RESPIRATORY SYSTEM DEVELOPMENT PROF. DR. AYŞE GÜLER EROĞLU

PRENATAL LUNG GROWTH 1-EMBRYONIC (DAY 26 TO DAY 52) 2-PSEUDOGLANDULAR (DAY 52 TO WEEK 16) 3-CANALICULAR (WEEK 16 TO 28) 4-SACCULAR (WEEK 28 TO 36) 5-ALVEOLAR (WEEK 36 TO TERM)

THE DEVELOPMENT OF THE AIR WAYS

1-EMBRYONIC PERIOD The lung first appears as a ventral outpouching of the primitive gut. The primary bronchi elongate into the mesenchyme and divide into the two main bronchi.

2-PSEUDOGLANDULAR STAGE The mesenchyme differentiates into cartilage, smooth muscle, connective tissue. All the major conducting airways including terminal bronchioles have formed.

3-CANALICULAR PERIOD The respiratory bronchioles have developed. The glandular appearance is lost.

4-SACCULAR STAGE The pulmonary capillaries proliferate and the epithelium thins. Gas exchange is possible. Cuboidal (type II) and thin (type I) epitelhial cells line the air space.

5-ALVEOLAR STAGE The walls of true alveoli develop The surface area for gas exchange increase

THE DEVELOPMENT OF PULMONARY VASCULATURE

1-EMBRYONIC PERIOD The main pulmonary artery arises from the sixth branchial arch.

2-PSEUDOGLANDULAR STAGE The pulmonary arteries are evident alongside the conducting airways. Supernumerary arteries are evident by 12 weeks of gestation. All preacinar arteries have formed by 12 weeks of gestation.

3-CANALICULAR PERIOD The lung develops a rich vascular supply that is closely associated with the respiratory bronchioles.

4-SACCULAR STAGE The saccules develop and capillaries can be found within the walls of the air spaces. Gas exchange can occur.

SURFACTANT PRODUCTION Surfactant is a mixture of phospholipids. Surfactant decreases surface tension at the air-liquid interface. The prematurely born infants with hyaline membran disease had abnormal surface tension at the air- liquid interface.

SURFACTANT PRODUCTION The source of pulmonary surfactant is the mature type II epithelial cell. Type II epithelial cell first appears during the saccular stage, however it is immature. Many drugs and hormones (steroids, throid and peptide hormones) can influence its biosynthesis and accelerate lung maturation.

THE CONTROL OF LUNG DEVELOPMENT The control of lung development is strongly influenced by interactions between the pulmonary epithelium and mesenchyme.

FETAL LUNG LIQUID SECRETİON The fetal lung liquid secretion is well established by the second half of gestation. It is produced at a rate of 4-5ml/kg/hr

THE LUNG AT BIRTH 1. The lung’s epithelium must change from fluid secretion to fluid absorbtion. 2. The distal lung units must fill with and retain the inhalated air. 3. The blood flow must increase approximately twentyfold.

1. Fetal lung liquid is cleared at birth by several mechanisms a. One third is squeezed out during the birth process. b. The remainder is absorbed by the epithelium. Failure of normal lung water clearance at birth results in respiratory distress (transient tachypnea of the newborn or wet lung syndrome)

2. The distal lung units must fill with and retain the inhalated air The ability to retain air at end- expiration depends on the presence of the pulmonary surfactant. Pulmonary surfactant decreases surface tension at the air-liquid interface.

3. Marked increases in the capasity and distensibility of the pulmonary vasculature a. Inflation of the lung with air results in mechanical distension of the vessels. b. Improvement in oxygenation removes hypoxic vasoconstriction. c. The rise in PaO2 induces granulocytes in the lung to release massive quantities of kinin (dilate the pulmonary vascular bed)

POSTNATAL LUNG GROWTH The postnatal growth of the lung continues into the adolescent years and perhaps beyond.

POSTNATAL LUNG GROWTH The lung of the newborn is not a miniature of the lung of the adult. During growth tracheal diameter approximately triples, alveolar dimensions increase about fourfold and alveolar numbers increase about tenfold while body mass increases twentyfold.

POSTNATAL LUNG GROWTH The preaciner blood vessels and airways increase in size after birth. The number of alveoli rapidly increases from 20 million to 200 million by the third year of life, but then multiplication slows. New blood vessels appear within the acinus.

POSTNATAL LUNG GROWTH Branching of conventional arteries continues until 18 months of age, supernumerary arteries continue to appear until 8 years of age. Throughout the chidhood there is an increase in the concentration of arteries to alveoli.