Formation of interalveolar pores in the rat lung.

Abstract

The aim of this morphological investigation was to obtain more information about the structural and cellular mechanisms of interalveolar pore formation in postnatal lung development. Assuming that alveolar pore formation is related to the general thinning of interalveolar walls observed in the postnatal period, we have focused our attention on the topographical relationship between epithelial cells and connective tissue in the septum. Thereby we tried to formulate a uniform concept of pore formation. After fixation with glutaraldehyde and osmiumtetroxide, tissue blocks of rat lungs aged 44 days were embedded in Epon. Serial sections were obtained in order to analyse precisely pores and supposed sites of pore formation (type II cells and thin spots in transsections of interalveolar walls). We made the following observations: there are pores with or without type II cells in the neighbourhood, and "pre-pores" with either fully transseptal granular pneumocytes, or thin spots in the interalveolar wall consisting of one or two layers of type I cell epithelium or of type II and type I cells without intervening connective tissue. From these findings we deduce that there is a general principle of interalveolar pore formation which consists in the formation of transseptal interepithelial cell contacts (i.e., between cells of type II and type I or type I and type I), promoted by the thinning of interalveolar walls in the stage of microvascular maturation. Within the zone of contact the cells thin out and give way to form an interalveolar opening.