Embryology of the Lungs

structures found in conduction portion

• URT:

  • nasal cavities

  • nasopharynx

  • oropharynx

  • larynx

• LRT:

  • trachea

  • bronchi

  • bronchioles

  • terminal bronchioles

structures found in respiratory portion

• respiratory bronchioles

• alveolar ducts

• alveolar sacs

• alveoli

growth of lung bud

lung bud at week 4

• foregut material is pulled anterioraly to form a lung bund

• groove is formed→ tracheoesophageal groove

• lung bud starts to pinch off, which pinches off to form tracheoesophageal septum

• bifocation of primitive trachea into bronchial buds

growth of lung bud- week 5

• developing trachea has fully pinched off from foregut anterior to oesophagus

• complete septation of the two structures→ trachea is separate to oesophagus

tracheoesophageal fistulas

• occur when oesophagus and trachea are linked as opposed to completely separating

tracheoesophageal fistual with oesophageal atresia

• difficult to detect prior to birth

• stomach still able to fill with amniotic fluid via connection of oesophagus to trachea→ normal appearance on prenatal scan

• upper portion of oesophagus becomes distended as also fills with amniotic fluid→ can give normal stomach appearance on scan

tracheoesophageal fistula repair

• surgery required soon after birth

• connection between trachea and oesophagus is closed

• upper and lower parts of oesophagus are connected

• if gap is large→ patient will need to wait few months for oesophagus to grow more:

  • until then, fed through tube directly to stomach

vacterl

• describes group of anomalies- often occur together in newborn babies:

  • Vetebral (spinal) defects

  • Anorectal atresia (failure of anus and lower end of gut to form)

  • Cardiac (heart) defects

  • Tracheoesophagal fistula with or without Eoshageal atresia

  • Renal (kidney) anomalies

  • Limb defects

formation of the pleura

• derived from mesoderm layer or trilaminar disc

• lateral plate mesoderm forms two layers:

  • parietal division

  • visceral division

• in thoracic cavity these form parietal and visceral pleura

stages of lung development

1. pseudoglandular

2. canalicular

3. saccular

4. alveolar

pseudoglandular stage

• week 5- week 16

• right and left primary bronchi

• secondary (lobar) bronchi:

  • 3 on right

  • 2 on left

• tertiary bronchi

• terminal bronchioles

canalicular stage

• week 16- week 26

• respiratory bronchioles

• primitive alveoli→ cuboidal cells

• pulmonary capillaries

saccular stage

• week 26-birth

• increase in number of primitive alveoli

• primitive alveoli begin to mature

• cuboidal cells become type I and type II pneumocytes:

  • type I→ flat cells (gaseous exchange)

  • type II→ cuboidal cells (surfactant)

alveolar stage

• week 36- 8yrs

• increased number of primitive alveoli become specialised

• mature alveoli develop septa→ increases surface area

• at birth→ 100mln primitive alveoli

• 8 years→ 300mln primitive alveoli

before birth breathing

• before birth

  • breathing in amniotic fluid

• during birth

  • air breathed in→ amniotic fluid ‘sucked up’ by pulmonary capillaries

  • all that should remain is surfactant

infant respiratory distress syndrome

• child born during canalicular stage (~24 wk) have low chance of survival

• alveoli not well developed

• low levels of surfactant→ alveoli prone to collapse

  • baby unable to take in high volume of air, breathing rate increases

• mechanical ventilation necessary:

  • causes damage to alveoli

  • leads to bronchopulmonary dysplasia

bronchopulmonary dysplasia

• damaged lung tissue

• slightly collapsed alveoli

pulmonary agenesis

• complete absence of one or both lungs:

  • can be just a lobe

• disruption of lung bud during formation

pulmonary hypoplasia

• either one or both lungs do not fully develop

• level or respiratory distress results from degree of hypoplasia

• can be found in association with congenital diaphragmatic hernia