CELL-SPECIFIC EXPRESSION OF FIBRONECTIN IN ADULT AND DEVELOPING RABBIT LUNG

Fibronectin (FN), a glycoprotein component of the extracellular matrix , plays a role in tissue morphogenesis and tissue-specific differentia tion through its effects on cell adhesion, cell shape, and cytoskeleta l organization. Immunohistochemistry has been used to show that during lung development FN deposition changes, yet the cell-specific sites o f pulmonary FN synthesis have not been determined. Because cellular FN synthesis is reflected by FN mRNA abundance, we performed in situ hyb ridizations to identify pulmonary tissue with the capacity to synthesi ze FN. Both in situ mRNA hybridization and immunohistochemical stainin g were performed on tissue sections from lungs of adults and late gest ation fetal and neonatal rabbits. In adults, FN transcripts and immuno staining were clearly seen in endothelial cells, smooth muscle cells, and chondrocytes. During lung development, FN transcripts were virtual ly ubiquitous: except in airway epithelium. There was a gradual decrea se in FN mRNA abundance with advancing fetal age, but low levels of FN mRNA persisted in neonatal and adult lungs. In contrast, parenchymal immunostaining increased throughout fetal development and remained ele vated in the newborn. FN immunostaining was lower in adult lung. In al l tissues examined, airway epithelial cells contained no FN transcript s above background. However, immunostaining was detected in airway bas ement membrane zones and on luminal surfaces of some epithelial cells. The lack of FN transcripts in airway epithelial cells suggests that F N synthesis does not normally occur in this cell type and that its ass ociated FN immunostaining is from another source. The colocalization o f FN mRNA and protein in pulmonary endothelial cells, smooth muscle ce lls, and chondrocytes in adults strongly suggests that these cells are sites of FN synthesis.