5 Different Phases Of Fetal Lung Development by Dr. Srikanta J T

The Stages of Fetal Lung Development

Fetal lung development can be divided into five phases. Let us know them in detail:

1. Embryonic Phase

During this phase, the cells or tissues that will take the form of the larynx and the trachea from the layer differentiate to form organs of the foregut. The lung bud develops from the trachea. Two little buds are formed from the trachea, one becomes the left lung, and the second one becomes the right lung eventually.

The separation of the two lung buds happens when the esophagus-tracheal ridges fuse to form the esophageal-tracheal septum. At around 8-weeks, the lung buds subdivide and form the bronchopulmonary segments.

• Lung buds are lined by epithelium that is derived from endoderm, which is differentiated into:
• Respiratory epithelium that lines the airways
• Specialized epithelium that lines the alveoli
• The innervation of the lungs is derived from the ectoderm.
• The pulmonary blood vessels, smooth muscle, cartilage, and other connective tissues originate from the mesoderm

2. Pseudoglandular Phase

The pseudo glandular stage happens between the seventh and sixteenth weeks of embryonic development. Further branching of the lung buds into several tiny units characterizes this phase. Eventually, every bud grows into an independent respiratory unit. It comprises a bronchiole and various capillary vessels.

They provide blood to the lungs for oxygen. During this stage, the principal separation of lung epithelium takes place. By the thirteenth week of gestation, cilia appear in the proximal airways.

By the sixteenth week of pregnancy, branching out of these bronchial tubes stops, and by this time the fetus has the same number of branches found in an adult lung, despite the fact that the tubes themselves are still very tiny

3. Canalicular Phase

The Canalicular stage takes place between the 16th and 25th week. During this time, the gas-exchanging segment of the lung is formed and vascularized. Lung tissue keeps on growing, adding millions of little new air sacs, termed alveoli, to the growing lung. As each lung grows further, arteries and vein branches follow its growth, supplying blood to the new tissues. In due course, small capillaries grow around each alveolus.

This brings the growing baby’s blood closely in contact with the lining of the cells. An air and blood barrier develops and this permits oxygen to enter the blood and carbon dioxide to come out of respiratory capillaries into the lungs for exhalation. Different kinds of tissues develop in the fetus’s lungs during the canalicular phase.

By 20 to 22 weeks of gestation, two types of special cells, known as type I and type II cells, develop and line the alveoli. In due course of time, air-transporting tissues can be distinguished from the tissues carrying gas

4. Saccular Phase

The saccular stage includes the period from 26 weeks of gestation until the term that is around the 36th week of gestation. During this stage, more alveoli are formed. Therefore, the tissues between them compact distinctly. The lining of the alveoli turns out to be considerably thinner.

During this period, the lining cells start to take on the qualities required for their function after birth. Type I cells turn out to be very thin, which helps to allow the exchange of gases between air and the blood after birth. Type II cells develop the features they need to deliver a highly specialized chemical called surfactant.

Surfactant is a soapy liquid that contains both protein and fat, which helps to keep the lung tissue delicate and prevents it from:

• Sticking to itself
• Tear during exhalation or lung compression

Surfactant is essential during delivery because it permits the amniotic fluid in the lungs to clear out and fill the lungs with proper air. Surfactants form a coating on the inner surface of each alveolus and avoid it from crumpling. From as early as the 24th week of pregnancy, the type II cells contain a small quantity of surfactant.

From that time until birth, as a part of preparing the baby for his first breath outside the womb, these cells manufacture and store additional surfactants. Babies born prematurely are more liable to pulmonary issues and breathing issues as the lungs will collapse if they are born before the proper development of surfactant.

[Read : Common Health Issues Of Premature Babies]

5. Alveolar Phase

The alveolar phase is the final phase of lung development. The duration of this phase starts just before birth and extends into early childhood. Apart from the extra production of surfactants, the development of lung development during this period is characterized by the formation of more alveoli.

This permits the tissues carrying gas of the lungs to grow and makes them proficient in moving more air as the baby grows. At birth, only about one-eighth to one-sixth of the adult numbers of alveoli are present in the baby’s lungs. Air sacs continue to form until the child is around 8 years old. Therefore, around 95 percent of alveoli are formed after birth.

The process of lung development starts quite early on in the fetal stage. This development continues till the child is about 8 years at which age, the lung development completes and the lungs are completely formed and mature.

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