The endocytic compartment is a complex intracellular heterogeneous organelle, which consists of an extensive network of tubulo-vesicular membranes extending from the cell surface to the perinuclear area. It is involved in the uptake, sorting, and processing of nutrients, metabolites, hormones, and growth factors; the maintenance of the characteristic distribution of the plasma membrane functional domains in polarized epithelial cells; and the recycling of receptors and ligands. Although the boundaries of the endocytic compartment are not completely defined, several probes, marker proteins, subcellular fractionation, and accurate morphological information have topologically delimited and characterized the ''early/sorting'' as well as the multivesicular ''late'' endosomes. However, the ''recycling endosomes,'' characterized by the enrichment in recycling receptors and depletion of ligands that are routed to lysosomes for degradation, lack a clear-cut definition and intracellular location. Recently, using isolated endosomes, progress has been made towards the biochemical and functional dissection of the endocytic compartment of the rat liver. The compartment of recycling emerges as a very complex fraction containing structures involved in recycling, transcytosis, and the caveolin cycle. The aim of this review is to emphasize the complexity of the ''receptor-recycling'' endocytic compartment in the hepatic trafficking pathways and put together the latest molecular data, which suggest that structures contained in this compartment might be also committed to shuttling the signal transduction machinery from the cell surface to the sorting endosomes.
The hepatic cell is a useful experimental epithelial-tissue model for studying the endocytic pathways for 2 reasons: first, hepatocytes are highly active metabolically and contain Abbreviations: LDL, low-density lipoprotein; pIgR, polymeric immunoglobulin receptor; GPI, glycosylphosphatidylinositol-anchored proteins; CURL, compartment of uncoupling receptors and ligands; MVB, multivesicular bodies; RRC, receptor-recycling compartment; TGN, trans-Golgi-network.