Lung development in a fetus begins throughout the embryonic phase, at about four weeks of gestational age. Very early in development, what will eventually be a baby is nothing more than a cell ball, wherever none of the cells are yet specialized.
An early differentiation phase separates cells into numerous different layers, one of which will enhance the organs of the gut. It is from these cells that the lungs develop, ultimately becoming capable of respiring on their own after a baby is born.
Lung developmental phases (PCTA):
- Pseudoglandular period (weeks 7–16)
- Canalicular period (weeks 16–24)
- Terminal sac period (week 24 to birth)
- Alveolar period (week 32–age 8 years)
Pseudoglandular period (weeks 7–16)
During this period, the developing lung resembles an exocrine gland. The numerous endodermal tubules (ETs) are lined by simple columnar epithelium and are surrounded by mesoderm containing a modest capillary network. Each endodermal tubule branches into 15–25 terminal bronchioles (TBs). During this period, respiration is not possible, and premature infants cannot survive.
Canalicular period (weeks 16–24)
During this period, the TBs branch into three or more respiratory bronchioles (RBs). The respiratory bronchioles subsequently branch into three to six alveolar ducts (ADs). The terminal bronchioles, respiratory bronchioles, and alveolar ducts are now lined by a simple cuboidal epithelium and are surrounded by mesoderm containing a prominent capillary network. Premature infants born before week 20 rarely survive.
Terminal sac period (week 24 to birth)
During this period, terminal sacs/primitive alveoli (TSs) bud off the ADs and then dilate and expand into the surrounding mesoderm. The terminal sacs are separated from each other by primary septae. The simple cuboidal epithelium within the terminal sacs differentiates into type I pneumocytes and type II pneumocytes. The terminal sacs are surrounded by mesoderm containing a rapidly proliferating capillary network. The capillaries make intimate contact with the terminal sacs and thereby establish a blood-air barrier with the type I pneumocytes. Premature infants born between week 25 and week 28 can survive with intensive care. Adequate vascularization and surfactant levels are the most important factors for the survival of premature infants.
Alveolar period (week 32–age 8 years)
During this period, terminal sacs are partitioned by secondary septae to form adult mature alveoli. About 20–70 million alveoli are present at birth. About 300–400 million alveoli are present by 8 years of age. The major mechanism for the increase in the number of alveoli is the formation of secondary septae that partition existing alveoli. After birth, the increase in the size of the lung is due to an increase in the number of respiratory bronchioles.
