Internal structure of lungs – alveoli and their microscopic structure, surfactant, development and maturation of lungs

Development

The larynx, trachea, bronchi and lungs begin to form around the 4th week. The Respiratory Diverticulum (Lung bud) appears as an outgrowth from the ventral wall of Foregut. The Lung bud, during its separation from the foregut (by tracheoesophageal septum), forms the Trachea and two bronchial buds. Epithelium of internal lining of the: Larynx, Trachea, Bronchi and the lungs has Endodermal Origin. Cartilage, muscular and CT components of Trachea and lungs are from the Splanchnic Mesoderm.

In the 5th week each bud enlarges to form the right and left main bronchi The right bronchi then form three secondary bronchi, and the left forms two. Lung buds develop laterally-caudally to their cavity - Peri-cardio-peritoneal canals (narrow). The canals lie on each side of foregut, gradually filled by expanding lung bud. The Pleuro-Peritoneal and Pleuro-peri-cardial folds separate the Peri-cardio-peritoneal canals from the peritoneal and pericardial cavities forming the Primitive pleural cavities. The Mesoderm covering the outside of the lung develops into Visceral Pleura. The somatic mesoderm layer, covering the inner body wall becomes Parietal Pleura. The Space between the pleura is called the Pleural cavity. While new subdivisions occur (tertiary) the lungs are located more caudally, so by the time of birth the trachea bifurcation will be at level of T4.

Maturation of the lungs

Up to the 7th month:

Bronchiole divides continuously into smaller canals (Canaliculi phase) and the vascular supply increases. Terminal bronchioles divide 🡪 respiratory bronchioles 🡪 each divide into 3-6 alveolar ducts 🡪 ducts end in terminal sacs (primitive alveoli –surrounded by flat alveolar cells, with close contact to capillaries)

By the end of 7th month:

There is a sufficient numbers of mature alveolar sacs and capillaries are present to guarantee good gas exchange, enabling the infant to survive.

During the last 2 months:

Number of terminal sacs (promotive alveolar sacs)increases. Cells lining the sacs are known as Type I Alveolar epithelial 🡪 become thinner. Type II alveolar epithelial cells are producing Surfactant, phospholipid-rich fluid, and lowering surface tension at air-alveolar interface. Mature Alveoli are not present before birth.

Before birth:

The lungs are filled with fluid that contains a high chloride concentration, little protein, some mucus and surfactant from alveolar epithelial cells (type II) 🡪 the amount of surfactant increases during the last 2 weeks. As respiration begins at birth, most of the lung fluid is rapidly resorbed by the blood and lymph capillaries.

Alveolar sac

Alveolar sacs are large clusters of alveoli at the end of the alveoli duct. As alveoli are sites of gas exchange, each lung contains about 300 million alveoli.

Interalveolar septum: 2 adjacent alveolis’ membranes fused together, contains non fenestrated capillaries, dust cells (macrophages) and pores.

Alveolar epithelium made up of 2 types of cells

• Type 1 Pneumocytes: about 90% of epithelium cells. Responsible for blood-air barrier between alveoli and capillary lamina - consists of alveolar epithelium, continuous capillary and their fused basement membrane
• Type 2 – Pneumocytes: about 10% producing Surfactant. Cuboidal cells, surfactant is a compound which lowers surface tension, helps prevent alveolar collapse during expiration.

Surfactant: Located on the inner lining of the alveoli. Reducing the alveolar surface tension. The reduction of surface tension means that less inspiratory force is needed to inflate the alveoli, easing the work of breathing. Without surfactant, alveoli would tend to collapse during expiration