The sequential appearance of eGFP neg -SpC pos (immature type-II pneumocytes), eGFP pos -SpC pos and eGFP neg -SpC pos (mature type-II pneumocytes) may suggest that the eGFP pos -SpC pos population is an intermediate step through which immature type-II pneumocytes must pass before developing into mature functionally competent type-II pneumocytes. This conclusion is compatible with the finding that mice with canonical BMP pathway disrupted due to deletion of the Smad1 gene accumulate in their developing lungs high numbers of periodic acid-Schiff (PAS) positive immature type-II pneumocytes [19] .