A massive accumulation of mononuclear phagocytes in the rat liver was found after the injection of glucan, a beta-1,3-polyglucose. Portal vessels and central veins contained large numbers of rounded and elongated cells which were adherent to the endothelium. By scanning electron microscopy most of these cells exhibited prominent lemellopodia, raised ridge-like profiles and blebs, the typical features of mononuclear phagocytes. Peroxidase cytochemistry revealed that whereas most cells in portal vessels were monocytes with peroxidase positive and negative granules, the majority of cells in central veins were macrophages exhibiting peroxidase activity in nuclear envelope (NE) and endoplasmic reticulum (ER). In sinusoids monocytes and macrophages were seen side by side. The major finding of the present study was that some cells, adherent to the endothelium or portal vessels or to the lining of sinusoids, exhibited a peroxidase pattern intermediate between monocytes and Kupffer cells, i.e. strong peroxidase activity in cytoplasmic granules, as well as a weak to moderately positive peroxidase reaction in NE and ER. These intermediate cells also contained peroxidase-negative granules with halo, which are usually seen in monocytes. Furthermore, examination of serial ultrathin sections of typical Kupffer cells revealed numerous peroxidase-positive granules and peroxidase-negative granules with halo in their cytoplasm. Finally, dividing Kupffer cells with positive peroxidase reaction in ER were found. These in vivo observations provide ultrastructural and cytochemical evidence in support of the concept of derivation of Kupffer cells from monocytes, demonstrating in addition that Kupffer cells are capable of self-replication in situ.

Mice were injected intravenously with beef liver catalase (mol wt 240,000) and very small doses of horseradish peroxidase (mol wt 40,000) and the site of localization of these enzymes in the kidney was studied by ultrastructural cytochemistry. 1 min after injection, catalase was present in glomerular capillary lumina and there was minimal permeation of the basement membrane. After 5–180 min, staining of the basement membrane increased progressively but was usually less than that in capillary lumina. At all time intervals the inner (sub-endothelial) layer of the basement membrane contained more reaction product than the lamina densa and the outer (subepithelial) layer. Catalase permeated the entire thickness of the basement membrane and extended up to the slit pore but not beyond the level of the slit diaphragm and was not seen in the urinary space or tubular lumina. Horseradish peroxidase permeated the whole thickness of the basement membrane within 2 min after injection; however, gradients of staining from the inner to outer layers of the basement membrane were frequently seen. The findings with both enzymes indicate that ( a ) the basement membrane restricts the passage of proteins over a wide range of molecular size with increasing impediment for larger molecules and ( b ) the slit pore functions as an additional barrier for molecules that cross the basement membrane.