 electron microscopi bovin lung normal blood-air barrier lung 20 healthi matur cattl fix oso embed polyest resin studi electron microscopi blood-air barrier variabl thick consist continu epitheli alveolar line cytoplasm epitheli cell contain membran-bound aggreg granul secretori structur epithelium rest basement membran turn continu rest adjac capillari endotheli basement membran continu merg connect tissu capillari endothelium fenestr continu cellular membran epithelium endothelium contain numer caveola pinocytot vesicl cell alveolar wall includ fibroblast lymphocyt macrophag occasion mast cell elastin observ 