 light electron microscop studi develop respiratori tissu rat 1 light microscop observ develop rat lung presenc glandular canalicular alveolar stage 2 three stage identifi electron microscopi part lung time 40 45 mm length 3 glandular stage lung tissu immatur appear light microscopi electron microscopi individu cell immatur respect organel glycogen immatur cell 4 canalicular stage lung tissu vascular 5 stage develop 'duct' 'air' space line continu complet epithelium blood vessel complet endothelium 6 lamel inclus bodi epitheli endoderm cell earli stage develop 7 micropinocytot vesicl larg number epitheli endotheli cytoplasm suggest foetu indic absorpt amniot fluid alveolar space 8 mechan alveolar distens discuss natur remain uncertain 9 respiratori tissu rat fulli differenti birth fact human infant discuss 10 adult blood-air barrier consist epithelium zona diffusa endothelium vari thick project perform whilst receipt grant medic council canada gratitud express gratitud express sylvia smith type manuscript 