CL4-DEVELOPMENT OF RESPIRATORY SYSTEM
LARYNX
•
LUNGS
Maturation of the lungs
cartilages and muscles originate from mesenchyme
of the 4th and 6th pharyngeal arches
As a result of
rapid proliferation
•
By the end of 7 months
•
Maturation of the Lungs is divided into four stages:
1. Pseudoglandular period
2. Canalicular period
3. Terminal sac period
of this
4. Alveolar period
mesenchyme, the
laryngeal orifice
changes in
appearance from a
sagittal slit to a Tshaped opening
4 weeks old→
respiratory
diverticulum (lung bud)
appears as an
outgrowth from the
ventral wall of the
foregut
Subsequently, when mesenchyme of the two arches
The spaces for the lungs, the pericardioperitoneal
transforms into the thyroid, cricoid, and arytenoid
canals, are narrow
cartilages, the characteristic adult shape of the
laryngeal orifice can be recognized
They lie on each side of the foregut and are
gradually filled by the expanding growth of the lungs
At about the time that the cartilages are formed, the
laryngeal epithelium also proliferates rapidly, resulting in
a temporary occlusion of the lumen
pericardioperitoneal canals
Subsequently, vacuolization and recanalization produce a
Pleuropericardial fold
pair of lateral recesses, the laryngeal ventricles
Endoderm
cartilaginous, muscular, and
the lungs
and lungs
Pericardial
connective tissue
components of the trachea
These recesses are bounded by folds of tissue that
differentiate into the false and true vocal cords
When the diverticulum expands caudally two longitudinal
these ridges fuse to form the tracheoesophageal septum
the remaining spaces after the formation of
pericardial & peritoneal cavities form the primitive
pleural cavities
•
Mesoderm→
covers
outside of the lung→ into
the visceral pleura.
•
somatic
mesoderm
layer→cover body wall
from the inside→ parietal
pleura
mesenchyme of the 4th and 6th pharyngeal arches,
While all of these new subdivisions are occurring and
all laryngeal muscles are innervated by branches of
the bronchial tree is developing, the lungs assume a
the 10th cranial nerve, the vagus nerve
more caudal position, so that by the time of birth,
Superior laryngeal nerve→ derivatives of 4th
pharyngeal arch
then the foregut is divided into
Terminal sac
period
Alveolar period
•many more Terminal sacs form and cells lining the sacs, known as type I
alveolar epithelial cells, become thin
•Capillaries begin to bulge into the sacs (developing alveoli) and establish
close contact with them
•Scattered among type I cells, type II alveolar epithelial cells develops
at the end of the 6th month. These cells produce surfactant, a
phospholipid-rich fluid capable of lowering surface tension
•thus premature infant after 7 month is able to survive because
sufficient numbers of capillaries are present to guarantee adequate gas
exchange
•Mature alveoli have well-developed epithelial endothelial (capillary)
contacts
•At birth, only about 1/6th of the adult number of alveoli are present
•The remaining alveoli (approx. 95%) are formed during the first 10 years
of postnatal life through the continuous formation of new primitive
alveoli
the bifurcation of the trachea is opposite the
fourth thoracic vertebra
Recurrent laryngeal nerve→ derivatives of 6th
Signals for branching, which emit from the
pharyngeal nerve
mesoderm, involve members of the fibroblast
growth factor family
a ventral portion→ trachea
+ lung buds
TRACHEA
•
During its separation from the foregut, the lung bud
forms the trachea and two lateral outpocketings, the
primary bronchial buds
BRONCHI
At the beginning of the fifth week, each of these
buds enlarges to form right and left primary bronchi
•
The respiratory primordium
maintains its communication
with the pharynx through the
laryngeal orifice
•
Because musculature of the larynx is derived from
ridges, the tracheoesophageal ridges, separate it from the
foregut
•Enlargement of lumen of bronchi and terminal bronchiole, and the lung
tissue becomes highly vascular
•Each terminal bronchiole divides into respiratory bronchioles, which in
turn divide alveolar ducts
•Development of some thin-walled terminal sacs (primitive alveoli) at the
ends of the respiratory bronchioles in the end of canalicular stage so
respiration is possible
•Although a fetus born toward the end of this period may survive if given
intensive care, it often dies because its respiratory and other systems
are still relatively immature
Peritoneal cavity
cavity
Initially, the lung bud is in open communication with the foregut
dorsal portion→
esophagus
Pleuroperitoneal
Canalicular
period
fold
Mesoderm→ splanchnopleuric
epithelium of the internal
lining of the larynx, trachea,
and bronchi, as well as that of
Pseudoglandular
period
•Continuous branching forming terminal bronchiole
•By 16 weeks, formation of all major elements of the lung except those
involved with gas exchange (respiratory bronchioles or alveoli are not
present)
•Respiration is not possible; hence, fetuses born during this period are
unable to survive
•
The right then forms
three 2° bronchi, and
the left, two thus
foreshadowing
the
three lobes of the
lung on the right side
and two on the left
With subsequent growth in caudal and lateral
directions, the lungs expand into the body cavity (A).
Branching is regulated by epithelial-mesenchymal
interactions between the endoderm of the lung buds
and visceral mesoderm that surrounds them
Before the bronchial tree reaches its final shape→
an additional 6 divisions form during postnatal life
By the end of the 6th month, approximately 17
generations of subdivisions have formed
creating the bronchopulmonary segments of the
adult lung
During further development, secondary bronchi
divide repeatedly in a dichotomous fashion, forming;
• 10 tertiary (segmental) bronchi in the right lung
• 8 tertiary bronchi in the left lung
Canalicular period
Terminal sac period
Alveolar period