Embryology of Respiratory System Flashcards
Hilum of the Lung
Root of lung; where structures enter and leave the lung
e.g. Pulmonary arteries and veins/bronchi
When and where does the respiratory diverticulum form in a foetus
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
What germ layers are the lower respiratory system from (larnx, trachea, bronchi and lungs)
Endoderm as the respiratory diverticulum comes from the foregut
How does the trachea develop/separate in a foetus
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
What does the laryngotracheal tube form
hmmm I wonder
Believe it or not, the answer is the larynx and trachea
I know, how shocking
Which part of the laryngotracheal tube forms which structure (very simple answer)
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
What would be the germ layers seen in a cross section of the laryngotracheal tube
** just remember that the mesoderm gives rise to connective tissue and endoderm to epithelial
Stages of lung developmemt
Pseudoglandular stage
Canalicular period
Terminal sac period
Alveolar period
Pseudoglandular stage
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
Pericardioperitoneal Canal
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
Figure out what the fuck is happening here
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
Describe the connection of the pericardioperitoneal canal with the peritoneal (abdominal) cavity
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
Canalicular period
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
Describe the vascularisation and epithelia of terminal sacs in the canalicular period
They start to become quite well vascularised; they are composed of cuboidal epithelia (which does change)
Terminal Sac Period
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
Compare more immature alveolar cells with more mature ones (simply)
Right is more mature
Has more squamous cells
When do type II pneumocytes begin to form and what is their role
End of the 6th month; they produce surfactant (phospholipid-rich fluid) which reduces surface tension
Alveolar Period
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
What percentage of mature alveoli form before birth
5%
Dont actually memorise, just be vaguely aware
Changes in lungs before birth
Surfactant production increases, especially in last two weeks
Breathing movements occur, probably to stimulate lung development
Amniotic fluid aspirated
Changes in lungs at birth
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
Lungs of a stillborn in water
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
Summarise the 4 periods of lung maturation in a foetus
Four embryonic components of the diaphragm
Transverse Septum
Pleuroperitoneal Membranes
Dorsal Mesentery of Oesophagus
Muscular ingrowth from lateral body walls
Transverse septum - What/where is it; What germ layer is it; is it continuous; Vaguely when does it form
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
Pleuroperitoneal Membranes - What do they form from; When do they form; What part do they form
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
Dorsal Mesentery of Oesophagus - What part of the diaphragm do they form; what else do they form; what are they derived from
Forms the median region of the diaphragm
Forms muscle bundle anterior to aorta (Crura of diaphragm)
Derived from myoblasts that migrate into it
How does the primordial diaphragm form
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.
Muscular ingrowth form lateral body walls
Myoblasts migrate in from peripheral tissue that muscularise the diaphragm that continues during development
Describe the diaphragm of a newborn
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
Respiratory Distress Syndrome
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
Treatment for Respiratory Distress Syndrome
Artificial surfactant and treatment with glucocorticoids to stimulate surfactant production
Oesophageal atresia and tracheoesophageal fistulas
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
Atresia
Narrowing or withering away of a structure
Fistula
Abnormal opening or passage
Congenital cysts of the lungs
Terminal bronchi abnormally dilated
looks like a honeycomb on an x-ray
Leads to poor fluid drainage of lungs
Usually at lung periphery
Congenital Diaphragmatic Hernia
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
What happens if abdominal contents herniate into thoracic cavity in a case of a congenital diaphragmatic hernia
Structures like intestines can enter the thoracic cavity, taking up space, compressing the lungs and making them hypoplastic or causing heart failure
When does the lobing of the lungs begin
Originates from the very first divisions of the lung buds (basically very early on)
