Lung and Digestive system

Question 1

Give an overview of lung development.

Answer 1

Lungs develop from the lateral fold, which mainly form the gut. The gut is divided in 4 regions:

1. Pharyngeal
2. Foregut
3. Midgut
4. Hindgut

The development of the lungs start at 4 weeks and it is derived from the foregut, which is of endodermal origin. The foregut will develop at first in the respiratory divertaculum, which is regulated by Retinoic acid (mesoderm part) and TBX4 (endoderm), which induce bud formation when upregulated. These two regulatory compunds will cause the formation of the laryngotracheal orifice, the opening between the trachea and larynx.

Steps:
1. Embryonic phase:
The foregut invagination form the bronchiol buds (primary, secondary - form lung lobes, and tertiary - bronchopulmonary
segments)
2. Pseudoglandula phase:
The bronchiol buds expand in the pericardioperitoneal cavity forming ciliated cells (week 10), cartilage (week 11) and
terminal bronchioles (week 16)
3. Canalicular:
During this phase vascularization of the lungs is formed, as well as the epithelium specialization in pneumocytes type I,
cells responsible for the proliferation of capillaries. In addition amniotic fluid invade the lungs, which is then absorbed at
birth.
4. Saccular:
The brionchioles subdivide to produce terminal sacs ( mature type II cells), which are responsible for gas exchange. In
addition, the surfactant appears, reducing the surface tension.
5. Alveolar:
Succuli keeps forming and they are finally divided by the septa in two parts.