The prenatal development of the lungs can be divided into four phases or periods: (1) pseudoglandular period (5th week to nearly 4th month), (2) ca-nalicular period, (4th month to 6th month), (3) terminal sac period (6th month to birth), and (4) alveolar period (late fetal period to childhood).
In the pseudoglandular period the segmental bronchi continue to branch in a dichotomous manner untill the smallest subdivisions of the conducting air tubes down as far as the terminal bronchioles are formed. These air tubes end blindly and are lined by simple cuboidal epithelium. Therefore, a histological section of the lung at this stage gives the false appearance of an exocrine glands (hence, the name pseudoglandular period). Respiration is not possible because no alveoli have been formed and fetuses born during this period can not survive.
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During the canalicular period
the blood vessels develop and the lung tissue becomes highly vascularised. Each terminal bronchial gives rise to two or more respiratory bronchioles, each of which then divides into three to six alveolar ducts. Respiration becomes possible at the end of the canalicular period. However, most of the fetuses born at this stage die because the respiratory and nervous systems are still immature.
During the terminal sac period the lung vascularity increases and primitive alveoli (called terminal sacs) are formed. The alveolar epithelium differentiates into type I cells (ie, squamous alveolar cells) and type II cells (ie, great alveolar cells). The type II cells secrete the surfactant, which functions to lower the surface tension at the air-alveolar interface. The blood capillaries become closely related to the primitive alveoli and adequate gaseous exchange now becomes possible. A premature infant born at about 28 weeks (or after) has very good chances of survival.
In the alveolar period mature alveoli are produced and more intimate contact is established between the alveolar squamous cells and endothelial cells of the lung capillaries. During the last two months of the fetal period and for several years after birth, the number of alveolar ducts and terminal sacs increases steadily. Mature alveoli are formed as the squamous alveolar cell of the terminal sacs and alveolar ducts become extremely thin. The adjacent
blood capillaries now bulge into the lumen of the alveolar sacs. The intimate contact between the epithelial cells of the capillaries established the blood-air barrier. It is important to note that 95% of the mature (true) alveoli develop after birth. Formation of new alveoli continues upto the age of 10 years.
The fetal chest exhibits breathing movements before birth although these are not continuous. These movements cause aspiration of some amniotic fluid into the lungs. The fetal breathing movements increase as the time of birth approaches; these movements are regarded to condition the respiratory muscles and to stimulate the lung development. Before birth the lungs are filled with amniotic fluid. After birth this fluid is cleared away and the lungs become filled by air. Most of the amniotic fluid filling the lungs is expelled through nose and mouth by pressure on the chest during delivery. The remainder of the fluid is absorbed into blood vessels and lymphatics of the lung.