Lung development was studied in late prenatal, 1-, 7-, 14-, and 21-days postnatal and adult cats. Cats were born with a few alveoli, and the lungs appeared to have patches of primitive air spaces (saccules). The saccules of prenatal kittens were thick walled, very cellular, and lined by type II pneumocytes. Eosinophils were observed in the septum, intraepithelially, and in the alveolar space of growing cats. Secondary septa were flanked by a double capillary network and divided saccules into multiple shallow alveoli. Septation was irregular and time dependent and not completed by day 231 of postnatal life. Elastic fibers accumulated at the tip of the septa, seemingly playing an important role in alveolar formation. Type II pneumocytes were located at the base of the secondary septa in growing cats, thus strengthening secondary septa to withstand the stresses of respiration. Pores of Kohn were not observed in growing cats.
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