Ultrastructural changes of the filtration barrier during mouse nephron development.

The development of mouse renal corpuscles localized in the juxtamedullary zone of renal cortex was assessed with the aid of light and electron microscopy. The observations have confirmed the existence of four developmental phases of the nephron. The analysis of filtration barrier components has shown that during the development of the kidney the height of maturing podocytes becomes markedly reduced, and that adherence and occluding junctions formed between them shift from their apices towards their bases. Ladder-like junctions have also been observed between podocytes. The number of their foot processes steadily grows, they become narrower and filtration slit diaphragms begin appear between them. Fusion of podocytes basement membranes and endotheliocytes leads to formation of a doubly thick filtration membrane. As the tuft of the capillary glomerulus grows, newly formed fragments of the basement membrane in the form of loops and pouches start to be evident in its close neighborhood. Large endotheliocytes become greatly flattened and numerous fenestrae appear in their cytoplasm.