This study investigated the histomorphological features of developing rabbit respiratory acini during the postnatal period. On the 1st day
of postnatal life, the epithelium of terminal bronchiole consisted of clear cells which intercalated between few ciliated and abundant nonciliated
(Clara) cells. At this age, the rabbit lung was in the alveolar stage. The terminal bronchioles branched into several alveolar ducts,
which opened into atria that communicated to alveolar sacs. All primary and secondary inter-alveolar septa were thick and showed a doublecapillary
network (immature septa). The primitive alveoli were lined largely by type-I pneumocytes and mature type-II pneumocytes. The type-
I pneumocytes displayed an intimate contact with the endothelial cells of the blood capillaries forming the blood–air barrier (0.90 ± 0.03 μm in
thickness). On the 3rd day, we observed intense septation and massive formation of new secondary septa giving the alveolar sac a crenate
appearance. The mean thickness of the air–blood barrier decreased to reach 0.78 ± 0.14 μm. On the 7th day, the terminal bronchiole epithelium
consisted of ciliated and non-ciliated cells. The non-ciliated cells could be identified as Clara cells and serous cells. New secondary septa were
formed, meanwhile the inter-alveolar septa become much thinner and the air–blood barrier thickness was 0.66 ± 0.03 μm. On the 14th day, the
terminal bronchiole expanded markedly and the pulmonary alveoli were thin-walled. Inter-alveolar septa become much thinner and single
capillary layers were observed. In the 1st month, the secondary septa increased in length forming mature cup-shaped alveoli. In the 2nd
month, the lung tissue grew massively to involve the terminal respiratory unit. In the 3rd month, the pulmonary parenchyma appeared morphologically
mature. All inter-alveolar septa showed a single-capillary layer, and primordia of new septa were also observed. The thickness of
the air–blood barrier was much thinner; 0.56 ± 0.16 μm. TUNEL assay after birth revealed that the apoptotic cells were abundant and distributed
in the epithelium lining of the pulmonary alveoli and the interstitium of the thick interalveolar septa. On the 7th day, and onward, the
incidence of apoptotic cells decreased markedly. This study concluded that the lung development included two phases: the first phase (from
birth to the 14th days) corresponds to the period of bulk alveolarization and microvascular maturation. The second phase (from the 14th days
to the full maturity) corresponds to the lung growth and late alveolarization.