maturation is crucial for recognizing potential developmental issues and the functionality of the respiratory system post-birth.

Q & A

What are the main structures of the respiratory system discussed in the video?

The main structures discussed are the airways (respiratory tree), the lungs, and the pleural cavities.

What is the respiratory tree?

The respiratory tree refers to the branching network of airways starting from the trachea down to the bronchioles and alveolar sacs.

How does the trachea divide and what does it lead to?

The trachea divides into the right and left main primary bronchi, which enter the right and left lungs, respectively.

What is the function of the conducting zone in the respiratory system?

The conducting zone's function is to transport air towards the lower airways without gas exchange.

What is the respiratory zone and where does gas exchange occur?

The respiratory zone includes respiratory bronchioles, alveolar ducts, and alveolar sacs, where gas exchange takes place.

What are the two layers of the pleura and their respective attachments?

The two layers are the visceral pleura, which is attached to the lung, and the parietal pleura, which is attached to the chest wall.

What germ layer does the pleura originate from?

The pleura originates from the mesoderm, one of the three primary germ layers.

What is the respiratory diverticulum and its role in lung development?

The respiratory diverticulum is an outgrowth from the primitive gut tube that forms the lung bud, which eventually develops into the respiratory tree.

What is the tracheoesophageal septum and its importance?

The tracheoesophageal septum is a structure that separates the trachea and esophagus, preventing a tracheal-esophageal fistula.

What are the stages of lung maturation mentioned in the video?

The stages of lung maturation are the pseudoglandular stage, canalicular stage, saccular stage, and alveolar stage.

How does the process of gas exchange begin at birth?

At birth, the baby takes the first breath, filling the lungs with air, resorbing the fluid, reducing pulmonary vascular resistance, and closing cardiac shunts, allowing blood to flow through the lungs for gas exchange.

Outlines

00:00

😀 Development of the Respiratory System Structures

The first paragraph introduces the respiratory system's development, focusing on the final structures such as the respiratory tree, lungs, and pleural cavities. It outlines the airways from the trachea to the alveolar sacs, distinguishing between the respiratory zone for gas exchange and the conducting zone. The paragraph also explains the embryological origins of the airways' walls, including the epithelial lining from the endoderm and other components from the mesoderm. The development process begins at week four with the respiratory diverticulum, which will form the trachea, larynx, and lung bud, and continues with the formation of bronchi and bronchioles. It also touches on the tracheoesophageal septum's role in separating the trachea from the esophagus, a critical development to prevent tracheoesophageal fistula.

05:01

😀 Lung Branching and Maturation Stages

The second paragraph delves into the branching of the respiratory system, detailing the formation of secondary and tertiary bronchi, which supply individual lung lobes separated by fissures. It describes the lung's segmentation into bronchopulmonary segments, with 10 on each side, and the overlapping histological stages of lung development: the pseudoglandular, canalicular, saccular, and alveolar stages. The pseudoglandular stage (5 to 17 weeks) is characterized by the lung's gland-like appearance, while the canalicular stage (16 to 26 weeks) sees the formation of new canals and an increase in capillaries. The saccular stage (26 weeks to birth) involves the formation of primitive alveoli with type 1 and type 2 cells, the latter producing surfactant to reduce surface tension. The alveolar stage (from birth to eight years) is when alveoli mature and proliferate, increasing in number for efficient gas exchange. The paragraph concludes with the physiological changes at birth that enable the lungs to take over gas exchange from the placenta.

10:05

😀 Conclusion and Call to Action

The final paragraph serves as a conclusion to the video, encouraging viewers to like and subscribe for more educational content. It provides a brief recap of the respiratory system's development and invites the audience to engage with the channel for future videos.

Keywords

💡Respiratory System

The respiratory system is the body's mechanism for breathing and gas exchange. It includes organs like the lungs, trachea, and bronchi. In the video, the development of this system is the central theme, discussing how it forms from the trachea to the alveoli, where gas exchange occurs.

💡Airways

Airways refer to the channels through which air passes during respiration. The script describes the respiratory tree, starting from the trachea down to the bronchioles and alveolar sacs, highlighting the conducting and respiratory zones within the airways.

💡Pleural Cavities

Pleural cavities are the spaces in the chest that house the lungs and are lined by the pleura. The video explains that the lungs are encased within these cavities, which are formed by the development of the mesodermal layer during embryogenesis.

💡Germ Layers

Germ layers are the three primary tissue layers in embryonic development: ectoderm, mesoderm, and endoderm. The script mentions these layers in the context of their contribution to the formation of different parts of the respiratory system, such as the epithelial lining from the endoderm and the mesodermal origin of the pleura.

💡Respiratory Diverticulum

The respiratory diverticulum is an outgrowth from the primitive gut tube that eventually gives rise to the lungs. The script describes its formation during week four of embryonic development and how its upper and lower portions contribute to the trachea, larynx, and lung bud.

💡Tracheoesophageal Septum

The tracheoesophageal septum is a structure that separates the trachea and esophagus. The video explains the process of its formation, which is crucial for the proper separation of the respiratory and digestive tracts, and the potential issue of a fistula if this process goes awry.

💡Bronchial Buds

Bronchial buds are the embryonic structures that develop into the bronchi of the lungs. The script details how these buds grow and differentiate into the various branches of the respiratory tree, including the primary, secondary, and tertiary bronchi.

💡Pseudoglandular Stage

The pseudoglandular stage is a histological stage in lung development characterized by the formation of terminal bronchioles with a gland-like appearance. The video describes this stage as occurring between weeks 5 to 17 of gestation, highlighting the importance of mesenchyme and capillary formation.

💡Surfactant

Surfactant is a substance produced by type 2 cells in the lungs that reduces surface tension within the alveoli. The script explains its role in preventing the collapse of small alveoli after birth, when air enters and creates an air-fluid interface.

💡Alveolar Stage

The alveolar stage is the final stage of lung maturation, occurring from birth to eight years of age. The video describes how the primitive alveoli mature and proliferate during this period, increasing in number to facilitate efficient gas exchange.

💡Gas Exchange

Gas exchange is the process by which oxygen is taken into the body and carbon dioxide is expelled. The script explains that this process begins at birth when the lungs fill with air, the pulmonary vascular resistance drops, and the cardiac shunts close, allowing blood to flow through the lungs for the first time.

Highlights

Introduction to the development of the respiratory system with a focus on the final structures.

Description of the respiratory tree, including the trachea, bronchi, bronchioles, and alveolar sacs.

Differentiation between the respiratory zone for gas exchange and the conducting zone for air passage.

Explanation of the pleural cavities and the two layers of pleura: visceral and parietal.

Embryological origins of the respiratory system from the three germ layers: ectoderm, mesoderm, and endoderm.

Development process starting from the primitive gut tube and the formation of the respiratory diverticulum.

Formation of the trachea, larynx, and lung bud from the diverticulum.

Formation of the tracheoesophageal septum and its importance in separating the trachea and esophagus.

Development of the pleural cavities from the pericardial peritoneal canals.

Differentiation of the bronchial buds into bronchi and bronchiolar buds into bronchioles.

Description of the secondary and tertiary bronchi, and their role in supplying individual lung lobes.

Explanation of the histological stages of lung development: pseudoglandular, canalicular, saccular, and alveolar stages.

Importance of type 1 and type 2 cells in the alveoli for gas exchange and surfactant production.

The maturation and proliferation of alveoli from birth to eight years of age.

The transition at birth from fluid-filled lungs to air-filled lungs and the initiation of gas exchange.

The physiological changes at birth that enable the lungs to take over gas exchange from the placenta.

Encouragement to like and subscribe for more educational content on the respiratory system.

Transcripts

hello and welcome to bite size med this

video is on the development of the

respiratory system

so first let's look at the final picture

the structures that are going to develop

and then we'll go backwards and see how

that happens

so what are the structures i'm talking

about the airways that's the respiratory

tree

the lungs and the pleural cavities

so we're going to be looking at the

development from the lower airways

starting from the trachea downwards

the trachea divides into the right and

left main primary bronchi

these are going to enter the right and

left lungs they then divide into

secondary bronchi and tertiary bronchi

the tertiary bronchi divide into

bronchioles which lead into terminal

bronchioles

and then into respiratory bronchioles

each of the respiratory bronchioles

leads into a few alveolar ducts which

then lead into alveolar sacs

so this portion is the respiratory zone

where gas exchange takes place

while the rest is the conducting zone

which just conducts

air towards the lower airways

now that was the tree the lungs are

inside the pleural cavities

that's lined by pleura there are two

layers

one layer is sort of stuck to the lung

inside and that's the visceral pleura

and the one that stores the chest wall

that's the parietal pleura

now the pleura is mesodermal in origin

remember those three germ layers

the ectoderm the mesoderm and the

endoderm

now let's remove the ectoderm these two

they form different

portions the airways have different

structures in their walls

now what's actually in the wall would

differ depending upon what level of

airway we're looking at

overall some of the things that can be

there would be an epithelial lining

there can be smooth muscle cartilage and

of course connective tissue

the epithelial lining is from the

endoderm while all the other stuff is

from the mesoderm

so those are the embryological

derivatives

so now that we know it's going to form

let's look at how they form

we start at week four this is the

primitive gut tube

from the upper portion that's the

cranial portion there's an

outgrowth this is a diverticulum the

respiratory diverticulum

the lower or the caudal portion of this

diverticulum

is going to form the lung bud

the diverticulum is surrounded by

splanchnic mesenchyme

to form all those mesodermal derivatives

we talked about earlier

so the upper portion of this

diverticulum would contribute to the

trachea and the larynx

but the lung bud that's gonna form the

respiratory tree

that's how it is so the bronchial birds

will form bronchi

and the bronchiolar buds form

bronchioles

so from the lung but there's the right

and left primary bronchial buds

but before we go any further you can see

right now the trachea

and the gut tube which is going to form

the esophagus they are currently

connected

now that has to close how for that we

need to flip it over and look at it from

above

so this is the developing trachea in

front and the esophagus behind

there are folds that are going to form

longitudinally

these are the tracheoesophageal folds

they fuse in the midline to form the

tracheoesophageal septum

so now both of them are separate with a

ventral trachea and a dorsal esophagus

now what would happen if something went

wrong here a fistula

the communication remains that's the

tracheal

esophageal fistula so once the septum

forms

the trachea is now separate these

bronchial buds

with their mesenchyme they push into

canals

called the pericardial peritoneal canals

now these canals are going to form the

pleural cavities

so if you look at the name you can see

that they connect the pericardial

and the peritoneal cavities now as the

buds grow into these canals

the communications between these

cavities that has to close

and that happens by folds which then

form membranes

so to separate from the pericardial

cavity there's a pleural pericardial

membrane

and from the peritoneal cavities there's

a plural peritoneal membrane

so that's how they separate but remember

the pleural cavity is lined

so the portion that's sort of attached

to the lung that's the visceral pleura

that will also develop from the splunk

neck mesoderm

the parietal pleura is towards the body

wall so that's from the somatic mesoderm

now the bronchial buds they form the

right and left primary main bronchi

these are going to branch again and form

secondary bronchi

secondary bronchi are low bar bronchi

which means they supply individual lung

lobes

the lungs are divided by fissures

antelopes

there are three on the right and two on

the left so that means there are three

secondary bronchi on the right

and two on the left these secondary

bronchi then branch into tertiary

bronchi

now these are segmental bronchi so they

each supply

a bronchopulmonary segment these

segments are both anatomical and

functional

there are 10 on each side though

sometimes they can fuse on the left so

there might be lesser

and now we're at seven weeks so this

whole thing was during the embryonic

stage of development

now further branching is divided into

stages and these are histological stages

and they overlap

so the division of weeks for each stage

is different in different books

so i just picked the one that repeated

the most and was the easiest

to understand this first we're going to

bring back the respiratory tree

so from the tertiary bronchi we have the

bronchioles and then the terminal

bronchioles

the respiratory bronchioles alveolar

ducts and then alveolar sacs

now we're going to split them into two

each the first

two the next two and the alveolar sacs

the stages overlap so it can't actually

be divided like this

but this is just something i did to make

it easier to remember

the tertiary bronchi they form

bronchiolar butts which give rise to the

bronchioles

and they lead into terminal bronchioles

now remember that these are histological

stages so these are the terminal

bronchioles with their lining

and around that is the mesenchyme which

gives rise to connective tissue

and also importantly is where

capillaries form

so at this point the lung sort of looks

like a gland

so it's called a pseudoglandular stage

and that's from

5 to 17 weeks though i think the easier

to remember one is between

6 and 16 weeks

now the next two steps that's the

respiratory bronchioles

leading into the alveolar ducts this is

the canalicular stitch

so from the terminal bronchioles we have

the respiratory bronchioles

and each respiratory bronchiole leads

into a few alveolar ducts

so new smaller canals are forming and

the canals which were formed in the

previous steps like the bronchi and the

bronchioles

they're going to get wider so it's all

about those canals that's why it's the

quinolicular stitch

and the capillaries that were formed in

the previous stage they are going to

increase

now so this is from around 16

to 26 weeks the alveolar ducts they lead

into sacks called the alveolar sacs

this is the terminal sac stage or the

saccular stage

from 26 weeks to birth these alveoli are

primitive

now the lining flattens so the cells are

flatter these are the type

1 cells why is that important because

they're needed for gas exchange

the capillaries start to come really

close to the alveolus

so it makes sense that the type 1 cells

are thin to make it easier for gas

exchange

there are also type 2 cells now these

form surfactant

that's important because once air enters

the alveoli

there's an air fluid interface that can

create surface tension

which can cause small alveoli to

collapse

surfactant is a phospholipid it lines up

inside each alveolus and reduces the

surface tension

it gets synthesized during this stage

but just synthesis isn't enough

we need a sufficient amount so the

levels increase with gestation

so you think we'll be done here but no

these are primitive allele

and they have to mature and proliferate

we need lots of alveoli

there are millions at birth but we need

millions and millions more

so that's the alveolar stage from birth

to eight years

where they mature and proliferate so

that's the four stages of

lung maturation the pseudoglandular

stage when it looks like a gland

the canalicular stage when it looks more

like a lung

the saccular stage when the alveolar

sacs form

and the alveolar stage when the alveoli

proliferate

and no we're still not done yet for gas

exchange we need circulation as well

so what happens at birth during fetal

life

the lungs are filled with fluid and the

alveoli are collapsed

the pulmonary vascular resistance is

high the heart has shunts

like the ductus arteriosus and the

foramen ovale

to bypass the lungs because the

oxygenation is being done by the

placenta

at birth the baby takes a breath the

lungs fill with air and the fluid gets

resorbed

the pulmonary vascular resistance drops

the cardiac shunts close

and blood flows through the lungs so now

their function of gas exchange begins

and that is how the respiratory system

develops if this video helped you give

it a thumbs up and subscribe to my

channel

thanks for watching and i'll see you on

the next one