Embryology

Respiratory system

When the embryo is 4 weeks old, an outgrowth from the ventral wall of the foregut will appear. This outgrowth is called the respiratory diverticulum (lung bud).
The appearance and location of the lung bud are dependent upon the increase in retinoic acid which is produced from the adjacent mesoderm.
The epithelial lining of the larynx, trachea, bronchi, as well as the lungs, is entirely of an endodermal origin. While the cartilaginous, muscular, and connective tissue components of the trachea and lungs are derived from the splanchnic mesoderm surrounding the foregut. Initially, the lung bud is in open communication with the foregut.
As the respiratory diverticulum expands caudally, two longitudinal ridges will form, the tracheo-oesophageal ridges separating it from the foregut into a dorsal portion, the oesophagus, and a ventral portion called the trachea and lung bud.
The respiratory primordium maintains its communication with the pharynx through the laryngeal orifice.
Any defect in these processes could lead to oesophageal atresia or tracheo-oesophageal fistula.

Larynx :

The internal lining of the larynx originates from the endoderm, while the cartilages and muscles originate from mesenchyme of the 4th and 6th pharyngeal arches
As a result of the rapid proliferation of this mesenchyme, the laryngeal orifice changes in appearance from a sagittal slit to a T-shaped opening. Subsequently, the mesenchyme of the two arches transforms into thyroid, cricoid, and arytenoid cartilages. The characteristic adult shape of the laryngeal opening can be recognized.
At the time of cartilages formation, the epithelial lining of the larynx will proliferate rapidly in a way that causes the closure of the lumen.
Later on, recanalization of the lumen occurs, and becomes patent. Besides, two lateral recesses, laryngeal ventricles, will occur.
These recesses are bounded by folding tissues forming the false and true vocal cords.
Since the muscles of the larynx are derived from the mesenchyme of the 4th and 6th pharyngeal arches; therefore, the nerve supply will be by the tenth cranial nerve (vagus n.).
The superior laryngeal nerve innervates the derivatives of the 4th pharyngeal arch, while the recurrent laryngeal nerve innervates the derivatives of the 6th pharyngeal arch.


Trachea, bronchi, and lungs :

During the separation from the foregut, the lung bud forms the trachea, and two bronchial buds.
At the beginning of the 5th week, each of these buds enlarges forming the right and left main bronchi. The right then forms three secondary bronchi, which later on form three the lobes of the right lung, while the left one forms two secondary bronchi, which later on form the two lobes of the left lung.
The pericardio-peritoneal canals, which lie on each side of the foregut, become filled with the expanding lungs; therefore, the spaces become narrow
Later on, the pleuro-peritoneal & pleuro-pericardial folds separate the pericardio-peritoneal canals from the peritoneal and pericardial cavities respectively, and the remaining spaces from the primitive pleural cavity.
The mesoderm which covers the outside of the lungs develops into the visceral pleura. The somatic mesoderm covering the body wall from the inside becomes the parietal pleura, and the space between these two pleurae is called the pleural cavity.
Further development of the lungs leads to the division of the secondary bronchi repeatedly, leading to the formation of 10 tertiary (segmental) bronchi on the right side, and 8 ones on the left side. These will create the broncho-pulmonary segments of the adult’s lung.
By the end of the sixth month, about 17 generation of subdivisions have been formed. Before the bronchial tree reaches its final shape, an additional 6 divisions is formed during the postnatal life.

Maturation of the lung :

Up to the seventh prenatal months, the bronchioles are divided continuously into more many smaller canals, and the vascular supply increases steadily.
The terminal bronchioles divide into respiratory bronchioles, and the respiratory bronchioles will divide into 3-6 alveolar ducts. The ducts end in the terminal sacs (primitive alveoli) that are surrounded by flat alveolar cells in close contact with the neighbouring capillaries.
By the end of the seventh month, sufficient amount of alveoli and capillaries are present, which are sufficient to keep the infant survived.
During the last 2 months prenatally, there are cells lining the sacs, called type I alveolar epithelium, which becomes thinner, so that the surrounding capillaries protrude into the alveolar sacs. This intimate contact between the epithelial and endothelial cells makes the blood-air barrier. Besides, at the end of the sixth month, type II alveolar epithelium appears, and begins to produce the surfactant.

Surfactant : is a phospholipid-rich fluid capable of lowering the surface tension at the air-alveolar interface.

Before birth, the lungs are full of fluid containing high chloride concentration, little protein, and some mucus from the bronchial glands.
As the concentration of the surfactant increases during the 34th week of gestation, some of this surfactant enters the amniotic fluid, an acts on the macrophages of the amniotic fluid. Once this is activated, macrophages migrate into the chorion to the uterus. It begins to produce immune system’s proteins, including interleukin-1 beta (1L-1β).
The production of the interleukins will stiumulate prostaglandin secretion, amd stimulates the uterus to contract (beginning of labour).
Foetal breathing movements begin before birth, causing the aspiration of the amniotic fluid. This movement is very important for stimulating lungs development, and conditioning of the respiratory muscles.
Due to this movement, the amniotic fluid will fill the lungs.
During birth, most of the fluid is reabsorbed, while the remaining fluid in the trachea and bronchi will be expelled to the outside.
Here (during birth), the surfactant is very important for respiration of the newborn baby, and prevents the collapse of the alveoli by lowering the surface tension.