RESPIRATORY PRIMORDIUM

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RESPIRATORY PRIMORDIUM

• It is formed in the (4th ) week as median
  outgrowth
• at the caudal end of the Pharynx (Foregut).

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LARYNGOTRACHEAL GROOVE

• The respiratory primordium is defined at (26-27)
  days as a Laryngotracheal groove caudal to the
  4th pair of Pharyngeal Pouches.

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LARYNGOTRACHEAL DIVERTICULUM

• By the end of the (4th week) the laryngotracheal
  groove has Evaginated to form the pouch like
  LaryngoTracheal Diverticulum.

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LARYNGOTRACHEAL DIVERTICULUM

• The Endoderm of the Foregut will form
• The Epithelial lining and glands of the larynx,
  trachea, bronchi and lungs.
• The surrounding Splanchnic mesoderm will form
• The connective tissue, cartilage and smooth
  muscles of these structures.

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LUNG BUD

• It is the expanded distal end of the
  laryngotracheal diverticulum.
• Initially, it is in open communication with the
  foregut.

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TRACHEOESOPHAGEAL SEPTUM

• Two longitudinal Tracheoesophageal folds (ridges)
  develop in the diverticulum.
• They approach each other and fuse to form the
  tracheoesophageal septum.

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TRACHEOESOPHAGEAL SEPTUM

• The foregut will be divided into
• Dorsal portion (Esophagus).
• Ventral portion (trachea and lung bud).
• The respiratory primordium keeps its
  communication with the pharynx through the
  Laryngeal Inlet.

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LARYNX

• CARTILAGES
• Derived from proliferation of mesenchyme in the
  4th and 6th pharyngeal arches.
• This mesenchyme is derived from Neural Crest
  cells.

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CARTILAGES

• The mesenchyme of the two arches transforms into
  Arytenoid, Thyroid and Cricoid cartilages.

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CARTILAGES

• This proliferation changes the appearance of the
  laryngeal orifice (primordial Glottis) from a
  narrow slit into a T -shaped laryngeal inlet.

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EPIGLOTTIS

• It is developed from the caudal part of the
  Hypobranchial Eminence.
• It reaches its adult form during the first three
  years after birth.

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EPITHELIAL LINING

• It is derived from the Endoderm of the cranial
  part of the laryngotracheal tube.
• Its rapid proliferation causes temporary
  occlusion of the laryngeal inlet.
• Recanalization occurs during the 10th week.

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EPITHELIAL LINING

• The ventricles of the larynx form during this
  recanalization.
• They are bounded by two folds of mucous membrane
  
• Vocal Folds (Cords) and Vestibular Folds.

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MUSCLES

• They are derived from myoblasts in the 4th and
  6th pharyngeal arches.
• They have nerve supply from the laryngeal
  branches of the Vagus nerves that supply these
  arches.

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TRACHEA

• The Endoderm of the laryngotracheal tube distal
  to the larynx gives rise to
• Epithelial lining and Glands.
• Splanchnic Mesoderm gives rise to
• Connective tissue, Cartilages and Muscles.

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BRONCHI

• Bronchial Buds develop as two outpouchings of
  the lung bud.
• They grow laterally into the pericardio
  peritoneal canals (primordia of the pleural
  cavities).

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PRIMARY (MAIN) BRONCHI

• It is the enlarged connection between the trachea
  and the bronchial buds.
• Right Primary Bronchus
• Larger and more vertical than the left(this
  embryonic picture is maintained till adult life).

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SECONDARY (STEM) BRONCHI

• They develop from subdivision of the Primary
  (main ) Bronchi.
• They supply the lobes of the lung.
• On the Right
• They are Superior, and Inferior (subdivide into
  Middle and lower).
• On the Left
• They are Superior and Inferior.

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TERTIARY (SEGMENTAL) BRONCHI

• In the Right Lung (10) in number.
• In the Left Lung
• (8) or (9) in number.
• BronchoPulmonary segment
• It is the segmental bronchus with its surrounding
  mass of mesenchyme.

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TERTIARY (SEGMENTAL) BRONCHI

• At (24) weeks
• (1)17 order of branches are formed.
• After birth (7) orders of branches are formed.
• (2) Respiratory Bronchioles have developed .

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PLEURA

• VISCERAL
• Is derived from the Splanchnic mesoderm.
• PARIETAL
• Is derived from the Somatic mesoderm. It lines
  the thoracic body wall.

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DEVELOPMENT OF THE LUNGS

• It depends on
• (1) Adequate thoracic space.
• (2) Fetal breathing movements before birth. It
  stimulates lung development by creating pressure
  gradient between the lungs and the amniotic
  fluid.

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MATURATION OF THE LUNGS

• (3) Adequate volume of amniotic fluid.
• Maturation of the lung passes through four
  periods.

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(1)PSEUDO GLANDULAR PERIOD

• (5-17) WEEKS
• 1. Glandular appearance of the lung (as an
  exocrine gland).
• 2. All major elements of the lung are formed.
• 3. No Respiratory Bronchioles or Alveoli.

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PSEUDO GLANDULAR PERIOD

• 4. Respiration is Not possible.
• 5. Fetuses born during this period are Unable to
  survive.

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(2) CANALICULAR PERIOD

• (16-25) WEEKS
• 1. The lumina of Bronchi and Terminal Bronchioles
  are larger.
• 2. Respiratory Bronchioles Each terminal
  bronchiole divides into two or more respiratory
  bronchioles.

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CANALICULAR PERIOD

• 3. Alveolar ducts The respiratory bronchioles
  divide into (3-6) tubular passages.
• 4. Many Blood vessels are developing in the
  mesenchyme surrounding the bronchi and terminal
  bronchioles.

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CANALICULAR PERIOD

• 5. (Some) Terminal sacs appear at the ends of
  the respiratory bronchioles.
• 6. Lung tissue is well vascularised.

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CANALICULAR PERIOD

• 7. Respiration is Possible toward the end of this
  period.
• 8. Fetuses born May survive if given intensive
  care.
• They often die because of immaturity of the
  respiratory and other systems.

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(3) TERMINAL SAC PERIOD

• (24 WEEKS- BIRTH)
• 1. Many terminal sacs develop.
• 2. Their epithelium becomes very thin.
• 3. Capillaries begin to bulge into these
  developing alveoli.

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TERMINAL SAC PERIOD

• 4. This establishes intimate contact between the
  Epithelial and Endothelial cells.
• 5. The Cuboidal Endodermal epithelium becomes
  thin Squamous (Type 1 Alveolar epithelial cells) .

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TERMINAL SAC PERIOD

• Type 1 Alveolar cells (Pneumocytes) permit
  adequate gas exchange Blood Air Barrier (for the
  survival of premature fetuses).

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(6) PULMONARY SURFACTANT

• Production
• From Type (11) Alveolar cells, these are rounded
  secretory epithelial cells scattered among the
  squamous cells.
• STRUCTURE
• A complex mixture of phospholipids.

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PULMONARY SURFACTANT

• TIME of production
• It begins by (20) weeks.
• It increases during the last (2) weeks before
  birth.
• FUNCTIONS
• It forms a monomolecular coat over the internal
  walls of the terminal sacs.

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PULMONARY SURFACTANT

• (a) It lowers the surface tension at the air-
  alveolar interface by counteracting the tension
  forces.
• (b) It facilitates the expansion of the terminal
  sacs.
• Deficiency causes Respiratory Distress because
  of collapse of the primitive alveoli.

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(4) ALVEOLAR PERIOD

• Late Fetal period- Childhood
• 1. Each respiratory bronchiole terminates in a
  cluster of thin walled terminal sacs (future
  alveolar ducts).
• 2. The epithelial lining of the terminal sacs
  becomes Extremely Thin squamous.

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ALVEOLAR PERIOD

• 3. Loose connective tissue with large pulmonary
  lymphatic vessels separates the terminal sacs.
• 4. The AlveolCapillary Membrane (Respiratory
  Membrane) is sufficiently thin to allow gas
  exchange.

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AUTONOMOUS GAS EXCHANGE

• It requires
• (1) Adequate amount of Surfactant.
• (2) The lung is changed from secretory to gas
  exchanging organ.
• (3) Existence of parallel pulmonary and systemic
  circulations.

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ALVEOLI

• 1. A full term newborn has (50) million alveoli
  (1/6) of the adult number.
• 2. On chest radiographs, the lungs of newborn
  infants are denser than adult lungs.
• 3. 95 of alveoli develop after birth( first 8
  years after birth) .

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ALVEOLI

• 4. Immature alveoli have the potential for
  forming additional alveoli.
• 5. The primordial alveoli become mature by
  increasing in size.

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MATURATION OF THE LUNGS

• 1. Cranial segments mature faster than caudal
  ones.
• 2. Growth of the lungs after birth is primarily
  due to an increase in the Number of respiratory
  bronchioles and alveoli and not to an increase in
  the Size of the alveoli.

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TRACHEOESOPHAGEAL FISTULA

• Etiology
• Incomplete division of the cranial part of the
  foregut into respiratory and esophageal parts.
• Defective tracheoesophageal septum causes
  abnormal communication between the trachea and
  esophagus.

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TRACHEOESOPHAGEAL FISTULA

• It is the most common anomaly of the lower
  respiratory tract especially in Male infants.
• It is usually associated with other congenital
  anomalies.

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TRACHEOESOPHAGEAL FISTULA

• Most common variety
• Blind ending of the superior part of the
  esophagus (esophageal atresia) and joining of the
  inferior part to the trachea near its
  bifurcation.

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TRACHEOESOPHAGEAL FISTULA

• Manifestations
• Cough and choke during swallowing because of
  accumulation of excessive saliva in the mouth and
  upper respiratory tract.
• Gastric contents may reflux from the stomach
  through the fistula into the trachea and lungs.