The lung development can be divided into four developmental stages. The first is the pseudoglandular stage (5th-17th week). During that stage, the lung buds proliferate and branch in the surrounding splanchnic mesenchyme. All major elements of the lung are formed during this stage. The canalicular stage (16th-25th week) is characterized by the widening of the bronchi and terminal bronchioli. The terminal bronchiole divides to form two or more respiratory bronchioles that then divide into several alveolar ducts. The lung tissue becomes highly vascularized and the airway epithelium lining changes from columnar to cuboidal. Respiration is possible when the cuboidal respiratory epithelium is replaced by thin, flat cells closely associated with blood capillaries. Many thin-walled terminal sacs develop during the terminal sac stage (24th week birth).

1. Respiratory bronchiolus
2. Blood capillaries
3. Cuboidal epithelium
4. Terminal bronchiolus

1. Thin squamous epithelium
2. Blood capillary
3. Cuboidal epithelium
4. Terminal sac

By the 24th week the terminal sacs are lined mainly by squamous epithelial cells of the endodermal origin, type I pneumocytes, and rounded secretory epithelial cells, type II pneumocytes. Next stage in lung development is the alveolar stage (late fetal period to childhood). It is characterized by the increase of respiratory bronchioles and primitive alveoli.

1. Blood capillary
2. Mature auveolus
3. Terminal bronchiolus
4. Thin squamous epithelium