The lower respiratory organs (larynx, trachea, bronchi and lungs) begin form during the fourth week. The endoderm lining gives rise to the epithelium and the glands. The connective tissue, cartilage and smooth muscle develop from the mesoderm surrounding the foregut.
By the end of fourth week, a pouchlike laryngotracheal diverticulum (lung bud) forms. As this diverticulum elongates, its distal end enlarges to form a globular respiratory bud. The laryngotracheal diverticulum soon separates from the primordial pharynx. Tracheoecophageal septum forms and divides the cranial part of the foregut into the laryngotracheal tube and a dorsal part which include primordium of the orophrarynx and esophagus.
The globular respiratory bud divides into primary bronchial buds. Later secondary broncial buds form. Early in the fifth week the connection of each bronchial bud with the trachea form the primordium of a main bronchus. The main bronchi subdivide into secondary bronchi and segmental bronchi. As the lungs develop, they acquire a layers of visceral pleura and parietal pleura from the mesoderm.
Maturation of lungs is divided into four stages:
Pseudoglandular period (6-16 weeks)
By 16 weeks, all the major elements of the lungs have formed, except those involved with gas exchange.
Canalicular period (16-26 weeks)
During this period the lumina of the bronchi and the terminal bronchioles become larger and the lung tissue becomes hightly vascular. By 24 weeks, each terminal bronchiole has given rise to two or more respiratory broncioles. Respiration is possible when terminal sacs are formed at the end of this period.
Terminal Sac period (26 weeks to birth)
More terminal sacs develop and capillaries begin to bulge into these developing alveoli. Pneumocytes type 1 ( gas exchange occur) and pneumocytes type 2 cells which produce surfactant are developed. The production of surfactant begins by 20 weeks but increases particularly during the last two weeks before birth. Surfactant reduce surface tension and facilitates expansion of the terminal sacs by preventing collaption during exhalation.
Alveolar period (32 weeks to 8 years)
Terminal sacs represent the future alveolar ducts. New alveoli may be added until 8 years of age.
HEART…just a little bit
Paired endothelial strands, angioblastic cords, appear in the cardiogenic mesoderm during the third week of development. These cords fold the heart tubes that fuse to form the tubular heart late in the third week. Three paired veins drain into the tubular heart of a week 4 embryo: Vitelline veins, Umbilical veins and common cardinal veins.
As lateral embryonic folding occurs, the heart tubes approach each other and fuse to form a single tube. A primordal myocardium is formed from mesoderm. At this stage the primordial myocardium is separated from endothelial tube by connective tissue called cardiac jelly. The endothelial tube becomes the internal endothelial lining of the heart, the endocardium and the primordial myocardium becomes the muscular wall of the heart, the myocardium. The epicardium is derived from the mesothelial cells. The tubular heart elongates and develops ventricles, atria and truncus arteriosus.