The internal KDR pool is composed mainly of early endosomes and Rab4⁺ recycling endosomes. Unstimulated HUVECs were either stained for endogenous EEA1 (A), transfected with GFP-Rab4 (B) or GFP-Rab11 (C), or stained for endogenous CD63 (D; all in green). Cells were costained for endogenous KDR in each case (red). Nuclei are stained with DAPI (blue). Inset panels show magnified portions of each image, as indicated (dashed squares). Bar represents 10 μm. (E) The percentage of total KDR⁺ vesicles that colocalize with each of the 4 endosomal markers (mean ± SD; n ≥ 14). (F) A model of the canonical pathways of endocytic sorting. Receptors enter the cells through clathrin-mediated endocytosis and are trafficked in EEA1⁺ early endosomes to an EEA1⁺/Rab4⁺ sorting endosomal compartment. Receptors can be recycled from here either through the Rab4-dependent short-loop or through the long-loop via the perinuclear recycling compartment (PNRC), marked by Rab11. RTKs such as the EGF and PDGF receptors are not recycled but are instead sorted to CD63⁺ late endosomes and then to the lysosomal compartment for degradation.