Lung development and maturation of lungs. By Dr Saneed Khaliq

1. MATURATION OF LUNGS
Dr. Saneed Khaliq
MBBS, MPHIL ANATOMY (3rd batch)
Bahria University Medical and Dental College Karachi

2. DEVELOPMENT OF BRONCHI AND LUNGS
• During 4th week a respiratory bud
appears at the caudal end of
tracheoesophageal diverticulum
• The bud soon divides into two
outpouchings, the primary bronchial
buds
• Soon Secondary and tertiary
bronchial buds develop

3. • During 5th week these
enlarge and form the
primordia of the main
bronchi
• The main bronchi
subdivide into secondary
bronchi that form lobar,
segmental, and
intrasegmental branches

4. • Secondary bronchi divide
repeatedly forming tertiary or
segmental bronchi
 Right lung: forms ten
 Left: forms eight to nine
• Creating the bronchopulmonary
segments of the adult lung
• By the end of the 24 weeks ,
approximately 17 generations of
subdivisions have formed

5. MATURATION OF THE LUNGS
• Maturation of the lungs is divided
into four histologic stages
1. Pseudoglandular
2. Canalicular
3. Terminal sac
4. Alveolar stages

6. PSEUDOGLANDULAR STAGE
(6-16 weeks)
• Developing lungs somewhat resembles an
exocrine gland during this period
• By 16 weeks all major elements of the
lung have formed except those involved
with gas exchange
• Terminal bronchioles are formed, BUT no
respiratory bronchioles or alveoli are
present
• Respiration is not possible at this stage
• Fetuses born during this period cannot
survive

7. CANALICULAR STAGE
(16-26 weeks)
• During the canalicular stage, the lumina of
bronchi and terminal bronchioles become
larger and the lung tissue becomes highly
vascular
• Each terminal bronchiole divide into 2 or
more Respiratory bronchiole
• Respiratory bronchiole divide into Alveolar
ducts
• Lung is well vascularized
• Fetus born at end of this stage (26 weeks)
may survive but earlier may not survive

8. TERMINAL SAC STAGE
(26 weeks - Late fetal period)
• During this stage, many more terminal sacs
develop and their epithelium becomes very
thin
• Capillaries bulge into the developing sacs
• Epithelium of terminal sac become very thin
(simple squamous)
• Close contact develops between epithelium
of sac & capillary to permit adequate
exchange of gases
• Fetus born prematurely at this stage
survives

9. TERMINAL SAC STAGE
(26 weeks - Late fetal period)
• The terminal sacs are lined mainly by
squamous epithelium called type I
pneumocytes
• Type I cells are responsible for gas
exchange
• Scattered among the squamous epithelial
cells are rounded secretory epithelial
cells called type II pneumocytes
• Type II pneumocytes are responsible for
production of pulmonary surfactant
preventing atelectasis
• Deficiency of surfactant results in RDS
or hyaline membrane disease

10. ALVEOLAR STAGE
(Late Fetal Period to 8 Years)
• The epithelial lining of the sacs attenuates to
a thin squamous epithelial layer
• The type I pneumocytes become so thin
that the adjacent capillaries bulge into the
alveolar sacs
• Formation of true alveoli more and more
• The lungs are capable of respiration because
the alveolocapillary membrane
sufficiently thin to allow gas exchange

11. ALVEOLAR STAGE
(Late Fetal Period to 8 Years)
• Approximately 95% of mature alveoli
develop postnatally
• Alveolar development is largely completed
by 3 years of age, but new alveoli are added
until approximately 8 years of age
• Initially, the septa are relatively thick, but
they are soon transformed into mature thin
septa that are capable of gas exchange

12. References:
• Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2016). The developing human: Clinically
oriented embryology (10th edition.). Philadelphia, PA: Elsevier