Development and Remodeling of the Vertebrate Blood-Gas Barrier
Figure 2
Schematic diagrams showing the steps in attenuation of the epithelium in the mammalian lung. (a) At the pseudoglandular stage, lung tubules are lined with high columnar homogeneous cells with the intercellular tight junctions placed high up towards the tubular lumen (open arrows). Notice also that the cells are devoid of microvilli (open arrowheads). (b) As the epithelium attenuates, cells develop lamellar bodies (open arrowheads) and there is lowering of intercellular tight junctions as the cells become stretched and also the intercellular spaces widen (closed arrows). The epithelial cells at this stage are no longer columnar but cuboidal and the tight junctions have been lowered to the basal part of the epithelium (open arrows). (c) The cells destined to become squamous pneumocytes (AT-I cells) become thinner (closed arrow), extrude their lamellar bodies (open arrowhead) and approximate blood capillaries (BC) so that a thin BGB is formed. Other cells differentiate to ultimate AT-II pneumocytes (closed arrowheads) and have well developed lamellar bodies. Notice also the depressed position of tight junctions (open arrows). Fibroblasts (f) are abundant in the interstitial tissue and are important in laying down collagen.
