The connective tissue compartment in the terminal region of the developing rat lung. An ultrastructural study

The present study was designed to analyze the connective tissue compartment in the terminal region of the developing rat lung. Lungs of rat fetuses (at 17, 19 and 21 days of gestation) and of newborns (2, 4, 7, 10, 14 and 21 days after birth) were examined by several ultrastructural techniques; staining with tannic acid for elastic fibers (EFs), collagen fibers (CFs) and basement membranes (BMs); staining with ruthenium red for glycosaminoglycans; and immunoelectron microscopy for visualization of collagen type IV, laminin and fibronectin (FN). Dynamic changes in the levels and distribution of the components of the extracellular connective tissue were observed in parallel with the formation and maturation of the alveoli. The terminal regions of the respiratory tract of the rat fetus and of the lungs of early newborns contained few CFs and EFs, and the Bms in these lungs were incomplete. However, strong immunolabeling of FN was observed (in BMs, CFs and in spaces among fibroblasts). Starting from 4 to 7 days newborns had increasing numbers of Cfs and EFs and BMs became more extensive. In rat newborns, immunolabeling specific for FN was weak and irregularly distributed. Furthermore, the present results provide a summary of the changes in connective tissue that occur in the rat lung during different stages of its development, from the pseudoglandular stage, through the canalicular and saccular stages, to the alveolar stage.