Development of the Respiratory System

During the 4 th week the respiratory diverticulum develops as an outgrowth from the ventral wall of the foregut. Laryngeotracheal groove forms caudal to 4 th pair of pharyngeal pouch Tracheoesophageal ridges form and eventually separate the 2 tubes, esophagus and laryngeotracheal tube

Larynx The lining is derived from endoderm The cartilage, muscle and connective tissue are derived from splanchnic mesoderm of the 4 th – 6 th arch that came from neural crest mesenchyme cells The cranial end mesenchyme produces arytenoid swellings which grow toward the tongue, forming the laryngeal inlet(vocal cords)

Larynx Rapidly growing epithelia temporarily closes the lumen but this will reopen at about the 10 th week, forming the laryngeal ventricle. This is bounded by the vocal and vestibular folds. The epiglottis develops from the hypobranchial eminence (3 rd & 4 th pharyngeal arch) Innervated by vagus nerve The larynx and epiglottis will continue to grow until the third year after birth

Trachea Endoderm becomes the epithelial linings and the glands The cartilage, muscle and connective tissue are derived from splanchnic mesoderm of the 4 th – 6 th arch that came from neural crest cells Some tracheal cartilage from pharyngeal arch cartilage

Bronchi and lungs Lung bud develops fourth week –This soon divides into two branches (bronchial buds) The bronchial buds elongate during the 5 th week, becoming the primary bronchi which go to each lung The right side will be wider and more vertical and allows an easier passage of foreign material than does the left side.

Bronchi and lungs The secondary bronchi form next. Three on the right side, two on the left. Secondary bronchi go to each lobe of the lungs and therefore are referred to as lobar bronchi The tertiary (segmental) bronchi form by 7 th week By 24 th week, 17 orders of branching occurs, seven additional orders will be present after birth

Maturation of the lungs Pseudoglandular (weeks 5-17) –Most of the elements of the respiratory system are present, but those elements involved with gas exchange have not developed yet. Canalicular (weeks 16-25) –Respiratory bronchials are present (week 24) –Primordial alveolar ducts appear (terminal sacs) They start as cuboidal epithelia but will flatten to become squamosal –The lungs will be well vascularized

Maturation of the lungs Terminal sac (week 24-birth) –Terminal sac epithelia flatten –Gas exchange can occur –Type I pneumocytes –squamous epithelia – gas exhange occurs with these cells –Type II – secretory cells – cuboidal or columnar Secretes surfactant which coats the lungs and allows greater stretch of the terminal sacs – to allow for sac expansion Alveolar –Terminal sacs continue to thin

Needed for gas exchange Surfactant – a phospolipid fluid –Necessary for expansion of the lungs during inspiration –Helps increase surface area for possible gas exchange Thin membranes capillaries and terminal sacs Pulmonary and systematic circulation

Alveolar number will continue to increase until age 8

Breathing will begin immediately after birth Breathing motions occur before birth allowing the lungs to fill with amniotic fluid The fluid is cleared through the nose or mouth, through the capillaries, and through the lymphatic vessles, arteries and veins

Important for lung development Space for growth inside the thoracic cavity Early breathing movements Amniotic fluid volume