RESPIRATORY SYSTEM At around 4th week respiratory system starts forming initially as an outgrowth from the ventral wall of the foregut. So, the epithelium lining the derivatives such as larynx, trachea, bronchi and alveoli is endodermal in origin. The cartilaginous and muscular portions of the trachea and lungs are derived from splanchnic mesoderm surrounding the foregut.

The trachea and lung buds are split off from the foregut by the esophagotracheal septum which divides the foregut into the respiratory diverticulum anteriorly and the esophagus posteriorly. Communication between these two is maintained by laryngeal orifice.

LARYNX Muscles and cartilages of larynx originate from mesenchyme of 4th and 6th pharyngeal arches.

Trachea Respiratory primordium forms a midline structure – the trachea and two lateral outpocketings the lung buds. The right lung bud eventually divides into three branches left lung bud into two branches.

PLEURA The mesoderm which covers the outside of the lung develops into the visceral pleura. The somatic mesoderm layer covering the body wall from the inside becomes the parietal pleura. The space between these two pleura is the pleural cavity.

BRONCHI During further development, the main bronchi divide repeatedly to from lobar bronchi and segmental bronchi.

MATURATION OF LUNGS Respiration becomes possible when some of the cells of the cuboidal respiratory bronchioli change into thin, flat cells, which are associated with blood and lymph capillaries. Surrounding spaces around these (inside these) are called terminal sacs or primitive alveoli.

Alveolar epithelial cells (Type I) – cells lining the sacs which are very thin and allow capillaries to protrude into the sacs. These are present mostly after 7th month and till after birth. Mature alveoli are not present before birth.

Alveolar epithelial cells (Type II) – these develops at the end of 6th month which mainly produce surfactant (substance lowering the surface tension). Without this surfactant the alveoli may collapse (ATELECTASIS). Surfactant is very important for premature babies as collapse may cause Respiratory Distress Syndrome (RDS).

Growth of the lungs after birth is mainly due to an increase in the number of respiratory bronchioli and alveoli. New alveoli formation will be there till the first ten years of the postnatal life.

By week 24, they divide another 14 times and the respiratory bronchioles have developed. They will divide an additional 7 more times before birth. As the bronchi develop, the surrounding mesenchyme synthesizes the surrounding cartilages, smooth muscle, connective tissue and capillaries.

LUNG DEVELOPMENT 1) Pseudoglandular period (5-17 weeks) By week 17 all major elements of the lungs have formed except for those involved with gas exchange. The lungs look like an endocrine organ. No respiration is possible!

2) Canalicular period (16-25 weeks) The lumen of the bronchi and terminal bronchioles become larger and the lungs become vascularized. By week 24, respiratory bronchioles have developed and respiration becomes possible, although the chances of survival are slim.

3) Terminal sac period (24 weeks to birth) More terminal sacs develop and capillaries enter into close relationship with them. They are lined with Type 1 alveolar cells or pneumocytes. Type II pneumocytes secrete surfactant counteracting the surface tension forces and facilitating expansions of the terminal sacs. Surfactant reaches adequate levels 2 weeks before birth.

Adequate pulmonary vasculature and sufficient surfactant are critical to the survival of premature infants.

4) Alveolar period (late fetal period to 8 years) 95% of the mature alveoli develop after birth. A newborn infant has only 1/6 to 1/8 of the adult number of alveoli and the lungs look denser in an x-ray. Developing lungs at birth are half filled with amnotic fluid. The fluids in the lungs are cleared:

through mouth and nose by pressure on the thorax during delivery. into the pulmonary capillaries. into the lymphatics and pulmonary arteries and veins.

LUNG ABNORMALITIES Ectopic lung lobes--which may arise from trachea or esophagus. Congenital cysts of the lung – Which are formed by dilatation of the terminal or larger bronchi. These may cause chronic lung infections.