Postnatal development of the pig kidney: ultrastucure of the glomerulus and the proximal tubule.

The detailed anatomy of the structures forming the glomerular filtration barrier and the proximal tubule was studied during postnatal development of the pig kidney. Development of the glomerular filtration barrier included three processes: Formation of visceral epithelial foot processes, flattening and fenestration of the endothelium and formation of a common epithelial and endothelial basal lamina. During formation of foot processes the distribution pattern of aggregated filaments in the epithelial cells was changed, suggesting that the filaments participate in the elongation of the foot processes. In the immature glomerulus the endothelial fenestrae were closed by thin diaphragms which disappeared during differentiation of the endothelium. Microvilli were observed in the proximal tubule cells in an early developmental stage, presumably before onset of glomerular filtration. Few cellular organelles were seen in the tubule anlage but the number of mitochondria, apical vacuoles and lysosome-like bodies increased markedly during maturation. Dense bodies identified as peroxisomes first appeared in a late development stage. The formation of nephrons continued up to about 3 weeks of age; after this time the morphological development was a differentiation of nephrons already present. The present observations, together with results of an accompanying functional study, demonstrate that changes in functional parameters reflect the structural development.