Embryology of Respiratory System

Question 1

Hilum of the Lung

Answer 1

Root of lung; where structures enter and leave the lung

e.g. Pulmonary arteries and veins/bronchi

Question 2

When and where does the respiratory diverticulum form in a foetus

Answer 2

From around 4 weeks, a little lung bud (pouch) forms on the ventral wall of the foregut

It is behind the heart and superior to the stomach and developing liver

Question 3

What germ layers are the lower respiratory system from (larnx, trachea, bronchi and lungs)

Answer 3

Endoderm as the respiratory diverticulum comes from the foregut

Question 4

How does the trachea develop/separate in a foetus

Answer 4

Key Points:
Top end of the tube is the Laryngotracheal tube and the bottom end is the respiratory divericulum

As development progresses, ridges form on either side to he tube to form the oesopagotracheal septum

Leads to the tube separating from the foregut, forming the oesophagus dorsally and the largynotracheal tube ventrally

Simultaneously the lung buds are expanding

Question 5

What does the laryngotracheal tube form

Answer 5

hmmm I wonder

Believe it or not, the answer is the larynx and trachea

I know, how shocking

Question 6

Which part of the laryngotracheal tube forms which structure (very simple answer)

Answer 6

The superior part forms the larynx

The section inferior to that forms the trachea

The actual inferior part forms the lungs***

*** not part of the answer, just extra information

Question 7

What would be the germ layers seen in a cross section of the laryngotracheal tube

Answer 7

** just remember that the mesoderm gives rise to connective tissue and endoderm to epithelial

Question 8

Stages of lung developmemt

Answer 8

Pseudoglandular stage
Canalicular period
Terminal sac period
Alveolar period

Question 9

Pseudoglandular stage

Answer 9

5-16 weeks of embryonic development

Terminal bronchioles form

By the end, all major components of lungs form except those needed for gas exchange

Note that even at this stage they are in a pleural sac

Question 10

Pericardioperitoneal Canal

Answer 10

Pleural cavities are not initially separate from each other; the lung bud is growing within the cavities

The cavity is named the Pericardioperitoneal Canal (continuous with the heart and lungs) and hence they are connected to one another indirectly via other cavities

Question 11

Figure out what the fuck is happening here

Answer 11

So basically that’s what you get when you look down from the the laryngeotracheal tube to the rest of the embryo

Right is later in development

Basically this shows that the pericardio-peritoneal canal is just one continuous cavity

The lung buds on either side of the foregut grow into the cavity

Also it shows how the pleuro-pericardial fold moves to separate the pleural and pericardial cavity

Later we see the lungs have expanded (thats what those rings on either side are), each with their own pleural cavity

Question 12

Describe the connection of the pericardioperitoneal canal with the peritoneal (abdominal) cavity

Answer 12

The pericardioperitoneal canals (which form the pleural cavities) remain connected to the peritoneal (abdominal) cavity until closed by fusion of the pleuroperitoneal folds during formation of the diaphragm

Question 13

Canalicular period

Answer 13

16-26 weeks

Lumens of the bronchi and terminal bronchioles enlarge

Tissue becomes vascularised

By 24 weeks, each terminal bronchiole has formed 2 or more respiratory bronchioles

Towrds the end, first terminal sacs (primitive alveoli) form at the end of the respiratory bronchioles

Question 14

Describe the vascularisation and epithelia of terminal sacs in the canalicular period

Answer 14

They start to become quite well vascularised; they are composed of cuboidal epithelia (which does change)

Question 15

Terminal Sac Period

Answer 15

26 weeks - birth

Many terminal sacs form and form type I pneumocytes

Capillaries bulge into the primordial alveoli to allow gas exchange

Type II pneumocytes form between type I

Question 16

Compare more immature alveolar cells with more mature ones (simply)

Answer 16

Right is more mature

Has more squamous cells

Question 17

When do type II pneumocytes begin to form and what is their role

Answer 17

End of the 6th month; they produce surfactant (phospholipid-rich fluid) which reduces surface tension

Question 18

Alveolar Period

Answer 18

8 Months to childhood

Increased production of surfactant

Primordial alveoli increase in size, type I epithelial cells become thinner and capillaries get even closer

Postnatal increase in lungs size is generally from increased divisions to form respiratory bronchioles and continued primordial alveoli production

Question 19

What percentage of mature alveoli form before birth

Answer 19

5%

Dont actually memorise, just be vaguely aware

Question 20

Changes in lungs before birth

Answer 20

Surfactant production increases, especially in last two weeks

Breathing movements occur, probably to stimulate lung development

Amniotic fluid aspirated

Question 21

Changes in lungs at birth

Answer 21

Lungd are filled with fluid so it is removed by:

• Pressure on thorax during delivery
• Absorbed into circulation
• Absorbed into lymphatics

Thin coating of surfactant is left, lining alveolar membranes

Question 22

Lungs of a stillborn in water

Answer 22

1st breath not taken so no air in lungs and lungs are full fluid; sink in water at autopsy

Once 1st breath is taken, lungs full of air and will float

Question 23

Summarise the 4 periods of lung maturation in a foetus

Answer 23

Question 24

Four embryonic components of the diaphragm

Answer 24

Transverse Septum
Pleuroperitoneal Membranes
Dorsal Mesentery of Oesophagus
Muscular ingrowth from lateral body walls

Question 25

Transverse septum - What/where is it; What germ layer is it; is it continuous; Vaguely when does it form

Answer 25

Like a shelf below the heart; the liver develops within it. It is the primordium of central tendon of diaphragm

Mesoderm

It is not continuous though; allowing lung and peritoneal cavity to be continuous

Forms early in development

Question 26

Pleuroperitoneal Membranes - What do they form from; When do they form; What part do they form

Answer 26

Forms from the lateral wall of pleural and pleuroperitoneal cavities

First appear in 5th week

Forms the posterior and lateral part of the diaphragm by fusing with the transverse septum and dorsal mesentery in week 7

Question 27

Dorsal Mesentery of Oesophagus - What part of the diaphragm do they form; what else do they form; what are they derived from

Answer 27

Forms the median region of the diaphragm

Forms muscle bundle anterior to aorta (Crura of diaphragm)

Derived from myoblasts that migrate into it

Question 28

How does the primordial diaphragm form

Answer 28

Formation of the primordial diaphragm occurs by fusion of the pleuroperitoneal membranes, dorsal mesentery of oesophagus and septum transversum. This partitions the thoracic and abdominal cavities.

Question 29

Muscular ingrowth form lateral body walls

Answer 29

Myoblasts migrate in from peripheral tissue that muscularise the diaphragm that continues during development

Question 30

Describe the diaphragm of a newborn

Answer 30

The diaphragm has a very muscular part; a small region which is membrane; the dorsal mesentery where the oesophagus, aorta and Inferior VC lie; a central tendon on which the heart sits

Question 31

Respiratory Distress Syndrome

Answer 31

Syndrome in babies whose lungs are not fully developed caused mostly by a lack of surfactant in the lungs

If there is not enough surfactant, RDS occurs because the surface tension will be too high and risks alveoli collapsing during expiration

Question 32

Treatment for Respiratory Distress Syndrome

Answer 32

Artificial surfactant and treatment with glucocorticoids to stimulate surfactant production

Question 33

Oesophageal atresia and tracheoesophageal fistulas

Answer 33

Caused by abnormal separation of oesophagus and laryngeotracheal tube (trachea) by oesophagotracheal septum

Associated with congential heart defects

Luckily it is easy to detect and has good surgery outcomes

Question 34

Atresia

Answer 34

Narrowing or withering away of a structure

Question 35

Fistula

Answer 35

Abnormal opening or passage

Question 36

Congenital cysts of the lungs

Answer 36

Terminal bronchi abnormally dilated

looks like a honeycomb on an x-ray

Leads to poor fluid drainage of lungs

Usually at lung periphery

Question 37

Congenital Diaphragmatic Hernia

Answer 37

Hole that forms in the diaphragm

Can be easily stitched up and dealt with

However, if abdominal contents herniate up, then a problem can occur

90% on left

Question 38

What happens if abdominal contents herniate into thoracic cavity in a case of a congenital diaphragmatic hernia

Answer 38

Structures like intestines can enter the thoracic cavity, taking up space, compressing the lungs and making them hypoplastic or causing heart failure

Question 39

When does the lobing of the lungs begin

Answer 39

Originates from the very first divisions of the lung buds (basically very early on)