The development of the respiratory system starts with the development of the larynx. The cranial portion of a 29-day-old embryo with four pharyngeal arches is cut on the frontal (coronal) plane starting from the 1st arch. The pharyngeal pouches, laryngeal orifice, esophagus, lung buds and pericardioperitoneal canals are visible in transparency. The pharyngeal arches and pouches, including the laryngeal orifice, are rotated toward the spectator and magnified in 3D. The lateral lingual swellings, tuberculum impar, foramen cecum, copula, epiglottal and arytenoid swellings are shown. From the 29th to the 42nd day, the epiglottal and arytenoid swellings, tongue and cervical sinus develop. From the 42nd day to the 12th week, the epiglottis, arytenoid cartilage and laryngeal orifice achieve their definitive forms. The development of the tongue, circumvallate papillae and lingual follicles is also animated.

The next animated chapter is dedicated to the development of the lungs and pleural cavities.

An approximately 20-day-old embryo shows the pericardial celom and celomic canals. On the 21st day, cephalocaudal folding of the embryo occurs, and the pericardial cavity is shifted into the future thorax. The pericardial cavity and the pericardioperitoneal canals are rotated toward the spectator in the next diagram, which reveals the opened yolk sac, percardioperitoneal canals, anterior intestinal portal, future midgut, pericardial cavity, endocardial tubes and foregut. From the 21st to 24th day, the pericardial cavity grows and communicates with the pericardioperitoneal canals. The posterior cardinal and umbilical and vitelline veins converge towards the heart. The 1st aortic arches are also shown. The vitelline duct forms.

In the next sequence the pericardial cavity along with the heart is cut out and removed. The heart is sectioned at the level of the truncus arteriosus and the sinuatrial junction. The diagram shows the pericardial cavity, openings of the pericardioperitoneal canals, pleuropericardial folds, lung buds and several other structures. The extent of the pericardioperitoneal canals in an approximately 24-day-old embryois is demonstrated.

The next sequence is dedicated to the growth of the lung bud into the pericardioperitoneal canals and the separation of the pleural cavities from the pericardial cavity. From the 24th day to approximately the 5th week, the pleuropericardial membranes approach one another, and the septum transversum grows with the liver bud to approach the pleuroperitoneal folds. The lung bud grows and divides into bronchial buds, which ramify within the cranial portion of the pericarioperitoneal canals. The evolution of other structures is also animated.

The cranial shift of the sinuatrial junction and its transformation into the sinus venarum, the fusion of the pleuropericardial membranes and the closure of the pericardioperitoneal canals by the pleuroperitoneal folds (i.e. the formation of the definitive pleural cavities) are demonstrated in an embryo at the end of the 8th week. In transparence, the dorsal ramifications of the bronchial bud from the pleuropericardial membranes can be followed.

The next sequences illustrate the development of the lungs under high magnification.

An approximately 25-day-old embryo shows the pericardial cavity, pericarioperitoneal canals and heart, among other structures. The heart is cut out and removed in an approximately 26-day-old embryo. The foregut (i.e. future esophagus) appears between the two pericardioperitoneal canals. The lung bud sprouts from the foregut. In the lower part of the diagram, the liver bud and septum transversum are visible.

Next, the lung bud and esophagus are cut transversally. In an approximately 27-day-old embryo, the lung bud starts to growth caudally. One day later, the lung bud continues to grow and divides into two bronchial buds. At the same time, the septum transversum enlarges laterally, while the pleuroperitoneal folds grow toward the septum transversum. The positions of the mesothelium, endoderm and splanchnopleuric mesoderm are indicated. From the 28^th day to the 5th week, the bronchial buds ramify, and the pericardioperitoneal canals gradually close after the fusion of the pleuroperitoneal folds with the septum transversum. The diagram shows the trachea, two primary bronchi, three secondary bronchi on the right side and two secondary bronchi on the left side.

The next sequence depicts the phases of lung maturation, beginning with the pseudoglandular phase.

From an approximately 5-week-old embryo to an approximately 16-week-old fetus, the dichotomous branching of bronchial buds produce secondary and tertiary bronchi, bronchioles and terminal bronchioles, and the periocardioperitoneal canals close. The respiratory bronchioles form through the division of the terminal brochiole.

The canalicular phase of lung maturation is shown under high magnification in a 20-week-old fetus. A terminal bronchiole divides into two respiratory bronchioles. The cuboidal epithelium of the respiratory bronchiole is surrounded by the splanchnopleuric mesoderm with blood vessels and the mesotelium, which give rise to future visceral pleura. On the bottom of the diagram, the somatopleuric mesoderm, also lined by the mesothelium, forms the future parietal pleura. In an approximately 26-week-old fetus, the future respiratory bronchioles lengthen with their end portions, and the future alveolar ducts appear.

The terminal sac phase takes place from the 26th week through birth. The histologic delimitation of the respiratory bronchioles and the formation of the terminal sacs and future alveolar ducts are illustrated in a fetus just before birth.

The next sequence shows the alveolar phase of lung maturation, which lasts from the 8th intrauterine month through the 10th year. After the first inspirations, the expansion of the terminal sacs (i.e. primitive alveoli) and the formation of respiratory bronchioles and alveolar ducts are animated with the differentiation of the type I and II aveolar cells. At the same time, both viseral and parieral pleura develop.

In order to demonstrate the formation of the alveolar wall, a segment of the terminal sac and future pleurae of an approximately 26-week-old fetus is framed and observed under high magnification. The diagram shows the cuboidal future alveolar epithelium over blood capillaries and mesenchymal cells of the splanchnopleuric mesoderm. Facing the latter lie the mesenchymal cells of the somatopleuric mesoderm. From the 26th week to a fetus just before the first inspiration, the cubolidal epithelium flattens and forms type I alvelar cells, while type II alveolar cells synthesize the surfactant. Simultaneously, the capillaries form below the alveolar epithelium, and a monocyte escapes from one capillary. The splanchno- and somatopleuric mesenchymal cells differentiate into the cells of the visceral and parietal pleura. The animation continues with the formation of the postnatal aveolar wall lined by flattened type I alveolar cells. The monocyte transforms into the alveolar macrophage and phagocytizes an inhaled particle.

The completion of lung development from the 20th week to the 10th year is presented under moderate magnification. The anatomic subdivision of the fully developed lung is shown.

Lung development is also animated by a transversal section of an approximately 25-day-old embryo, which is cut at level of the sinus venosus and the lung bud and observed form above. The diagram shows the sectioned lung bud, pericardioperitoneal canals, pericardial cavity, pleuroperitoneal folds, pleuropericardial folds with common cardinal veins, umbilical and vitelline veins, septum transversum, common atrium of the heart, mesocardium, sinus venosus and other details. An approximately 6-week-old embryo shows the separation of the pericardial cavity from the pericardioperitoneal canals by the pleuropericardial folds. The pericardioperitoneal canals and the pericardial cavity separate from the peritoneal cavity by the pleuroperitoneal folds and the septum transversum, and the lung bud sprouts within the pericardioperitoneal canals. Finally, the pleuropericardial folds fuse with the mesopulmonum and mesocardium, and the diaphragm forms. At the end of the 8th week, the pericardial cavity separates from the pleural cavities and the peritoneal cavity. The lungs fill the pleural cavities, and the right common cardinal vein transforms into the superior vena cava.

As malformations of the respiratory system, tracheoesophageal malformations (including 4 cases of tracheoesophageal fistulae) are animated and compared with normal development. The following lung and bronchial malformations are partially animated: bilateral lung agenesis, unilateral lung agenesis, lung aplasia, lung hypoplasia, supernumerary lobes, ectopic lung lobes and congenital lung cysts.

(This animation is essential for students of medicine, veterinary medicine, and biology as well as for departments of anatomy, histology, embryology, physiology and cell biology; it is also recommended for students of stomatology and departments or clinics of physiology, pediatrics and pulmonology.)