Posted by TA ANNA on September 04, 2009 at 14:52:44:
Hey everyone. Here are my notes on the 4 stages of lung maturation. This is basically right out of your text/Dr. M's notes. She also has a lung development document posted on Angel, so you may want to merge my notes with that document. Also, our ppt from today's review has been posted on Angel under "lecture materials". Good luck with the studying, and have a wonderful weekend!
o PSEUDOGLANDULAR PERIOD (6-16 weeks)
Lungs histologically resemble an exocrine gland.
All major elements of lung have formed, except those involved with gas exchange.
No respiratory bronchioles or alveoli present.
Fetuses born in this period CANNOT SURVIVE.
o CANALICULAR PERIOD (16-26 weeks)
Lumen of bronchi and terminal bronchioles become larger and the lung tissue becomes highly vascular.
By 24 weeks, terminal bronchioles have given rise to respiratory bronchioles, each which divides into primordial alveolar ducts.
Respiration IS possible at end of this stage because some terminal sacs (primordial alveoli) have formed, so gas exchange can occur.
Surfactant production begins by 20 weeks, but it is only present in small amounts.
Fetuses born in 24-26 weeks require intensive care due to pulmonary surfactant deficiency.
Lung tissue is highly vascular.
o TERMINAL SAC STAGE (26 weeks-birth)
Terminal sacs are future alveolar ducts
Blood air barrier is established…intimate connection between type 1 alveolar cells and capillaries.
Surfactant production (type 2 alveolar cells) greatly increased a few weeks before birth.
o ALVEOLAR PERIOD (32 weeks to 8 yrs)
At the beginning of the alveolar period, each respiratory bronchiole terminates in a cluster of thin-walled terminal sacs, which represent future alveolar ducts. The alveolocapillary membrane is sufficiently thin to allow gas exchange.
Requirements to transition to autonomous gas exchange: Surfactant production, transformation of lungs into gas-exchanging organs, establishment of pulmonary and systemic circulations.
About 95% of mature alveoli develop postnatally.
Postnatal lung growth is due primarily to an increase in the number of respiratory bronchioles and primordial alveoli rather than the size of the alveoli. (Immature alveoli have the ability to form additional primordial alveoli).
Adults have 300 million alveoli. 150 million primordial alveoli present at birth.