The Air-Blood Barrier
By light microscopy it is not generally apparent whether cells in the interalveolar septa are endothelial, epithelial or interstitial. In pathological states which widen the septal interstitium, a distinct epithelium can sometimes be recognised, but its existence in normal lung remained a matter of some controversy until the advent of the electron microscope. Fine structural studies established that there was a very thin but complete epithelium which extended throughout the alveoli, alveolar sacs and ducts and into the respiratory and terminal bronchioles. At a variable point within the bronchioles there is an abrupt transition from flattened to columnar epithelium. The epithelium of the respiratory portion of the lung is therefore continuous with that in the airways, this whole lining being derived from the original endodermal bud. The epithelium is separated from underlying mesenchymal derivatives by a supportive basement membrane.
KeywordsAlveolar Epithelium Pulmonary Trunk Diffuse Alveolar Damage Hyaline Membrane Disease Phosphatidic Acid Phosphatase
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