The development of the rat glomerular capillary wall was studied by morphometry at the electron microscopical level. The total area of the glomerular basement membrane (GBM) was 28 times larger in mature than in immature animals corresponding to an increase of 1.7 times when relating the increase to kidney weight. The area of endothelial pores was 1% of the GBM area in early developmental stages, but increased to 9% in adult glomeruli. The epithelial slit area was 1% of the GBM area in immature and 10% in mature glomeruli. The maturation of the epithelial slit area preceded that of the endothelial pores area. During the period of glomerular development studied here total GFR per kidney weight increases 22-fold as compared to the 1.7-fold increase in GBM area. This suggests that the increase in GBM area determines the increase in GFR to only a small extent and that a permeability change in the glomerular capillary wall contributes significantly to the development of GFR. The permeability increase may be due to physical-chemical changes in the GBM as well as increases of endothelial pore and epithelial slit areas. © 1980, Academic Press, Inc.. All rights reserved.
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