I n his pioneering work on the ultrastructure of the hepatic sinusoid 30 years ago, Wisse (1) observed numerous ''smoothwalled macropinocytic vesicles'' in the sinusoidal liver endothelial cells (LEC), suggestive of active pinocytic scavenging of blood plasma proteins. The subsequent finding that the specific activities of lysosomal enzymes in LEC were as high as in the Kupffer cells (2), and for some enzymes even higher, substantiated Wisse's notion. In a series of papers over the next 20 years, it was shown that LEC eliminate an array of soluble macromolecular physiologic and foreign waste products from the circulation by receptor-mediated endocytosis (3-7). It is now well established that the population of LEC in mammals serves as the most important site of elimination of an array of circulating soluble macromolecular waste products. To carry out this scavenger function, LEC express at least five types of specific receptors for endocytosis of major physiologic waste products: (i) the hyaluronan receptor for major matrix polysaccharides and proteoglycans such as hyaluronan and chondroitin sulfate (8); (ii) the collagen alpha-chain receptor for collagen alpha-chains of several types of collagen (9); (iii) the scavenger receptor for amino-terminal propeptides of types I and III procollagen (4), modified macromolecules such as atherogenic advanced glycation end (AGE) products (5) and oxidized LDL (6), oligodeoxynucleotides (10), and blood proteins modified by the process of blood clotting and platelet activation (11); (iv) the mannose receptor for carboxy-terminal propeptide of type I procollagen (7) and tissue plasminogen activator (12); and (v) the Fc-␥ receptor for IgG-antigen immune complexes (13). It is interesting to note that the LEC are able to clear all major categories of biological macromolecules (proteins, polysaccharides, lipids, and nucleic acids) by means of these five types of endocytosis receptors. The blood concentration of most of these substances is normally very low (ng͞ml) because of highly efficient elimination by endocytosis receptors on LEC.On the basis of several studies showing that LEC represent an important nonphagocytic scavenger cell in mammals, it has been suggested that the hepatic scavenger function is shared between Kupffer cells, which eliminate insoluble waste by phagocytosis, and LEC, which remove soluble colloidal or macromolecular waste (3). Hypothesizing that the scavenger function of LEC in mammals be represented by similar nonmacrophagic, nonphagocytic scavenger endothelia in animals of all vertebrate classes, we set out to screen animal species from the seven major vertebrate classes for the presence of scavenger cells equivalent of mammalian LEC.The findings from these studies strongly suggest the presence of a nonmacrophagic system of scavenger endothelial cells that clear the blood from an array of circulating waste macromolecules and colloids. These scavenger endothelial cells make up the backbone of the reticuloendothelial system (RES), and thus represent an import...