Figure 4: Transmission electron micrographs showing the various stages in attenuation of the avian conduit epithelium. (a) At E12 in the chick embryo, apical elongation of the epithelial cells results in formation of aposomes (stars) and this precedes constriction of the cell at a region below the aposome (arrowheads), due to squeezing by adjacent better endowed cells [37]. (b) In the ostrich embryo at E24 several attenuation processes are evident contemporaneously. In addition to development of lamellar bodies (open arrowhead), there is lowering of tight junctions (open arrows and circle) so that the aposome (star) is clearly delineated. ((c)-(d)) A second method of extruding the aposomes demonstrated in the ostrich involves formation of a double membrane separating the basal part of the cell from the aposome (arrowheads). With subsequent unzipping of the double membrane (open arrows in (c)), the aposome is discharged. Notice the still attached aposomes (stars) and the discharged ones (asterisks in (d)). (b)–(d) are modified from [36]. Closed arrows in (d) indicate microfolds formed after rapture of vesicles.