FORMATION OF INTERALVEOLAR PORES IN THE RAT LUNG

Background: The aim of this morphological investigation was to obtain more information about the structural and cellular mechanisms of inter alveolar pore formation in postnatal lung development, Assuming that a lveolar pore formation is related to the general thinning of interalve olar walls observed in the postnatal period, we have focused our atten tion on the topographical relationship between epithelial cells and co nnective tissue in the septum, Thereby we tried to formulate a uniform concept of pore formation. Methods: After fixation with glutaraldehyd e and osmiumtetroxide, tissue blocks of rat lungs aged 44 days were em bedded in Epon, Serial sections were obtained in order to analyse prec isely pores and supposed sites of pore formation (type II cells and th in spots in transsections of interalveolar walls). Results: We made th e following observations: there are pores with or without type II cell s in the neighbourhood, and ''pre-pores'' with either fully transsepta l granular pneumocytes, or thin spots in the interalveolar wall consis ting of one or two layers of type I cell epithelium or of type II and type I cells without intervening connective tissue. Conclusions: From these findings we deduce that there is a general principle of interalv eolar pore formation which consists in the formation of transseptal in terepithelial cell contacts (i.e., between cells of type II and type I or type I and type I), promoted by the thinning of interalveolar wall s in the stage of microvascular maturation. Within the zone of contact the cells thin out and give way to form an interalveolar opening. (C) 1996 Wiley-Liss, Inc.