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Development
of the lungs
Before birth, the lungs
are fluid filled. Lung development is divided into several
stages:
Embryonic (3-6
weeks)
Primitive bronchial buds form which enlarge to form two primary
bronchi. These will eventually develop into the left and right
lungs.
Pseudoglandular
period (6-16 weeks)
Bronchi and major blood vessels of the lung are formed. Bronchial
branching results in the formation of over 20 generations
(divisions) of airways.
Canalicular period
(16-28 weeks)
The diameter (size) of airways increases and the lung vasculature
(blood supply) continues to develop. The structure of a lung
becomes increasingly more complex as it grows. Primitive end
respiratory units start to form; these consist of a respiratory
bronchiole, leading to alveolar ducts and terminal sacs (primitive
alveoli) with associated capillaries. At the same time the
lung structure increases in complexity the cells which line
the terminal sacs begin to differentiate and surfactant
production begins at around week 22-28.
Saccular period
(28 weeks to term)
During the terminal sac period, numerous terminal sacs surrounded
by capillaries develop. Surfactant production starts to increase
significantly between 30 and 32 weeks.
Alveolar period
(first few years after birth)
Most alveoli are formed in the first and second year of life,
whilst between the ages of 3 and 8 years the alveoli increase
in size and to a lesser extent in number.
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