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The spatial organization of endoplasmic reticulum (ER) was examined in all segments of rat nephron. Tissues were fixed with glutaraldehyde, impregnated "en bloc" with osmium tetroxide, prepared for and examined by standard (80-100 kV) and high voltage (1 mEV) transmission electron microscopy. In all proximal tubule cells, ER forms a continuous and extensive network of canaliculi and abundant fenestrated saccules which surround mitochondria and cytoplasmic bodies; the cage-like structure of the fenestrated saccules was most evident around the spherical mitochondria of the S3 segment. In the cells of the distal straight and convoluted tubules, the network consists mostly of canaliculi with rare non-fenestrated saccules. The ER network of canaliculi is particularly rich in intercalated cells, in contrast with its rudimentary appearance in the adjacent principal cells of the collecting tubule. In fact, in these cells there are few isolated ER cisternae and they are rarely impregnated. The nuclear envelope is well impregnated in most cells throughout the various segments. Segmental variations in ER organization and its relative abundance are most likely related to the well, established functional heterogeneity of the nephron segments. Moreover, the extensive and unique organization among mitochondria, ER and the basolateral membrane suggests that these three organelles function as a unit which is related to active electrolyte transport. In addition, because of its transepithelial organization, ER may well constitute a transcellular pathway for molecules.

The morphologic interrelationships between mitochondria, the endoplasmic reticulum (ER) and other organelles were examined in rat kidney cells by stereomicroscopy of thick sections (0.3-1.0 micron) using either standard transmission (80-100 kV) or high voltage (1 mEV) electron microscopy. Mitochondria fit into three different categories: (1) elongated cylinders observed in S1 and S2 segments; (2) irregular lamina in the cortical ascending limb or plates in the distal straight and convoluted segments; (3) small spheres or short rods mostly in intercalated and principal cells of the collecting tubule. The chondrioma occupies a large volume in all cells except in the thin limb and in principal cells of the collecting tubule. This volume occupied by the chondrioma is likely to be related to metabolic functions, but its polymorphic configuration could also be explained by a passive adaptation of the mitochondria to the space left by the basilar membrane infoldings and the ER network, which was found to have an extensive three-dimensional organization which varies, as for the mitochondria, with the cell type. In fact, in the proximal nephron, mitochondria surrounded by the ER and the plasma membranes appear to form a functional unit. Organelle interrelationship is extensively modified during ontogeny or under hormonal influence. Disruption in organelle relationships or in their motility could probably have far reaching consequences which would be more deleterious than the pathological lesions observed at the level of a single organelle. The extent of the disassembly can be fully explored only with thick sections and stereomicroscopy.

We investigated the process of neonatal hyperprolinuria in dog and rat. Plasma proline varied only 2-fold in the puppy whereas prolinuria increased up to 12-fold from birth to the 10th day declining thereafter to reach adult values (