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660        seCtioN iii    RespiRatoRy  ` RESPIRATORY—EmbRYOlOgY                                                                                                       RespiRatoRy  ` RESPIRATORY—EmbRYOlOgY





                `  RESPIRATORY—EmbRYOlOgY

               Lung development      Occurs in five stages. Initial development includes development of lung bud from distal end of
                                       respiratory diverticulum during week 4. Every Pulmonologist Can See Alveoli.
                STAgE                STRUCTURAl DEVElOPmENT                    NOTES
                Embryonic            Lung bud Ž trachea Ž bronchial buds       Errors at this stage can lead to
                 (weeks 4–7)          Ž mainstem bronchi Ž secondary (lobar)    tracheoesophageal fistula.
                                      bronchi Ž tertiary (segmental) bronchi.
                Pseudoglandular      Endodermal tubules Ž terminal bronchioles.   Respiration impossible, incompatible with life.
                 (weeks 5–17)         Surrounded by modest capillary network.
                Canalicular          Terminal bronchioles Ž respiratory bronchioles   Airways increase in diameter.
                 (weeks 16–25)        Ž alveolar ducts. Surrounded by prominent   Respiration capable at 25 weeks.
                                      capillary network.                       Pneumocytes develop starting at 20 weeks.
                Saccular             Alveolar ducts Ž terminal sacs. Terminal sacs
                 (week 26–birth)      separated by 1° septae.
                Alveolar             Terminal sacs Ž adult alveoli (due to 2°   At birth: 20–70 million alveoli.
                 (week 36–8 years)    septation).                              By 8 years: 300–400 million alveoli.
                                     In utero, “breathing” occurs via aspiration
                                      and expulsion of amniotic fluid Ž  vascular
                                      resistance through gestation.
                                     At birth, fluid gets replaced with air Ž  in
                                      pulmonary vascular resistance.
                                      Embryonic                           Fetal                          Postnatal
                                      period                              period                         period

                                                                                                      Alveolar
                                                                                      Saccular
                                                                   Canalicular                  BIRTH
                                                 Pseudoglandular
                                           Embryonic                                       Surfactant
                                       2  4   6  8  10  12  14  16  18  20  22  24  26  28  30  32  34  36  38  40  2  4  6  8
                                                                                                   Weeks  Years

                                      L



                                      R





               Congenital lung malformations
                Pulmonary hypoplasia  Poorly developed bronchial tree with abnormal histology. Associated with congenital diaphragmatic
                                       hernia (usually left-sided), bilateral renal agenesis (Potter sequence).
                Bronchogenic cysts   Caused by abnormal budding of the foregut and dilation of terminal or large bronchi. Discrete,
                                       round, sharply defined, fluid-filled densities on CXR (air-filled if infected). Generally
                                       asymptomatic but can drain poorly, causing airway compression and/or recurrent respiratory
                                       infections.













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