Development of respiratory system in a foetus .

1. BSN 1
LECTURE 3
DEVELOPMENT OF THE
RESPIRATORY SYSTEM

2. The Respiratory System
• Basic functions of the respiratory system
– Supplies body with oxygen
– Disposes of carbon dioxide
• Four processes involved respiration
– Pulmonary ventilation
– External respiration
– Transport of respiratory gases
– Internal respiration

3. Functional Anatomy of the Respiratory
System
Respiratory organs
– Nose, nasal cavity, and paranasal sinuses
– Pharynx, larynx, and trachea
– Bronchi and smaller branches
– Lungs and alveoli

4. Organs of the Respiratory System

5. Organs of the Respiratory
System
Divided into
– Conducting zone
– Respiratory zone

6. The foregut can be divided into three parts:
1. The first part lies ventral to the developing
brain, and forms the primitive pharynx, which
has the branchial arches associated with it.
2. The second part lies dorsal to the heart, and
forms the lung bud and the oesophagus.
3. The third part lies dorsal to the septum
transversum and forms the stomach and other
related gastro-intestinal structures.

8. The respiratory system is a derivative of
the second part of the foregut.
a. It begins to develop in the beginning of the
fourth week (day 22)
b. It begins as a laryngo- tracheal groove on
the ventral aspect of the foregut, which
deepens and forms a repiratory
diverticulum.
c. Separates from the oesophagus

9. RESPIRATORY DIVERTICULUM
Tracheoesophageal
septum divides the
cephalic portion of
foregut into:
 Ventral portion-
Trachea and lung buds
 Dorsal portion-
Esophagus

10. The respiratory diverticulum bifurcates into right
and left bronchial buds on day 26-28.
Asymmetric branching occurs during the following 2
weeks to form secondary bronchi: 3 on the right
and 2 on the left forming the main divisions of the
bronchial tree.
The lung bud and its subsequent branches are of
endodermal origin.
They give rise to the epithelium lining all the
respiratory passages, the alveoli and the
associated glands.
The surrounding mesoderm, the splanchnopleure,
gives rise to all the supporting structures: the
connective tissue, cartilage, muscle and blood
vessels.

12. The pattern of branching is regulated by the
surrounding mesoderm.
• The mesoderm surrounding the trachea inhibits
branching whereas the mesoderm surrounding
the bronchi stimulates branching.
• Transplantation of part of the bronchial
mesoderm to replace part of the tracheal
mesoderm forms an ectopic lobe of the lung
arising directly from the trachea. Transplantation
of part of the tracheal mesoderm to replace part
of the bronchial mesoderm suppresses the
formation of a lobe.

13. Development of the lungs
passes through five stages:
• The sequence is most important rather
than the actual timing, which is variable in
the existing literature.
• week 4 - 5 embryonic
• week 5 - 17 pseudoglandular
• week 16 - 25 canalicular
• week 24 - 40 terminal sac
• late fetal - 8 years alveolar

14. Embryonic
• Endoderm - tubular ventral growth from
foregut pharynx.
• Mesoderm - mesenchyme of lung buds.
• Intraembryonic coelom - pleural cavities
elongated spaces connecting pericardial
and peritoneal spaces

15. Pseudoglandular stage
• week 5 - 17
• tubular branching of the human lung airways
continues
• by 2 months all segmental bronchi are present.
• lungs have appearance of a glandlike structure.
• stage is critical for the formation of all conducting
airways.
• lined with tall columnar epithelium, the more distal
structures are lined with cuboidal epithelium.

16. Canalicular stage
• week 16 - 24
• Lung morphology changes dramatically
• differentiation of the pulmonary epithelium results
in the formation of the future air-blood tissue
barrier.
• Surfactant synthesis and the canalization of the
lung parenchyma by capillaries begin.
• future gas exchange regions can be distinguished
from the future conducting airways of the lungs.

17. Saccular stage
Alveolar sac structure

18. Saccular stage
• week 24 to near term.
• most peripheral airways form widened airspaces, termed
saccules.
• saccules widen and lengthen the airspace (by the
addition of new generations).
• future gas exchange region expands significantly.
• Fibroblastic cells also undergo differentiation, they
produce extracellular matrix, collagen, and elastin. May
have a role in epithelial differentiation and control of
surfactant secretion
• The vascular tree also grows in length and diameter
during this time.

19. TERMINAL SAC PERIOD
(27TH week)
Type I Alveolar Epithelial Cells- cells of cuboidal
lined bronchioles change into thin, flat cells
=gas exchange between blood
=air possible in primitive alveoli
Type II alveolar epithelial cells produced surfactant
(phospholipid coat on alveolar membrane) which
lower surface tensiont air alvolar interface
=start to be produced at end of 6th month
=maximum production 2 weeks before
birth

20. Alveolar stage
• near term through postnatal period.
• 1-3 years postnatally alveoli continue to
form through a septation process increasing
the gas exchange surface area.
• microvascular maturation occurs during this
period.
• respiratory motions and amniotic fluid are
thought to have a role in lung maturation.

21. • Postnatally new alveoli continue to be formed
during childhood to the age of about 8 years.
• Clinical Implications
• Babies born before 28 weeks have very small
chances of survival because they lack pulmonary
alveoli. A few manage to survive following
intensive respiratory assistance.
• Babies born between 32 and 36 weeks usually
suffer from the respiratory distress syndrome
due to lack of surfactant secreted by pneumocytes
type II. The respiratory distress syndrome is
treated by:
• a. Intensive respiratory assistance
• b. Surfactant therapy including surfactant
lipoprotein and surfactant associated proteins A,B
and C.

22. Development of the Pleural
Cavities and Diaphragm
• .

23. Development of the Pleural
Cavities and Diaphragm
• .

24. Development of the Pleural
Cavities and Diaphragm
• The pleuro-peritoneal canals pass dorso-
lateral to the septum transversum.
Separation of the pleural and peritoneal
cavities occurs by growth of paired pleuro-
peritoneal folds (or membranes) at the
level of the septum transversum.

25. Development of the Pleural
Cavities and Diaphragm
• The pleuroperitoneal folds arise from the
posterolateral part of the pleuro-peritoneal
canals and grow towards the septum
transversum between the 5th and 7th
weeks. The left pleuro-peritoneal canal is
larger than the right, and closure occurs
slightly later.

28. • The muscle of the diaphragm is derived
from myoblasts that migrate into the
septum transversum from the cervical
myotomes C3,4,5, innervated by the
phrenic nerve.

29. Anomalies in the formation of the tracheal outgrowth result in
oesophageal atresia and tracheo-oesophageal fistula.
• The tracheal bud is the site of rapid cell
proliferation of the tracheal outgrowth from
the second (oesophageal part of the fore
gut. This is followed by apoptosis to convert
the outgrowth into a tubular structure. At
the same time, the second (oesophageal)
part of the foregut also proliferates rapidly,
causing obliteration of the lumen and
subsequent re-canalization by apoptosis. A
defect in programming of foregut
differentiation results in

30. Congenital Malformation
• oesophageal atresia or tracheo-oesophageal atresia
or both. These are therefore considered to be one defect
with different manifestations as shown in the diagrams
below. Note that atresia is commonly present. The
oesophagus often communicates with the trachea either
above or below the atresia.
• The clinical features of oesophageal atresia are:
a. Inability to swallow and choking on attempts at feeding.
b. Inability to introduce a naso-gastric tube
c. X-rays often shows absence of a stomach bubble, but
this may be present depending on the site of the atresia.

31. TRACHEO-ESOPHAGEAL FISTULA
Different Types
DUE TO FAULTY PARTITIONING OF FOREGUT BY
TRACHEOESOPHAGEAL SEPTUM

32. • Oeosophageal atresia is usually
accompanied by polyhydramnios during
pregnancy. This is due to inability of the
foetus to swallow amniotic fluid while it
continues to be formed by the kidneys.

33. Congenital anomalies of the
Respiratory system and diaphragm
• 1. Congenital diaphragmatic hernia is a
defect in the formation of the diaphragm. It
is usually due to failure of closure of the
pleuro-peritoneal canal.

34. Congenital anomalies of the
Respiratory system and diaphragm
• In congenital diaphragmatic hernia the
abdominal contents herniate into the thorax
and compresses the lung, causing severe
pulmonary hypoplasia, which is frequently
the cause of death.
The clinical features are:
• o Respiratory difficulty
• o Absent breath sounds on the affected side.

35. Congenital anomalies of the
Respiratory system and diaphragm
• o Impaired thoracic movements
• o Radiologically, coils of intestine are
visible in the thorax and the mediastinum
is shifted to the opposite side. Surgical
correction of the diaphragmatic defect is
possible, but there is a high mortality
depending mainly on the degree of
pulmonary hypoplasia.
•

36. Congenital anomalies of the
Respiratory system and diaphragm
• 3. Pulmonary agenesis and pulmonary
aplasia are defects in the development of
the lung buds or of the surrounding
mesoderm. They are both quite rare.

37. Congenital anomalies of the
Respiratory system and diaphragm
• 2. Pulmonary agenesis and pulmonary
aplasia are defects in the development of
the lung buds or of the surrounding
mesoderm. They are both quite rare.
• 3. Pulmonary hypoplasia is usually
secondary to pulmonary compression
during development. This is usually
secondary to congenital diaphragmatic
hernia and oligohydromnios
• d.

38. PHARYNGEAL ARCHES
• The pharyngeal arches (branchial arch, Greek,
branchial = gill) are a series of externally visible
anterior tissue bands lying under the early brain
that give rise to the structures of the head and
neck. Each arch though initially formed from
similar components will differentiate to form
different head and neck structures. In humans,
five arches form (1,2,3,4 and 6) but only four are
externally visible on the embryo.

39. • Each arch has initially identical structures:
an internal endodermal pouch, a
mesenchymal (mesoderm and neural
crest) core, a membrane (endoderm and
ectoderm) and external cleft (ectoderm).
Each arch mesenchymal core also
contains similar components: blood
vessel, nerve, muscular, cartilage

40. Pharyngeal Arch Development

42. branchial arch (Gk. branchia= gill)
arch consists of all 3 trilaminar embryo
layers ;
• ectoderm- outside
• mesoderm- core of mesenchyme
• endoderm- inside

43. Neural Crest
• Mesenchyme invaded by neural crest
generating connective tissue components
• cartilage, bone, ligaments
• arises from midbrain and hindbrain region

44. Arch Features
• Each arch contains: artery, cartilage,
nerve, muscular component
• Arches and Phanynx Form the face,
tongue, lips, jaws, palate, pharynx and
neck cranial nerves, sense organ
components, glands

45. • Humans have 5 arches - 1, 2, 3, 4, 6 (Arch
5 does not form or regresses rapidly)
• from in rostro-caudal sequence, Arch 1 to 6
from week 4 onwards
• arch 1 and 2 appear at time of closure of
cranial neuropore
• Face - mainly arch 1 and 2
• Neck components - arch 3 and 4 (arch 4
and 6 fuse)

46. Arch Features
– arch
– groove
• externally separates each arch
– also called a cleft
• only first pair persist as external auditory meatus
– pouch
• internally separates each arch
• pockets from the pharynx
– membrane
• ectoderm and endoderm contact regions
• only first pair persist as tympanic membrane

47. Structures formed from pharengeal arches
• Pharyngeal Arch 1 (Mandibular Arch) has 2 prominances
– smaller upper- maxillary forms maxilla, zygomatic bone and
squamous part of temporal
– larger lower- mandibular, forms mandible and
malleus,incus(auditory ossicles)
• Pharyngeal Arch 2 (Hyoid Arch)
– forms most of hyoid bone
– The stapes(3rd auditory ossicles)
– Styloid process of temporal bone
• Arch 3 and 4
– neck structures

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