The document summarizes the development of the respiratory system from the fourth week of gestation through childhood. It describes how the larynx, trachea, bronchi and lungs develop from the laryngotracheal groove and lung buds. It outlines the four periods of lung maturation from pseudoglandular to alveolar and discusses how the preacinar and intraacinar pulmonary arteries and veins develop along with the airways and alveoli. The document also notes some potential congenital anomalies that can occur during respiratory development.

1. DEVELO
PMENT
OF THE
RESPIR
ATORY
SYSTEM
Dr. Ademola A. Oremosu

2. Development of the lower
respiratory organs
– The lower respiratory organs (larynx, trachea, bronchi, and
lungs) begin to form during the fourth week of
development. The respiratory primordium is indicated at 26
to 27 days by a median outgrowth from the caudal end of
the ventral wall of the primordial pharynx — the
laryngotracheal groove

3. Stages of development
illustrated

4. –This rudiment of the
tracheobronchial tree develops
caudal to the fourth pair of
pharyngeal arches. The endoderm
lining the laryngotracheal groove
gives rise to the epithelium and
glands of the larynx, trachea, bronchi,
and the pulmonary epithelium

5. Successive stages of development

6. The connective tissue, cartilage, and
smooth muscle in these structures
develop from the splanchnic
mesenchyme surrounding the foregut
As this diverticulum elongates, it is
invested with splanchnic
mesenchyme and its distal end
enlarges to form a globular lung

7. DEVELOPMENT OF THE
LARYNX
–The epithelial lining of the larynx
develops from the endoderm of the
cranial end of the laryngotracheal
tube, The cartilages of the larynx
develop from the cartilages in the
fourth and sixth pairs of pharyngeal
arches

8. – The laryngeal cartilages develop from mesenchyme that is derived from neural crest
cells
– mesenchyme at the cranial end of the laryngotracheal tube proliferates rapidly,
producing paired arytenoid swellings
– These swellings grow toward the tongue, converting the slit- like aperture — the
primordial glottis — into a T shaped laryngeal inlet and reducing the developing
laryngeal lumen to a narrow slit

9. Development of the larynx

10. – The laryngeal epithelium proliferates rapidly, resulting in
temporary occlusion of the laryngeal lumen
– Recanalization of the larynx occurs by the tenth week
– The laryngeal ventricles form during this recanalization
process. These recesses are bounded by folds of mucous
membrane that become the vocal folds (cords) and
vestibular folds

11. – The epiglottis develops from the caudal part of the
hypobranchial eminence
– Because the laryngeal muscles develop from myoblast in
the fourth and sixth pairs of pharyngeal arches, they are
innervated by the laryngeal branches of the vagus nerves
(CN X) that supply these arches

12. Development of the trachea
– Endodermal lining of the laryngotracheal
tube distal to the larynx differentiates into
the epithelium and glands of the trachea and
the pulmonary epithelium.
– Cartilage, connective tissue, and muscles of
the trachea are derived from the splanchnic
mesenchyme surrounding the
laryngotracheal tube

13. Progressive development of
the trachea

14. Congenital anomalies of the trachea -
TOF

15. DEVELOPMENT OF THE
BRONCHI AND LUNGS
– The lung bud that developed at the caudal end of laryngotracheal tube during the
fourth week
– Divides into two outpouchings bronchial buds
– These endodermal buds grow laterally into the pericardioperitoneal canals, the
primordia of the pleural cavities
– Together with the surrounding splanchnic mesenchyme, the bronchial buds
differentiate into bronchi and their ramifications in the lungs

16. Development of the bronchi and the
lungs

17. Maturation of the lungs is divided
into four periods
– pseudoglandular period
– canalicular period
– terminal sac period
– alveolar period

18. PSEUDOGLANDULAR
PERIOD (5- 17 WEEKS)
– Resembles an exocrine gland during this period
– By 17 weeks all major elements of the lung have formed, except those involved with
gas exchange
– Respiration is not possible; hence, fetuses born during this period are unable to
survive.

19. Different periods of lung maturation

20. CANALICULAR PERIOD (16 - 25
WEEKS)
– overlaps the pseudoglandular period because cranial segments of the lungs mature
faster than caudal ones
– Lumina of the bronchi and terminal bronchioles become larger, and the lung tissue
becomes highly vascular
– By 24 weeks, each terminal bronchiole has given rise to 2 or more respiratory
bronchioles
– Respiration is possible toward the end of the canalicular period because some thin-
walled terminal sacs (primordial alveoli) have developed

21. TERMINAL SAC PERIOD
(24 WEEKS TO BIRTH)
– many more terminal sacs develop and their epithelium becomes very thin
– Capillaries begin to bulge into these developing alveoli
– The intimate contact between epithelial and endothelial cells establishes the blood-
air barrier
– permits adequate gas exchange for survival of the fetus if it is born prematurely

22. Terminal sac period (cont’d)
– By 24 weeks, the terminal sacs are lined mainly by squamous epithelial cells of
endodermal origin — type I alveolar cells or pneumocytes
– type II alveolar cells or pneumocytes — which secrete pulmonary surfactant,
– fetuses born prematurely at 24 to 26 weeks after fertilization may survive intensive
care
– may suffer from respiratory distress because of surfactant deficiency

23. ALVEOLAR PERIOD (LATE
FETAL PERIOD TO
CHILDHOOD)
– Structures analogous to alveoli are present at 32
weeks of gestation
– At the beginning of the alveolar period, each
respiratory bronchiole terminates in a cluster of thin-
walled terminal sacs
– Characteristic mature alveoli do not form until after
birth; about 95% of alveoli develop postnatally

24. Lung Unit
Acinus : That part of the lung supplied by a terminal bronchiolus
This includes respiratory bronchioli, alveolar ducts, and
alveoli. It is the respiratory region of the lung.
Lobule : The three to five terminal brochioli, with the acini they
supply, that cluster at the end of any pathway.
Bronchopulmonary segment : Each segment is supplied
by its own bronchus and artery, and draining to veins
that run at its periphery in intersegmental plane.

25. Laws of Lung Development
(Reid 1967a, Hoslop and Reid 1974a)
1 The heart tube is formed by the end of 3 weeks of gestation,
and 5 days later the lung primordia (bud) develops at the
caudal end of laryngotracheal sulcus.
2 Lobar bronchus of each lung develop at the 5 weeks of gestation.
3 Bronchial tree is developed by the 16th week of intrauterine life.
4 During early fetal life, main feature of arterial growth is an increase
in number of branches, whereas during late growth in size & length.
5 Alveoli develop mainly after birth, increasing in number until
the age of 8 years and in size until growth of chest wall finishes
with adulthood.
6 The preacinal vessels follow the development of the airways,
the intraacinar that of the alveoli.
Muscularization of intraacinar arteries does not keep pace with
the appearance of new arteries.

26. Preacinal Development of Pulmonary
Arteries & Veins
A 5 to 16 weeks’ gestation
1. Arteries
* The arteries develop as the airway.
* The supernumerary arteries appear at the same time as adjacent
conventional arteries.
2. Veins
* The venous pathways develop at the same time as the arteries.
B Changes after 16 weeks
1. Preacinar region
* During late fetal life : The proximal part increases in diameter faster.
* After Birth : They increases at the same rate during infancy and in the
intraacinar vessels, there is greater increase proximally.
2. Conventional & supernumerary arteries
* Both increase in size and each shows a linear relationship to age
* During childhood both increase same rate. (rapid in the first 18 months)

27. Intraacinar Development of
Pulmonary Arteries & Veins
A. Branching patterns
1. Before 16 weeks of gestation
* No alveoli are present.
2. After 16 weeks of gestation
* Airways develop beyond the terminal bronchiolus (respiratory bronchiole,
sacules)
* Both conventional , and supernumerary artery appear.
3. After Birth (during childhood)
* As new alveolar duct & alveoli appear and enlarge additional arteries form.
* Few new conventional vessel appear, but supernumeraries increase
considerably.
B. Vessel numbers
1. 1st 3 years of life
* Both arteries and alveoli per unit area increase in number.
2. After 5 years
* The arterial concentration decrease, but since the alveoli have increased
in size, the ratio stay the same.

28. Structures of Pulmonary
Arteries & Veins
A. Main pulmonary artery
* During fetal life : resembles the aorta both in its thickness and
configuration of elastic fibrils, (parallel, compact, and uniform in
thickness)
* These features up to about 6 months of age, when changes, starting at birth
* By the 2 years : the adult configuration (40~70% as thick as that of aorta)
B. Intrapulmonary arteries
* During fetal life : The large intrapulmonary arteries have the same structure
as the main pulmonary artery.
* Progressing peripherally along the arterial pathway, elastic lamina
decrease, replaced by a muscular structure, and the elastic laminae
further decrease until in the small arteries.
* Along any pathway the wholly muscular wall get thinner and eventually
the incomplete around the circumference and present only as a spiral.

29. Pulmonary Vascular Development
1 Arterial size : increase most rapidly during first 2 months of life,
but growth rate remained during first 4 years.
2 Arterial number : increase most rapidly in the first 2 months,
but subsequent multiplication at same rate as alveoli.
3 Arterial medial thickness : fall quickly during first several days
(3 days to 2 week) and continue to decrease. (adult level :
4-10 months)
4 Intraacinal artery becomes more muscular during childhood as
they increase in size. (adult level : 19 years of age)

30. Congenital anomalies
– Laryngeal atresia- rare
– Laryngeal web-uncommon, incomplete canalization of larynx
– Tracheoesophageal fistula-85% assoc. with esophageal atresia
– Laryngotracheoesophageal cleft
– Tracheal stenosis and atresia
– Tracheal diverticulum
– Respiratory distress syndrome
– Congenital lung cyst
– Agenesis, hypoplasia and accessory lung