Functional structure of the glomerular filtration barrier and the proximal tubuli in the developing foetal and neonatal pig kidney.

1. The detailed anatomy of the structures forming the glomerular filtration barrier and the proximal tubules was studied during development of foetal and neonatal pigs. 2. In the glomerular filtration barrier during ontogenetic development an increased number of fenestrae was noted in the endothelium. The condensation decreased in the glomerular basement membrane and an increase in the widths of the pedicles was observed. 3. The proximal tubular cells revealed a highly specialised organisation with an enlarged cell surface (microvilli) towards the lumen. In the apical part of the cell a zone containing vesicles probably pinched off from the base of the microvilli was observed. At the base of the cell elongated mitochondria and electron dense bodies were found. 4. An increase in the number of vesicles was observed in the proximal tubular cells from piglets receiving colostrum and exhibiting proteinuria. Judging from these results it seems probable that the nephrons are able to function already in the metanephros of the pig foetuses of 4 cm CR-length. 5. The results imply that there was no structural evidence of an immature glomerular filtration barrier being the cause of neonatal proteinuria. On the other hand an enhanced involvement of the cells of the proximal tubules was noted and this is interpreted to be due to the absorption of proteins from the primary urine after onset of colostrum ingestion by the newborn piglet.