1. embryology of the lungs Flashcards
what is the respiratory diverticulum
blind ending outgrowth from ventral wall of foregut
forms at 4 weeks
what origin is the epithelial lining of the trachea, larynx, bronchi and lungs?
endoderm
how does the trachea develop?
4 weeks
Oesophagotracheal ridges fuse to form oesophagotracheal septum
forms from the laryngotracheal tube (layrnx too)
(respiratory diverticulum forms lungs)
which layers are present in the trachea during formation
splanchnic mesoderm-> forms cartilage, connective tissue and muscle
endoderm-> epitherlium, glands of trachea, pulomonary epithelium
stages of lung maturation
Pseudoglandular stage (5-16 weeks)
Canalicular period (16-26 weeks)
Terminal sac period (26 weeks to birth)
Alveolar period (8 months to childhood)
Pseudoglandular stage
Terminal bronchioles form
- By the end of this period, all the major components
of the lung have formed, except those required for
gas exchange
how is the pleural cavity formed
- Pericardioperitoneal canals are initially connected to pericardial (primitive thoracic) and peritoneal (primitive abdominal) cavities
- The pericardioperitoneal canals become separated from the pericardial cavity by the pleuropericardial folds
- 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
Lumens of the bronchi and terminal bronchioles enlarge
- Tissue becomes vascularised
- By 24 weeks, each terminal bronchiole has formed 2 or more respiratory bronchioles
- Towards the end of this period, the first terminal sacs
form at the end of the respiratory bronchioles
Terminal sac period
- Many terminal sacs form - the primordial alveoli
- Epithelial cells of the terminal sacs become flat and thin - are type I alveolar epithelial cells
- Capillaries come into close contact with the flat epithelial cells and start to bulge into the primordial alveoli. This close contact, at the blood air barrier, will allow gas exchange
- Secretory, rounded epithelial cells start to form - are type II alveolar cells. Form in-between the flat type I alveolar cells (produce surfactant)
what is surfactant
- Produced by type II alveolar epithelial cells
- Phospholipid-rich fluid
- Forms a monomolecular film over internal walls of the terminal sacs and mature alveoli
- Lowers surface tension at the air-alveolar interface
- Produced from the end of 6th month, though at low levels
Alveolar period
Increased production of surfactant
- Only about 5% of mature alveoli form before birth
- Primordial alveoli increase in size, type I epithelial cells become thinner and capillaries form an even closer association as they mature
- Most postnatal increase in lungs size is due to increased divisions to form respiratory bronchioles and continued primordial alveoli production
Changes in lungs before birth
- Amount of surfactant produced increases before birth, mostly in the last 2 weeks of gestation
- Breathing movements occur before birth to stimulate lung development and respiratory muscles.
- Amniotic fluid is aspirated
Changes in lungs at birth
at birth, lungs half filled with fluid
This fluid is removed from the lungs by:
1) Pressure on the thorax during delivery,
expelling fluid through mouth and nose
2) Absorbed into circulation via the pulmonary
circulation
3) Absorbed into lymphatics
A thin coating of surfactant is left lining the alveolar
cell membranes
Lungs of a stillborn
1st breath not taken, so no air in lungs. Lungs are full of fluid and will sink if placed in water at autopsy.
Once first breath is taken, lungs full of air and will float.
what is the diaphragm
Musculotendinous dome-shaped partition that separates the thoracic and abdominal cavities
four embryonic components during formation of diaphragm
Transverse septum
Pleuroperitoneal membranes
Dorsal mesentery of oesophagus
Muscular ingrowth from lateral body walls
Transverse septum
- Mesodermal in origin
- Grows dorsally from ventrolateral body wall
- Forms early in development
- Forming liver embedded in tissue
- Caudal to pericardial cavity – partially separating it
from peritoneal cavity
- Primordium of central tendon of diaphragm
posterior side direct communication between peritoneal and pleuropericardial
Pleuroperitoneal membranes
Form from the lateral wall of pleural and peritoneal cavities
- First appear at the start of the 5th week
- Forms posterior and lateral parts of diaphragm, by fusing with the transverse septum and dorsal mesentery in the 7th week
Dorsal mesentery of oesophagus
- Will form the median region of the diaphragm
- Forms muscle bundles anterior to the aorta, the “Crura of the diaphragm”
- Derived from myoblasts that had previously migrated into the dorsal mesentery of oesophagus
what does the formation of primordal diaphragm occur by
fusion of the pleuroperitoneal membranes, dorsal mesentery of oesophagus and septum transversum. This partitions the thoracic and abdominal cavities.
Muscular ingrowth from lateral body walls
- Contributes muscle to peripheral region of diaphragm external to the region that is derived from the pleuroperitoneal membranes
occurs by 12th week
important defects and problems
The premature baby and Respiratory Distress Syndrome
Oesophageal atresia and tracheoesophageal fistula
Congenital cysts of the lung
Congenital diaphragmatic hernia
what can cause respiritory distress syndrome in a premature baby
treatments
not enough surfactant produced- high surface tension- alveoli collapse during expiration
Treatments: Artificial surfactant and treatment with glucocorticoids to stimulate surfactant production
