AbstractThe present study defined the ultrastructural features of peritubular capillary development. Two‐day‐old beagle puppies and adult dogs were perfused with 2.5% glutaraldehyde and routinely prepared for light and transmission electron microscopy. Some of the fixed tissue was subsequently used to make freeze‐fracture replicas.The outer cortex of the puppy kidney possessed large, thick‐walled vessels best termed sinusoidal capillaries instead of the small caliber vessels (peritubular capillaries) noted in the adult. These sinusoidal vessels showed extensive overlapping of the endothelium with isolated patches of fenestrae. Their luminal surfaces were irregular, owing to prominent ridges and sporadic bulges of endothelium. The basement membrane of most vessels was not present. Interstitial spaces were filled with mesenchymal cells and cells closely resembling pericytes. The diameter of the fenestrae of vessels throughout the cortex was similar; however, the number of fenestrae per micrometer of endothelium increased significantly from outer to inner cortex. Vessels of the inner cortex were also immature when compared to the adult. From these morphological findings, it was apparent that a true peritubular capillary system does not exist in the two‐day‐old puppy. Ultrastructural features of these vessels suggested reduced permeability char
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