Ultrastructural studies on selected elements of the extracellular matrix in the developing rat lung alveolus

As the respiratory system adapts to the extrauterine life, the extracellular matrix (ECM) plays an important structural and regulatory role during its development. Therefore the purpose of this investigation was to analyze ultrastructurally several elements of extracellular connective tissue during the process of rat lung development. Morphological observations were mainly focused on the terminal part of respiratory system. To outline different components of connective tissue network, several ultrastructural techniques were used (both histochemical and immunohistochemical). The distribution and amount of the following proteins were studied: laminin, collagen type IV, collagen fibrils (CFs), elastic fibers (EFs) and fibronectin (FN). Additionally localization of glycosaminoglycans (GAGs) was examined. The present study deals with four periods of lung development: pseudoglandular, canalicular, saccular and alveolar. In all these stages localization and amount of ECM components change rapidly. In early periods of lung development, the amount of connective tissue fibers was low, basement membranes (BMs) were incomplete, and FN was distributed nearly uniformly. Later when the process of lung alveoli formation begins, the number and thickness of both CFs and EFs rapidly increased, BMs became complete, the content and distribution of FN were irregular. In all stages of lung development GAGs were distributed in BMs and among connective tissue fibers. The results described in the present study summarize morphologically ECM changes occurring during formation of lung alveolus.