Figure 3: Micrographs from semithin sections ((a)–(d)) showing the coarse changes in the parabronchial epithelium and from paraffin sections showing staining for α-smooth muscle actin ((e) and (f)). ((a) and (b)) A close up of individual parabronchial tubes (PB) in the ostrich at E24, showing a cuboidal epithelium (open arrow in (a)) and a thickened columnar epithelium (open arrowhead in (b)). Note that in both cases, the nuclei remain in the basal region; the apical part of the cell becomes elongated thus reducing the parabronchial lumen (PB). ((c) and (d)) By E11 in the chick embryo (c), the parabronchial epithelium is pseudostratified and the apical parts of the cells appear club-like (open arrowheads in (c)). By E12, these apical parts are severed such that they appear to fall off into the parabronchial (PB) lumen (open arrowheads in (d)). Dark arrowheads in (c) show developing capillaries. ((e) and (f)) Chick lung stained for alpha-SMA at E8 and E19 respectively. These alpha-SMA positive cells (open arrows in (e)) surround the developing parabronchus (PB) while leaving some gaps (closed arrowheads in (e)) for future migration of atria. At E19, the atria are well formed and the alpha-SMA positive cells are restricted to the apical parts of the interatrial septa (open arrows in (f)). (a) and (b) are modified from [36] while (c)–(f) are from [15].