Morphologic and functional characteristics of subpopulations of murine lung fibroblasts grown in vitro.

Fibrotic development is a common response of the lung to toxic or deleterious insult. For example, the lung is the dose-limiting organ for irradiation of the thorax for primary or metastatic lesions, due in large part to latent fibrosis. The development of the fibrotic response reflects a cascade of cell-cell and cell- extracellular matrix interactions, the ultimate target of which is the fibroblast. There is increasing evidence of subpopulations of pulmonary fibroblasts, which may have differing roles in either the initiation or progression of fibrosis. Recently we described two fibroblast subpopulations from the murine lung, which differ in the presence or absence of the membrane antigen Thy-1 (Phipps et al., 1989). These Thy-1 + and Thy-1- subpopulations are stable and differ in certain functions, such as the production of cytokines and the display of Class I1 MHC antigens. To de- termine the morphologic development of the two subpopulations and their growth characteristics in vitro, cultures of the two cell subtypes were prepared for trans- mission and scanning electron microscopy at varying stages of growth. Thy-1 + fibroblasts are more spindle-shaped, contain intracellular lipid, exhibit abundant cell-cell contacts, and are capable of secreting large amounts of collagen and mod- est amounts of fibronectin. Thy-1- fibroblasts are more rounded and spread, con- tain no intracellular lipid droplets, possess more intracellular microfilaments and microtubules, and synthesize less collagen and more fibronectin than do Thy-1 + cells. There are no significant differences between the two subpopulations insofar as growth rates are concerned, but Thy-1 + fibroblasts possess an additional DNA peak during periods of early growth. The lung has become a major focus of investigative attention due to its susceptibility to the induction of fibrotic development. Indeed, pulmonary fibrosis can be readily induced by several mechanisms, such as are encountered with drug toxicity or x-irradiation. The latter is of considerable importance in that the lung, frequently within the radiation field during treatment for primary or secondary lesions, is the dose-limiting organ for thoracic irradiation. Of particular interest to our research group are the early stages of developing pulmonary fibrosis. Mindful that the whole process reflects a cascade of cell-cell and cell-extracellular matrix interactions, we have focused our attention on the septal fibroblast and subpopula- tions thereof. Heretofore, others have reported the po- tential existence of fibroblast subpopulations, two of which have been differentiated on the basis of intracel- lular lipid. The lipid-containing interstitial cell (LIC) has been described for rat fetal and neonatal lung by Brody and Kaplan (19631, Maksvytis et al. (1981,1982, 1984), and Vaccaro and Brody (19781, among others. This cell type is prominent during the early stages of lung growth and differentiation. Those interstitial cells that do not contain lipid (NLIC) possess higher concen- trations of microfilaments and microtubules and are therefore considered to be more contractile, potentially