Fig. 4. Abnormal ultrastructure of distal airway epithelium in SP-C/G6en mice. Transmission electron microscopy was used to determine the ultrastructural morphology of the distal airway epithelial cells in SP-C/G6en and SP-C/en mice at E19.5. Wild-type littermates show normal distal airway structure including AEC-1 (A, white arrowheads) and AEC-2 (A, black arrowhead) cells lining the airways, with small blood vessels closely adjoining the thin epithelial barrier (A, asterisk). In addition, numerous interalveolar septa were observed (B, bracket). In contrast, SP-C/G6en mice had AEC-2 cells lining all of their distal airways (C-E, black arrowheads). These AEC-2 cells contained large vacuoles reminiscent of the glycogen storage vesicles observed in immature AEC-2 cells in early lung development (Ten Have-Opbroek, 1991). However, many of these cells still contained lamellar bodies (F, black arrow) and secreted surfactant was observed in the airways (F, white arrow) showing that these cells were not arrested at an earlier stage of AEC-2 differentiation. Also, SP-C/G6en mice contained normal blood vessel development underlying the distal airway epithelium suggesting that pulmonary vascular development was not perturbed (C,D, asterisks).