Alterations in lung basement membrane during fetal growth and type 2 cell development.

We studied basement membrane development in the late fetal and in the neonatal rat lung, from the 18th day of gestation (term = 22 days) through the 8th postnatal day, with particular emphasis on the gas-exchange region of the lung. In the periphery of the lung, as type 2 cells differentiate, the continuous basement membrane develops openings beneath these cells. Basal cytoplasmic foot processes extend through these discontinuities into the underlying interstitium, often approaching interstitial cells closely. These discontinuities and extensive foot processes are associated only with type 2 epithelial cells and not with either differentiated airway cells or with the type 1 alveolar lining cells derived from type 2 cells. The type 2 cell basement membrane discontinuities and penetrating foot processes are maximal in the perinatal period and decrease in the week after birth. The appearance of openings in type 2 cell basement membrane and changes in distribution, linear density, and ruthenium red staining of anionic sites suggest that the epithelial basement membrane undergoes continuous remodeling throughout development, particularly in association with type 2 cell differentiation and growth of lung surface area. Epithelial cell foot processes may interact with underlying interstitial cells and affect the coordination of lung surface growth with the development of its connective tissue framework.