Embryology of the Respiratory System and clinical implications Flashcards
Stages of early respiratory system development
formation of trachea and bronchi
lung development
formation of pleural cavities
diaphragm development
What is the respiratory diverticulum?
lung bud
When and where does the respiratory diverticulum form?
around the 4th week
forms as blind-ending outgrowth from ventral wall of foregut
endodermal origin
When does the trachea develop?
during the 4th week
What happens for the trachea to form?
oesophagotracheal ridges fuse to form oesophagotracheal septum
laryngotracheal tube and oesophagus is separate
What does the laryngotracheal tube develop into?
larynx and trachea
Describe the structure of the laryngotracheal tube
inner lining: endoderm
outer lining: splanchnic mesoderm
What does the endoderm of the laryngotracheal tube give rise to?
epithelium and glands of trachea and pulmonary epithelium
What does the splanchnic mesoderm of the laryngotracheal tube give rise to?
cartilage, connective tissue and muscles
Stages of lung maturation
- pseudoglandular stage
- canalicular period
- terminal sac period
- alveolar period
When is the pseudoglandular stage?
5-16 weeks
What occurs in the pseudoglandular stage?
terminal bronchioles form
by the end: all major components of the lung has formed, except those needed for gas exchange
development of pleural cavities
Development of pleural cavities
pericardioperitoneal canals are connected to pericardial (primitive thoracic) and peritoneal (primitive abdominal) cavities
pericardioperitoneal canals become separated from the pericardial cavity by pleuropericardial folds
When do the pericardioperitoneal canals become completely separated?
remain connected to peritoneal (abdominal) cavity until closed by fusion of the peritoneal folds during formation of the diaphragm
When is the canalicular period?
16-26 weeks
What happens in the canalicular period?
lumens of bronchi and terminal bronchi enlarge
tissue becomes vascularised
by 24 weeks: each terminal bronchiole has formed 2 or more respiratory bronchioles
towards end: 1st terminal sacs form at end of resp. bronchioles
What are respiratory bronchioles characterised by?
cuboidal epithelium
When is the terminal sac period?
26 weeks - birth
What happens in the terminal sac period?
many terminal sacs form (primordial alveoli)
type 1 alveolar epithelial cells form
capillaries come into close contact with flat epithelial cells - start to blge into primordial alveoli
type 2 alveolar cells begin to form
Type 1 alveolar cells
epithelial cells of the terminal sacs become flat and thin squamos cells from cuboidal
line alveolar sacs
allows efficient gas exchange
Type 2 alveolar epithelial cells
rounded, secretory epithelial cells
line alveolar sacs
formed from end of 6th month
produce surfactant
What is surfactant
produced by type II alveolar epithelial cells
phospholipid-rich fluid
forms monomolecular film over internal walls of the terminal sace and mature alveoli
lowers surface tension at the air-alveolar interface so alveoli don’t collapse due to change in pressure
When is the alveolar period?
8 months - childhood
What happens in the alveolar period?
increased production of surfactant
primordial alveoli increase in size
type I cells thinner and capillaries even closer association
postnatal increase in lung size: increased divisions to form repiratory bronchioles and continued prmordial alveoli production
Changes in lungs before birth
amount of surfactant produced increases, mostly in last two weeks
breathing movements occur to stimulate lung development and respiratory muscles
amniotic fluid aspirated
Changes in lungs at birth
lungs half-filled with liquid, removed by:
- pressure on thorax during delivery
- absorbed into circulation via pulmonary circulation
- absorbed into lymphatics
thin coating of surfactant left lining alveolar cell membranes
Lungs of a stillborn
1st breath not taken, lungs have no air and are full of fluid and baby will sink in water
What is the diaphragm?
musculotendinous dome-shaped partition that separates the thoracic and abdominal cavities
What are the embryonic components of the formation of the diaphragm?
transverse septum
pleuroperitoneal membranes
dorsal mesentery of oesophagus
muscular ingrowth from lateral body walls
What is the transverse septum?
sheet of non-continuous tissue (mesodermal in origin)
primordium of central tendon of diaphragm (non-muscular)
Where and when does the transverse septum grow?
4th week
grows dorsally from ventrolateral body wall
forming liver is embedded in tissue
caudal to pericardial cavity, partially separating it from peritoneal cavity
ventral and lateral, not posterior
When and where do the pleuroperitoneal membranes form?
5th week
form from the lateral wall of pleural and peritoneal cavities
What do the pleuroperitoneal membranes form
forms posterior and lateral parts of the diaphragm
fuses with the transverse septum and dorsal mesentery in the 7th week
What does the dorsal mesentery form?
medial region of the diaphragm
muscle bundles anterior to the aorta (in adults: Crura of the diaphragm)
not yet mature/muscularised
What is the dorsal mesentery derived from?
myoblasts that have previously migrated into the dorsal mesentery of oesophagus
Formation of the mature diaphragm
muscular ingrowth from lateral body walls
contributes muscle to peripheral region of diaphragm external to the region that is derived from the pleuroperitoneal membranes
by 12th week
Give examples of defects and problems of the lungs and diaphragm
premature and Respiratory Distress Syndrome
oesophageal atresia and tracheoesophageal fistula
congenital cysts of the lung
congenital diaphragmatic hernia
Premature baby
not enough surfactant
high surface tension at blood-air interface
risk of alveoli collapsing during expiration
RDS
RDS
2% of newborns
rapid, laboured breathing
treatments (if known in advance): artificial surfactant and treatment with glucocorticoids to stimulate surfactant production
Oesophageal atresia and tracheoesophageal fistula
abnormal separation of the oesophagus and trachea by the oesophagotracheal septum
more common in males
associated with other defects like congenital heart disease
most common: proximal blind end of oesophagus
Atresia meaning
narrowing/withering away
Fistula meaning
abnormal opening or passage
Problem of most common oesophageal atresia and tracheoesophageal fistula
XS amounts of fluid in mouth/upper RT, gastric contents may reflux - into trachea and lungs
coughing and choking
pneumonia
Congenital cysts of the lungs
terminal bronchi abnormally dilated, usually at periphery
small and numerous/few and large
problems: drain poorly, chronic lung infections
Congenital diaphragmatic hernia
congenital absence of large area of diaphragm
due to: failure of fusion of pleuroperitoneal membrane with other components, posterolateral defect
problems: abdominal contents hemiated, hypoplastic lung, pressure on heart
