Proliferation of alveolar interstitial cells during postnatal lung growth. Evidence for two distinct populations of pulmonary fibroblasts.

In order to define the origin and fate of lipid-filled interstitial cells (LIC) in the neonatal rat lung, 4- and 11-day-old rats were injected with tritiated thymidine (3H-T) and killed 1 to 96 h later. Thymidine-labeled lung cells were identified in autoradiographs and analyzed ultrastructurally. Interstitial cells accounted for 61% of 3H-T-labeled cells at 4 days but only 26% at 11 days. The LIC had a different labeling index than did nonlipid interstitial fibroblasts (NLIC) at each age, and also had distinct morphologic characteristics. At no time point did labeled LIC lose their lipid and become NLIC or vice versa. These results suggest that at least 2 separate populations of interstitial fibroblasts exist in the neonatal rat lung, probably with distinct functions. This supports the concept of tissue fibroblast heterogeneity.