RESPIRATORY SYSTEM

• Respiratory system


  Consists of the nose, nasopharynx, larynx, trachea, extrapulmonary bronchi and lungs
• Respiratory system in birds


  • Trachea terminates in a flattened y-shaped tube, the syrinx
• Parts of the nose

  • Nasal cavity
  • Alar or wings
  • Bridge
  • External or anterior nares
  • Choanae or posterior nares
  • Nasal septum
  • Nasolacrimal duct
• Formation of the nasal cavity


  1. Surface ectoderm at the rostral end of the frontonasal prominence thickens to form bilateral nasal placodes

  2. Placodes become nasal pits by outward growth of the surrounding medial and lateral nasal processes (ectomesenchyme)

  3. Continued outgrowth of the medial and lateral nasal processes elongates the nasal pits and transforms them into a primitive nasal cavity that communicates bilaterally with the oral cavity

  4. Nasal and oral cavities are separated by the formation of the palate

  5. Nasal cavity opens caudally into the nasopharynx through the choanae or posterior nares
• Lining epithelium of the nasal cavity


  Derived from surface ectoderm (nasal placode)
• Lining of the rest of the respiratory system

  Comes from endoderm
• Paranasal sinuses


  Diverticula of the epithelial lining of the nasal cavity that develop postnatally
• Vomeronasal organ


  Specialized olfactory sense organ that develops from the epithelium in the floor of the nasal cavity
• Formation of the external nares (nostrils)

  1. Bilateral rostral openings of the nasal cavity

  2. Ectomesenchyme surrounding them forms cartilage of the nose

  3. Each lateral nasal process gives rise to the alar cartilage (wing of the nose)

  4. Right and left medial nasal processes fuse to form a primary palate rostrally and a nasal septum caudally
• Nasal septum


  • Consists of bone, cartilage, and loose connective tissue
• Nasal conchae


  • Cartilaginous ridges that arise from the wall of the nasal cavity and grow into each nasal cavity
• Formation of other parts of the nose

  1. Frontonasal prominence => bridge of the nose

  2. Depression between the lateral nasal process and the maxillary process deepens to form the nasolacrimal groove, which closes over to form the nasolacrimal duct
• Origin of the other organs of the respiratory system

  1. Larynx, trachea (and syrinx in birds), extrapulmonary bronchi and lung are derived from intraembryonic splanchnopleure composed of an invaginated pharyngeal endoderm, the laryngotracheal groove, and the closely applied splanchnic mesoderm

  2. Epithelial lining of the larynx, trachea, extrapulmonary bronchi and bronchial trees are derived from the laryngotracheal groove endoderm
• Formation of the laryngotracheal groove


  1. Derived from the invaginated endoderm in the median floor of pharynx at the level of PP 4

  2. Groove grows caudo-ventrally into the splanchnic mesoderm beneath the foregut and is separated dorsally from the esophagus by the formation of the transverse tracheoesophageal ridge, and elongates to form the larynx and the trachea
• Formation of the larynx

  1. Ectomesenchyme of VA4 aggregates and elongates in a transverse plane to form the epiglottal swelling

  2. Splanchnic mesoderm on the lateral sides of the laryngotracheal groove also aggregate and elongate in a longitudinal plane to form bilateral laryngeal swellings

  3. The epiglottal swelling and the two laryngeal swellings grow together to form the larynx with a T-shaped opening

  4. Epiglottal swelling becomes the epiglottic cartilage of the larynx

  5. The laryngeal swellings become the arytenoid, thyroid and cricoid cartilages of the larynx

  6. A diverticulum of the lateral laryngeal wall produces a lateral ventricle and vocal fold (except in cats and cattle)

  7. Laryngeal muscles develop from somite myotomes that migrate into VA4

  8. Lining epithelium of larynx is derived from endoderm of the laryngotracheal groove
• Formation of the trachea

  1. The laryngotracheal groove grows caudally into splanchnic mesoderm located ventral to the pharynx

  2. The endoderm of the laryngotracheal tube becomes the respiratory (lining) epithelium of the trachea

  3. The apposed splanchnic mesoderm of the laryngotracheal groove becomes the cartilage and connective tissue of the trachea

  4. Tracheal elongation shifts bronchi caudally into the thorax

  5. The blind caudal end of the laryngotracheal groove develops bilobed bronchial buds which grow to form the extrapulmonary bronchi

  6. In ruminants and swine the trachea gives off a tracheal bronchus that supplies the right cranial lobe of the lung
• Bronchi and lung development


  1. The fetal period can be subdivided into overlapping pseudo-glandular, canalicular, saccular and alveolar periods

  2. The postnatal period has a marked longitudinal growth of the respiratory bronchioles and the alveolar saccule, as well as additional alveolar development from the walls of these structures

  3. At birth, most of the fluid in the respiratory system is expelled through the mouth and nose, and the first inspiration fills the system with air, with remaining fluid removed by absorption
• Formation of the lung

  1. Outgrowths of each extrapulmonary bronchus form future lobar bronchi, each of which gives rise to more than dozen additional branches to form a bronchial tree

  2. The smallest branches are bronchioles (mammals) that give rise to lung alveolar ducts, sacs and alveoli (parabronchus, air vesicles and air capillaries in avian)

  3. The bronchial branchings continue to occur throughout the fetal period and into the postnatal period, with subsequent lung growth due to hypertrophy (increase size) of alveoli and air ways

  4. The endoderm of the laryngotracheal tube becomes the respiratory (lining) epithelium of the bronchial tree

  5. The apposed splanchnic mesoderm of the laryngotracheal groove becomes the cartilage and connective tissue and smooth muscle of the bronchial tree and blood vessels of the lung

  6. Continued branching of the bronchial tree results in lung tissue occupying more and more of the pleural cavity, coated by visceral pleura

  7. Initially, the bronchial tree branches are solid cords of cells that grow into the splanchnic mesoderm, eventually becoming hollow, dilated and sac-like (alveolar sac that becomes partitioned into alveoli)

  8. Some endodermal (lining epithelial) alveolar cells become cuboidal and secrete a phospholipid surfactant

  9. Fetal lungs contain fluid that facilitates the breathing movements that take place in utero, which drains or is absorbed as air is breathed at birth
• Changes in the position of lungs

  1. In very young embryos the lungs are situated dorsal to the heart

  2. In older embryos the lungs are situated dorsal to the heart and liver

  3. Eventually the lungs expand laterally and ventrally to enclose the heart
• Tracheoesophageal fistula results from a partial persistence of the laryngotracheal groove, causing refluxing of feed through the upper respiratory tract and aspiration pneumonia in newborns
• Barker foal syndrome (hyaline disease) results from a lack of production of pulmonary surfactant, causing gasping in newborns