Figure 1: Micrographs showing the changing pulmonary epithelium in the developing quokka lung. (a) At the canalicular stage, both cuboidal (closed arrowhead) and squamous epithelium (open arrowhead) are present. At the centre of the thick interstitium is a large blood vessel (V). (b) The cuboidal epithelium comprises of cells well-endowed with lamellar bodies (white arrows). These cells notably lack microvilli and may be described as pneumoblasts with a potential to form either of the two definitive alveolar pneumocytes (AT-I and AT-II). Note the large blood vessel (V) below the epithelium. ((c) and (d)) During the saccular stage the epithelial cells (E) possess numerous lamellar bodies (asterisk) and have become low cuboidal in the process of conversion to AT-I cells. AT-II cells converting to AT-I pneumocytes appear to do so by extruding entire lamellar bodies (closed arrowhead in (d)) and flattening out (arrow). Notice the already formed thin BGB (open arrowhead) and an erythrocyte (Er) in the conterminous capillary. ((e) and (f)) Immature interalveolar septa (E) are converted to mature ones through fusion of capillary layers (asterisk in (e)) and reduction in interstitial tissue. The process starts during the alveolar stage and continues during the microvascular maturation stage. Notice the thin BGB (square frames) and the thick side of the BGB in adults (open arrowhead in (f)). The Erythrocytes (Er) and a nucleus (N) belonging to a AT-I cell are also shown. (a)–(c) are from [11], (d) is from [12] while (e) and (f) were obtained from [13], all with permission from the publishers.