Dr. Ahmed Fathalla Ibrahim
LOWER RESPIRATORY
ORGANS
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LARYNX
TRACHEA
BRONCHI
LUNGS
PHARYNGEAL ARCHES
RESPIRATORY PRIMORDIUM
RESPIRATORY PRIMORDIUM
RESPIRATORY PRIMORDIUM
6
RESPIRATORY PRIMORDIUM
• Begins to form in the middle of 4th week
• Laryngotracheal groove: a median
outgrowth from the caudal end of the
floor (ventral wall) of primordial
pharynx, caudal to 4th pharyngeal
pouch
• Respiratory diverticulum: produced by
deepening (evagination) of the groove,
located ventral to caudal part of foregut
RESPIRATORY PRIMORDIUM
• Tracheal bud: it is the enlarged distal end of
diverticulum
• Longitudinal tracheoesophageal folds
develop in diverticulum, approach each other
& fuse to form tracheoesophageal septum
dividing cranial part of foregut into:
1. Ventral part: laryngotracheal tube:
primordium of larynx, trachea, bronchi &
lungs
2. Dorsal part: pharynx, esophagus
• Both parts maintain communication through
primordial laryngeal inlet
EMBRYOLOGICAL ORIGIN
• Endoderm lining laryngotracheal tube:
epithelium & glands of all respiratory organs
• Fourth & sixth pharyngeal arch cartilages: all
laryngeal cartilages EXCEPT epiglottis
• Sixth pharyngeal arch muscles: intrinsic
muscles of larynx
• Mesenchyme of hypopharyngeal eminence:
epiglottis
• Splanchnic mesoderm surrounding foregut:
connective tissue, cartilage, smooth
muscles, blood & lymphatic vessels of
trachea, bronchi & lungs
DEVELOPMENT OF BRONCHI
& LUNGS
DEVELOPMENT OF BRONCHI
& LUNGS
• During 5th week, tracheal bud divides into 2
primary bronchial buds
• Main bronchus: formed by primary bronchial
bud together with its surrounding splanchnic
mesoderm, divided into: secondary (lobar),
tertiary (segmental) & intrasegmental
branches
• By 24 weeks, 17 orders of branches have
formed & respiratory bronchioles have
developed
• After birth, 24 orders of branches are present
DEVELOPMENT OF BRONCHI
& LUNGS
• As the lungs expand:
1.They invaginate pleura & acquire a
visceral layer of pleura (derived from
splanchnic mesoderm)
2.They grow caudally into body wall & lie
close to heart. The thoracic body wall
becomes lined by a parietal layer of
pleura (derived from somatic
mesoderm)
MATURATION OF LUNGS
1.
2.
3.
4.
Pseudoglandular period
Canalicular period
Terminal saccular period
Alveolar period
MATURATION OF LUNGS
PSEUDOGLANDULAR
PERIOD (6-12 WEEKS)
• Lung resembles an exocrine gland
• Only bronchial & primordial terminal
bronchioles are formed
• Respiration is not possible
CANALICULAR PERIOD
(16-26 WEEKS)
• Lumina of bronchi & terminal
bronchioles enlarge
• Respiratory bronchioles & alveolar
ducts develop
• Lung tissue becomes highly vascular
• Respiration is possible
• Fetus born at this period often dies
because of immaturity of systems
TERMINAL SACCULAR
PERIOD (26 – BIRTH)
• Alveolar ducts give rise to terminal saccules
(primordial alveoli)
• Terminal saccules: cuboidal epithelium
begins to change into squamous
• Capillaries begin to bulge into alveoli to
establish blood-air barrier that permits
adequate gas exchange
• Terminal saccules are lined with:
1. Type I pneumocytes (across which gas
exchange occurs): main type of cells
2. Type II pneumocytes (secrete surfactant)
TERMINAL SACCULAR
PERIOD (26 – BIRTH)
Surfactant:
• Role: counteracts surface tension
forces & facilitates expansion of alveoli
• Production: begins by 20 weeks &
increases during last 2 weeks of
pregnancy
• By 26-28 weeks: surfactant is produced
in sufficient amount for survival of
prematurely born fetus
ALVEOLAR PERIOD
(32 WEEKS – 8 YEARS)
• Number of respiratory bronchioles and
terminal saccules (primordial alveoli)
increases
• Type I pneumocytes of terminal
saccules becomes extremely thin
squamous so that adjacent capillaries
bulge into it
• Alveolocapillary membrane is
sufficiently thin to allow gas exchange
ALVEOLAR PERIOD
(32 WEEKS – 8 YEARS)
• Lungs begin to work after birth
• Transition from dependence on
placenta for gas exchange to lungs
requires:
1.Production of sufficient amount of
surfactant
2.Development of alveolocapillary
membrane that is sufficiently thin
ALVEOLAR PERIOD
(32 WEEKS – 8 YEARS)
• Maturation of alveoli (about 95%)
occurs after birth
• From birth to third year: Increase in
size of lungs is mostly due to increase
in number of respiratory bronchioles &
primordial alveoli
• Immature alveoli have the potential for
forming additional primordial alveoli
• Immature alveoli increases in size &
become mature
TRACHEOESOPHAGEAL
FISTULA
TRACHEOESOPHAGEAL
FISTULA
• Most common anomaly
• Associated with esophageal atresia in
more than 85% of cases
• Often associated with polyhydramnios
• Cause: incomplete fusion of
treacheoesophageal folds resulting in a
defective tracheoesophageal septum &
faulty partitioning of foregut into
trachea & esophagus
TRACHEOESOPHAGEAL
FISTULA
Clinical manifesttaions:
• Infants cough & choke when
swallowing (due to accumulation of
saliva in upper respiratory tract)
• Infants regurgitate when swallowing
milk (milk fills esophageal pouch & is
regurgitated)
• Pneumonitis
RESPIRATORY DISTRESS
SYNDROME
• Also known as hyaline membrane
disease
• Affects about 2% of newborn
(especially premature newborn)
• Cause: deficiency of surfactant
• Manifestation: lungs are underinflated,
alveoli contain a fluid with a high
protein content that resembles a
hyaline membrane
RESPIRATORY DISTRESS
SYNDROME
• Predisposing factors:
1.Prolonged intrauterine asphyxia
2.Deficiency of thyroxine
• Treatment: glucocorticoid
OTHER ANOMALIES
• Stenosis (narrowing) or atresia
(obstruction) of trachea: due to unequal
partitioning of foregut into esophagus
& trachea
• Congenital lung cysts: disturbance in
bronchial development leads to dilation
of terminal bronchi & formation of
cysts filled with fluid or air
• Agenesis of lung: due to failure of
bronchial bud to develop