Figure 5: Transmission electron photomicrographs illustrating the additional mechanisms of epithelial attenuation that occur later during the attenuation process in the chiken lung. In all cases the rectangular frame delineates the BGB, BC is blood capillary and AC is air capillary. ((a) and (b)) are modified from [35] while the rest are from [37], with the kind permission of the publisher. ((a) and (b)) Formation of vacuoles (V) and vesicles (open arrows) and their subsequent rapture (fusion with the apical plasmalema) results in formation of numerous microfolds that resemble microvilli (closed arrowheads). Notice an aposome (asterisk in (b)) still attached to the cell apical membrane but still hanging above a vacuole (closed arrow). ((c) and (d)) Microfolds formed as a result of vesicle rapture (closed arrowheads) are severed so that by the time of hatching (d) there were virtually no microfolds. The BGB was similar to that of adults and the air capillaries (AC) were well developed.